domain: sde shs

text:
Daniil Kulikov

Orcid

The new technological Solution for sports activities in non-professional organizations in the context of the green business: challenges and perspectives

Keywords: environmental protection, new technological solution, artificial ice, modern ice arenas, modern ice rinks

In recent years, sports organizations related to winter sports have increasingly focused on environmental sustainability. The top management of sports clubs aims to use only ice arenas that adhere to the principles of the green economy, considering the impact of global warming. Major sports federations and clubs also recognize the rapidly growing sector of environmental science in the sports industry, as sports facilities have a significant negative impact on the environment. In this context, the use of artificial ice is considered a new way of providing ice services for the sport population with minimal environmental impact and reduced costs. This research aims to study the potential of using new technologies, particularly artificial ice, in accordance with the principles of a green economy, and to develop recommendations for sports clubs for the development of winter games on ice arenas in the future. The authors employed empirical methods such as questionnaires and structured interviews during the study. A total of 3 surveys were conducted with potential service consumers and providers of natural and synthetic ice: 132 ice hockey players, 12 representatives of ice arena management from Latvia and France, and 5 experts promoting the use of synthetic ice. The authors hope that their conclusions and recommendations will be of practical importance to investors, coaches, and managers of ice arenas, as well as all interested parties associated with winter sports and ice arenas focused on the principles of a green economy.

INTRODUCTION

One of the most important tasks for leaders of sports clubs, ice arenas, and companies serving sports events nowadays is to develop ways for further development based on national strategies and the foundations of building a "green" economy. A green economy aims to reduce environmental risks and ecological scarcities, and to achieve sustainable development without degrading the environment. It is closely related to ecological economics, but has a more politically applied focus (UNEP, 2011). Experts at the United Nations Environment (UNEP) view the green economy as economic activity "that enhances human well-being and social justice, while significantly reducing environmental risks and environmental degradation» (UNEP, 2021). Most sports organizations related to winter sports pay great attention to the environment.

Considering the global warming situation, top management of sports clubs strives to use only those ice arenas where the principles of the "green economy" are taken into account.

Managers of sports organizations initiate ongoing expert assessments, scientific research, and discussion of the impact of mass sports winter events on the environment, and introduce environmentally friendly modern technologies. This is understandable because commercialization and the Business Intelligence of the sports industry are becoming important features of modern society, as well as modern approaches in the environment sector (Cotts et al., D; Roper, K; Payant, R, 2010).

The increased interest of the leadership of sports organizations towards new technologies in the construction of new sports centers, expansion, and renovation of ice rinks is giving new opportunities for business. Leaders of major sports federations and clubs are also aware that the environmental impact of the sports industry is a rapidly growing sector of humanity, since sport facilities bring devastating environmental impact to our generation and affect our sustainable environment negatively. Some authors in their research have measured the impact and consequences of ice rink maintenance on the environment (Karampour, M. &2011; Schulz,J, 2012). In this case, the management of the clubs as customers and stakeholders is trying to use the concept of artificial ice, which can be categorized as a completely new way of ice service for the sport population with minimum impact on the environment and significant cost savings. The aim of this research is to study the perspectives of using new technologies, particularly artificial ice, in accordance with the concept of a green economy to develop recommendations for sports clubs for the development of winter games on ice arenas in the future. During the analysis, the authors draw attention to the destructive impact of large sports facilities on the environment, the need to collect independent research data, and the need to conduct data analysis to be able to conduct more thorough research and make better recommendations. The authors agree with I. Yatskiv's point of view that "The concept of Data Analysis is becoming increasingly popular as a business information management tool where it is expected to reveal knowledge structures that can guide decisions in conditions of limited certainty" [START_REF] Yatskiv | Methods of data analysis and business forecasting course[END_REF]. This study aims to determine if replacing natural ice with artificial (synthetic) ice will maintain the quality of sports activities for a non-professional group of population.

The following questions were formulated: RQ1: What is the environmental impact of sports facilities and massive winter sports events? RQ2: Are there any advantages to using artificial ice by sports clubs in ice arenas for hockey and winter sports? Can the quality of training, leisure activities, and sports games be maintained?

1.

Literature review

The need to transform the activities and development strategies of sports organizations is confirmed by the fact that, in several countries in the pan-European region, the strategy for building a "green" economy has become an integral part of a broader political strategy, such as a strategy for sustainable development (Belgium/Flanders, Czech Republic, France, Lithuania, Romania). In other cases, such strategies are formulated within the framework of environmental policy (Belgium/Flanders, Republic of Moldova, Seventh Environmental Action Program of the European Commission) or other policy directions, often within the framework of energy policy (Hungary, Italy) (Alinov, M,et al., Orunkhanov, M., [START_REF] Alinov | Model of the concept of transition of regions to the "green" economy. Sustainable development and green (low-carbon) economy of regions: problems, opportunities and prospects[END_REF].

The problems of global warming and water scarcity on Earth have made society think about the environmental friendliness of ice rinks. Ice rinks have been well-established for years, becoming a popular destination for families' leisure. However, when these rinks were built, energy-efficient solutions were not on the horizon. The issues of the green economy have made the management of sports clubs and ice arenas rethink the environmental friendliness of ice rinks. There has been much discussion centered around the influence of climate change on hockey. However, only a few people have discussed and measured how hockey and other businesses around the rink interact with the environment. The relationship may not seem significant at first within just one ice arena. However, it is far from being insignificant when we consider over 9,000 ice arenas that are operating on average for 11 months per year (Statista research, 2019).

The term resource management at sports facilities, particularly ice rinks, was analyzed by Rogstam and Mazzotti (Rogstam, J & Mazzotti, W, 2014). This research calculated that an average ice rink uses 1000 MWh/yr. On the other hand, the same authors concluded that inefficient ice rinks can use up to 2000 MWh/yr, whereas analytical research by Karampour (Karampour, M, 2011) calculated that the most efficient ice rink systems can use 700 MWh/ yr. By doing the math, 1,000 megawatts will be equal to about 6.6 billion kWh in a year, which is equivalent to the amount of power consumed by 900,000 homes in the Northeast but only 460,000 homes in the south. According to analytical evidence research by Schulz regarding water consumption by the machine that spreads the water on the ice surface, further machine called Zamboni, it takes up to 270 gallons of water, which equates to 1022 liters of water each time the machine prepares the ice for the skating session (Schulz, J, 2012). According to the U.S institute of medicine, it was determined that an adequate daily fluid intake is about 3.7 liters of fluids a day for men and about 2.7 liters of fluids a day for women (Institution of Medicine; IOM, 2005).

Therefore, just one resurfacing of the ice rink can maintain a healthy lifestyle for water intake for men for 276 days and 378 days for women. Zamboni has to resurface ice daily from 10-15 times per day, and the ice rink usually operates between 9-12 months. The ice-making water comes out below where the driver is seated on the Zamboni and is spread out over the ice, creating a smooth surface for skaters. However, there was a discussion on the CJAD 800 AM News talk Radio with Kretzschmar that there is an ecologically sustainable alternative ice rink solution that functions without water, energy consumption, and no carbon emission. Synthetic (artificial) ice has a total of 10 years warranty, which gives the opportunity to flip the surface on the other side and use the ice surface again for another 10 years [START_REF] Kretzschmar | Radio discussion regarding the sustainable modern ice rink technology[END_REF].

While reviewing worldwide ice rink statistics for 2019 regarding the number of ice rinks across the world that operate on a regular basis, the author found 7189 indoor ice arenas with natural ice and 9109 outdoor ice rinks that mainly require water resources to resurface the ice.

Analyzing climate change in North America (Canada) and the scope of winter sports, we found that over the past 100 years, the number of winter days has significantly decreased, but the average temperature has increased by 2°C on average. Reviewing scientific research, it was predicted that historically, Rideau Canal's skating season lasted around 61 days, which is 9 weeks. By 2020, it would shrink by 29% and will lose 67% (41 days) by 2050. By the end of the 21st century, with continuing high emission scenarios, the skating season will be reduced by 87% [START_REF] Suzuki | On thin ice. Winter sports and climate change[END_REF].

The resource costs for opening and subsequent maintenance are being addressed, among other things, by the International Olympic Committee and the International Ice Hockey Federation. The ultimate goal for the model is to use innovative technologies to save water, reduce emissions into the atmosphere, and lower overall energy used at hockey ice arenas. The main goal is to conserve the future opportunity and let future generations play hockey.

The management of sports clubs should provide training and scheduled performances for hockey players, skaters, and figure skaters throughout the year. This always requires special conditions for the ice surface of the rink, maintaining it constantly in perfect condition, and maintaining a special temperature regime. In the warm season or warm regions, it is necessary to build expensive indoor ice arenas, provide appropriate equipment, and hire service personnel.

One of the proposed solutions is synthetic ice. However, this option for the environment also requires research. It is estimated that synthetic ice has around 90% of the glide factor compared to natural ice (Kennedy, J & Watkins, W, 2012) . The majority of synthetic ice rinks like to use liquid surface enhancements, which help develop the quality of synthetic ice products and further reduce drag on the skate blade over the artificial surface. Nevertheless, most synthetic ice rinks allow skating without a particular liquid substance (Plastics on ice: How plastics advance ice hockey,2018). However, while assessing the perspective use of artificial ice sport services, the authors encountered the following questions:

there is not enough information on how much natural ice and artificial ice consume resources in Latvia and France, making it harder to compare perspectives of artificial ice from an economical and environmental point of view; there is not enough reliable and independent information from elite ice hockey players and their professional opinion regarding the quality of artificial ice; there is not enough independent professional opinion on whether artificial ice can be a good solution for young athletes who face a lack of training ice time all year round and especially in the summer time; there is no data regarding how the general public would potentially perceive the notion of changing the natural ice surface to artificial ice rinks for non-professional groups of athletes.

Research methodology

The aim of our research was to study the perspectives of using new technologies, in particular, artificial ice, in accordance with the concept of a green economy, to develop recommendations for sports clubs for the future development of winter games on ice arenas. During the study, the authors applied such empirical methods as questionnaires and structured interviews.

It was important for us to understand the attitude of various population groups towards artificial ice, including professional athletes, which was taken as a basis for identifying the prospects for the development of this entertainment sphere through the use of artificial ice for mass skating.

A total of three surveys were conducted with potential service consumers and providers for natural and synthetic ice. The complexity of the work lay in ordering all three groups of respondents, since in our opinion, representatives of ice rinks tend to consider natural ice an irreplaceable product, while representatives of the new artificial ice product actively propose a replacement for natural ice in the form of synthetic ice and see many-sided benefits when using artificial turf. However, in the authors' opinion, the most critical group of this study is professional athletes, who are ready to share their independent opinion based on their acquired experience of playing hockey on different surfaces, thereby assessing the prospects for using artificial surfaces.

In the first group, 132 athletes with experience in playing ice hockey were interviewed. In the second group, representatives of the management of an ice arena with natural ice were interviewed using the example of two countries, Latvia and France. In Latvia, six respondents were interviewed, who were specialists with work experience in the range of 8-16 years, while in France, six respondents with work experience in the range of 5-20 years were interviewed. In the third group, experts providing and promoting a new service of synthetic ice were interviewed, with five respondents having three years of experience in this service sector.

The survey was conducted anonymously, subject to privacy and data protection policies. Participation was optional.

Research results

The main group of respondents has experience playing hockey in the range of 5-25 years or more, which totals 92.4% of the surveyed respondents. Most of the surveyed respondents have the status of elite athletes (33.1%) or athletes of the second level (33.3%), who also belong to professional athletes performing in the second divisions. This selection of respondents is able to give a more structured and professional assessment of the new artificial ice service since the respondents in these groups have extensive experience in skating.

The study results show that the majority of professional athletes, as well as the predominant amount of the general public, are widely supporting the opportunity to skate on synthetic ice. One of the key survey criteria was to understand whether the surveyed respondents have experience skating directly on artificial ice. 72% answered positively to this question, which gives a clearer picture of the study when analyzing the subsequent questions.

Among the respondents, it was calculated that 61.6% had a positive experience while skating on artificial ice, and only 10.4% of the interviewees had negative experience with synthetic ice, whereas 28.0% were not able to measure this criteria as they had no skating experience with synthetic ice. During the study, we found that only 11.2% believed that this coverage can be assessed as excellent, 10.4% believed that this coverage met the criterion very well, and the majority of respondents (23.2%) agreed with the assessment well. The slightly lower indicators of the group who rated the artificial ice as normal were 16.8% of the total number of respondents (Figure 1).

Figure 1. Perception after skating on synthetic ice

Analyzing another factor which implies readiness to use synthetic ice among professional ice hockey players, the authors defined that 86.5% of professional ice hockey players are ready to substitute natural ice with synthetic ice during the summer time, whereas 13.5% are not ready to use synthetic ice under any circumstances (Figure 2). During this survey, the authors were able to reveal that artificial ice can be considered a promising direction for professional athletes in the summer, as well as for non-professional groups of the population. With the aim of finding a possible solution to the problem of lack of ice time for novice athletes, the authors found that 62.7% of respondents agreed with the opinion that artificial ice can replace natural ice but only partially. Only 9% of respondents believe that training on natural ice can be completely replaced by training on artificial ice, while 6% of respondents believe that skating on artificial ice can harm the development of the technical characteristics of a growing athlete.

In the final question, the respondents were asked to share their views on the prospect of using artificial ice. It is worth noting that more than 90% of respondents expressed positive aspects of using artificial ice, correlating the possibility of using artificial ice with a good tool to improve shooting technique, puck handling, as well as a number of expert opinions suggesting that during the off-season, this is a good opportunity to maintain muscle corset in the usual hockey mode, which helps reduce the risk of injury when returning to ice training after the off-season.

Taking the economic factor into account, the authors have defined that maintaining a low price for synthetic ice is a crucial criterion for ice rink managers today. It costs less than 10 euros per hour to maintain the synthetic ice surface, whereas the price for natural ice varies between 70-110 euros per hour during the summer. That's the main reason why many ice rink managers decide to melt the ice surface between 1-3 months as the temperature increases and the cost of maintaining natural ice also rises. (See Figure 3). While analyzing the costs for synthetic ice maintenance, the authors defined that its price will be less than 10 euros per hour. This is significantly less expensive compared to natural ice, and this criterion is not dependent on the season of the year. Therefore, the costs will remain stable despite the temperature level outside the arena (See Figure 4). Experts in the field of artificial ice were also asked to evaluate the quality of sliding on artificial ice compared to natural ice. Two (40%) of the respondents attributed the sliding quality of artificial ice as equal to 96-97%, while 1 (20%) considered that the quality of sliding is comparable to 98-99%, and also 1 (20%) of respondents attributed the quality of sliding to the criteria of 94-95% and 92-93% (See Figure 5). The authors decided to define the important criteria for the potential of synthetic ice among the general public. At this stage, it was concluded that 100% of the respondents said that nonprofessional skaters are not able to notice the difference between natural and synthetic ice.

Also, respondents, in their comments on the most common problems with artificial ice, noted that as a result of skating, plastic separates into chips (pieces of plastic). This factor reduces the quality of sliding and subsequently the lifetime of the arena since regular cleaning of the surface is required. However, it can be considered a positive factor that there is no need for specially trained workers to maintain such an arena since any worker, including a robotic machine, can clean the artificial ice. This significantly minimizes the cost and emission of maintaining artificial ice. During the survey, the authors found that 4 (80%) of experts said that synthetic ice does not have a negative impact on nature. From an environmental point of view, the use of artificial ice significantly decreases the devastating influence in the environmental sector compared to natural ice.

Conclusion

Ice hockey rinks are significant consumers of energy and water, resulting in harmful emissions to the atmosphere, which negatively impact the environment. In contrast, artificial ice rinks have no energy or water consumption and no carbon footprint, making them a green concept.

The authors found that artificial ice has positive prospects among potential consumers, with 86.5% of respondents willing to substitute natural ice for artificial ice for skating/ training purposes during the summer. Additionally, 61.6% of respondents from professional athletes and rink managers had a positive attitude towards synthetic ice, indicating that this modern ice concept can find appropriate use among potential consumers.

Given the lack of ice for training in Riga (Latvia), the authors suggest that it is possible to open a sports school for ice hockey and figure skating based on artificial ice, which can improve technical skills and strengthen the athlete's muscle corset, particularly in the summer when many rinks are melted, and training is limited.

From an economic perspective, the construction of synthetic ice is a much less costly project that pays for itself much more quickly. The use of synthetic ice is also possible 24 hours a day, 7 days a week, without the need for powerful energy generators and water resources. The concept of synthetic ice requires minimal technical means for its installation and maintenance, saving significant sums every day. Additionally, Euro projects aimed at supporting eco-business projects may offer bonuses for using this concept.

In summary, the authors' conclusions may be of practical importance to investors, coaches of sports clubs, and managers of ice arenas, as well as anyone interested in winter sports and ice arenas that are focused on the principles of a green economy.

Figure 2 .

 2 Figure 2. Readiness to skate on synthetic ice

Figure 3 .

 3 Figure 3. Price ratio for natural ice rink maintenance during the summer

Figure 4 .

 4 Figure 4. Price ratio for synthetic ice rink maintenance during the summer

Figure 5 .

 5 Figure 5. If natural ice have 100% glide, how would you evaluate natural ice vs artificial ice glide?

Acknowledgment

The study involved 132 athletes with experience in playing ice hockey, 12 representatives of the management of an ice arena with natural ice from Latvia and France, and 5 experts providing and promoting a new service of synthetic ice, who volunteered to participate. All participants received information about the study and gave voluntary consent to participate.


Sylva Drtikolová

Lumír Poláček

Lenka Půtová

Sylva Drtikolová Kaupová

Jaroslav Brůžek

Jiří Hadrava

Ivan Mikšík

Marine Morvan

Petr Velemínský

Early life histories of Great Moravian children -carbon and nitrogen isotopic analysis of dentine serial sections from the Early Medieval population of Mikulčice (9th-10th centuries AD, Czechia)

Keywords: Breastfeeding, physiological stress, diet, stable isotopes, Middle Ages

HAL is

Introduction

Since the pioneering work by [START_REF] Fogel | Nitrogen isotope tracers of human lactation in modern and archaeological populations[END_REF] described the relationship between the hair isotopic values of mothers and their breastfed babies, the reconstruction of breastfeeding and weaning behaviour has naturally attracted the attention of bioarchaeologists. The duration of breastfeeding, as well as the timing of the introduction of complementary food, clearly affect the health and physical well-being of the child both in the short term [START_REF] Lamberti | Breastfeeding and the risk for diarrhea morbidity and mortality[END_REF], Shamir 2016, [START_REF] Wilson | Weaning practices of the Makushi of Guyana and their relationship to infant and child mortality: A preliminary assessment of international recommendations[END_REF]) and the long term (Berti et Last but not least, the duration of breastfeeding has a considerable impact on women's health and fertility [START_REF] Bentley | Fertility changes with the prehistoric transition to agriculture: Perspectives from reproductive ecology and paleodemography[END_REF][START_REF] Jay | Breastfeeding and weaning behaviour in archaeological populations: Evidence from the isotopic analysis of skeletal materials[END_REF]). For all these reasons, information on this aspect of childcare helps substantially to understand the population dynamics and living conditions of past populations.

For more than a decade, researchers generally followed a cross-sectional approach, analysing carbon and nitrogen isotopes in the bone collagen of infants and young children of various ages, and comparing their values to female population averages (e.g. [START_REF] Fogel | Nitrogen isotope tracers of human lactation in modern and archaeological populations[END_REF][START_REF] Katzenberg | Weaning and infant mortality: Evaluating the skeletal evidence[END_REF][START_REF] Mays | The effects of infant feeding practices on infant and maternal health in a medieval community[END_REF], [START_REF] Pearson | Exploring the relationship between weaning and infant mortality: An isotope case study from Aşıklı Höyük and Çayönü Tepesi[END_REF][START_REF] Prowse | Isotopic and dental evidence for infant and young child feeding practices in an imperial Roman skeletal sample[END_REF][START_REF] Schurr | Stable nitrogen isotopes as evidence for the age of weaning at the Angel Site: A comparison of isotopic and demographic measures of weaning age[END_REF]), often attempting to link observed isotopic patterns with the mortality and morbidity profiles of subadults. Subsequently, however, some hidden pitfalls of this approach were addressed, such as the omission of the intra-population dietary variation in lactating mothers or potential mortality biases in weaning interpretations in terms of the osteological paradox [START_REF] Fuller | Stable carbon and nitrogen isotope variations in tooth dentine serial sections from Wharram Percy[END_REF][START_REF] Wood | The osteological paradox: Problems of inferring prehistoric health from skeletal samples [and comments and reply[END_REF]. A solution to these problematic issues was seen in the enforcement of an intra-individual approach, recovering dietary information from different periods of an individual's life. Samples were taken from different mineralized tissues (Herrscher 2013, Howcroft et Gradually, scientists' attention moved to dentine tissue, which, due to the absence of turnover, retains the isotopic signal from the period of tooth development throughout life [START_REF] Balasse | Detection of dietary changes by intratooth carbon and nitrogen isotopic analysis: an experimental study of dentine collagen of cattle (Bos taurus)[END_REF][START_REF] Richards | Stable carbon and nitrogen isotope values of bone and teeth reflect weaning age at the Medieval Wharram Percy site, Yorkshire, UK[END_REF], and thus allows the inclusion of adult individuals into studies while avoiding the risk of mortality biases. Sampling horizontal sections of dentine results in an isotopic profile covering the entire period of the tooth development (Beaumont et al. 2013). However, rather than resolving the issue of breastfeeding, the more detailed sampling methodology revealed that the dentine isotopic record results from a complex interaction of a number of dietary and non-dietary factors, among which physiological stress plays a key role (Beaumont et al. 2013[START_REF] Beaumont | Comparing apples and oranges: Why infant bone collagen may not reflect dietary intake in the same way as dentine collagen[END_REF], Craig-Atkins et al. 2018, [START_REF] King | Let's talk about stress, baby! Infant-feeding practices and stress in the ancient Atacama desert, Northern Chile[END_REF].

Potential sources of isotopic variation in dentine isotopic values

Breastfeeding

The isotopic effect of breastfeeding is a reflection of a phenomenon called the "trophic level effect" [START_REF] Ambrose | Experimental evidence for the relationship of the carbon isotope ratios of whole diet and dietary protein to those of bone collagen and carbonate[END_REF][START_REF] Deniro | Influence of diet on the distribution of carbon isotopes in animals[END_REF][START_REF] Deniro | Influence of diet on the distribution of nitrogen isotopes in animals[END_REF][START_REF] Schoeninger | Nitrogen and carbon isotopic composition of bone collagen from marine and terrestrial animals[END_REF], whereby heavier isotopes are discriminated against at each level of the food chain, resulting in isotopic enrichment of the consumers' tissues above those of their prey. In bone collagen, enrichment by 3-5 ‰ was observed with each trophic level for δ15N values (direct estimates for human give slightly higher estimates of 5.5-6 ‰, O'Connell et al. 2012) and by 1 ‰ for δ13C values. However, focusing on breastfeeding, direct observation of mother-infant pairs reported somewhat lower shifts in δ15N values associated with breastfeeding -between 2-3 ‰ [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF][START_REF] Herrscher | Longitudinal study of stable isotope compositions of maternal milk and implications for the palaeo-diet of infants[END_REF]).

Prior to birth, the foetus is an integral part of the maternal organism [START_REF] Fogel | Nitrogen isotope tracers of human lactation in modern and archaeological populations[END_REF]). Although a small offset has been observed in the tissues formed in-utero in mother-offspring pairs (de Luca et al. 2012, but see [START_REF] Herrscher | Longitudinal study of stable isotope compositions of maternal milk and implications for the palaeo-diet of infants[END_REF], much larger deviations occur once breastfeeding starts. Due to the trophic level effect, breastfed infants exhibit an elevation of both the δ13C and δ15N values above maternal values. During weaning, both δ13C and δ15N isotopic values of the newly formed tissues drops continuously, along with the decreasing dietary proportion of breast milk [START_REF] Fogel | Nitrogen isotope tracers of human lactation in modern and archaeological populations[END_REF][START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]).

Nutritional or physiological stress

As demonstrated by a number of controlled-feeding experiments on different animal species (for review see e.g. Reitsema 2013), as well as by direct observations of humans with eating disorders, pregnancy complications, and a number of serious diseases [START_REF] Fuller | Nitrogen balance and δ15N: Why you're not what you eat during nutritional stress[END_REF][START_REF] Mekota | Serial analysis of stable nitrogen and carbon isotopes in hair: Monitoring starvation and recovery phases of patients suffering from anorexia nervosa[END_REF][START_REF] Tea | Stable isotope abundance and fractionation in human diseases[END_REF], changes in nitrogen balance under physiological stress are responsible for notable isotopic shifts. To compensate for protein insufficiency under conditions of negative nitrogen balance, body tissues are catabolized and recycled. Thus, the fractionating processes of transamination and deamination are repeated, resulting in enrichment in the 15N of the newly formed tissues [START_REF] Reitsema | Beyond diet reconstruction: Stable isotope applications to human physiology, health, and nutrition[END_REF]. The impact of decreased protein bioavailability on carbon isotopic values is, however, less unequivocal. In some studies, the above-described shifts in δ15N values have been found to be accompanied by decreased δ13C values, this results from the combination of the altered ratio between routed dietary and endogenously synthesized amino acids and of the mobilization of fat stores, which are isotopically depleted [START_REF] Beaumont | Comparing apples and oranges: Why infant bone collagen may not reflect dietary intake in the same way as dentine collagen[END_REF][START_REF] Mekota | Serial analysis of stable nitrogen and carbon isotopes in hair: Monitoring starvation and recovery phases of patients suffering from anorexia nervosa[END_REF][START_REF] Neuberger | Signs of malnutrition and starvation-Reconstruction of nutritional life histories by serial isotopic analyses of hair[END_REF][START_REF] Schwarcz | Some Biochemical Aspects of Carbon Isotopic Paleodiet Studies[END_REF]. Based on these findings, a typical isotopic "stress pattern" has been described, which is commonly used especially in studies of dentine isotopic profiles, stressing the opposing covariance between δ15N and δ13C values during the stress episode (Beaumont et

Other factors

The next point potentially deconstructing the paradigm of the "typical isotopic stress pattern" is that there are a number of other factors of both dietary and physiological origin, concerning mother and/or child, which may evoke isotopic shifts emulating or overriding isotopic reflection of both breastfeeding and stress. The use of distinct weaning foods is quite common throughout the world (e.g. [START_REF] Sellen | Comparison of infant feeding patterns reported for nonindustrial populations with current recommendations[END_REF]), as are specific dietary rules imposed on pregnant or lactating women [START_REF] Baumslag | Breastfeeding: Cultural practices and variations[END_REF]. Alternatively, annual or seasonal variation in the isotopic composition of the mothers' diets, and therefore of breastmilk, can never be excluded. This argument is especially important in all the contexts with documented consumption of C4 plants and/or both marine and freshwater products [START_REF] King | Let's talk about stress, baby! Infant-feeding practices and stress in the ancient Atacama desert, Northern Chile[END_REF]). Further, although there is limited information on the isotopic composition of breastmilk, it seems that there are isotopic shifts in breastmilk values over the course of breastfeeding independent of maternal diet or health [START_REF] Herrscher | Longitudinal study of stable isotope compositions of maternal milk and implications for the palaeo-diet of infants[END_REF]), probably linked with changing macronutrient composition over the course of lactation (Czosnykowska-Łukacka et al. 2018). The practice of wet-nursing, well-known in continental Europe since Antiquity [START_REF] Fildes | The culture and biology of breastfeeding: An historical review of Western Europe[END_REF], can also affect the isotopic reflection of breastfeeding, when the isotopic values of the wet-nurse are different from those of the mother [START_REF] Herrscher | Comportements socioculturels liés à l'allaitement et au sevrage: Le cas d'une population grenobloise sous l'Ancien Régime[END_REF]).

For all the reasons mentioned above, it is evident that the reduction of isotopic variation in infants and young children to a simple typology of isotopic curves reflecting "weaning" or "physiological stress" (e.g. Craig-Atkins et al. 2018) is not viable. In this study, we thus attempt to avoid this descriptive approach and rather analyse the interplay between δ15N and δ13C isotope values in early life without such categorization, to compare the isotopic values and their shifts between biologically and socioculturally defined groups and to highlight the multiple potential interpretations of incremental profile shapes.

For that reason, we have explored the early life experiences of a sufficiently numerous (n = 78) population sample from the Great Moravian settlement agglomeration of Mikulčice, defining a number of simple questions: first, do all the individuals exhibit a isotopic peak in the early life period attributable to breastfeeding? Second, due to the well-described impact of early life experience on health and physical well-being in a long-term perspective [START_REF] Demmelmair | Long-term consequences of early nutrition[END_REF][START_REF] Mcdade | Life history, maintenance, and the early origins of immune function[END_REF][START_REF] Palou | Leptin intake during lactation prevents obesity and affects food intake and food preferences in later life[END_REF], we searched for the potential differences in isotopic profiles between individuals who died during M1 formation, those who died after M1 formation but before adulthood (i.e. during the second decade of the life) and finally individuals who survived to adulthood. To check for different parental investment in infants of different sexes, which are common in populations with high gender inequality (Eerkens and Bartelink 2013, Jayachandran and Kuziemko 2011), we explored the potential differences in isotopic profile between males and females. And finally, we explored the whether the level of parental effort and/or environmental risk differed between members of Great Moravian elites vs. common folk.

Materials and methods

Great Moravia (9th to beginning of the 10th centuries AD) was the first Slavic proto-state structure in Central Europe (Figure 1). Along with rapid political consolidation and the introduction of the first proto-urban centers, Christianisation probably extended into Moravia at the beginning of the 9th century [START_REF] Herold | Fortified settlements of the 9th and 10th centuries ad in Central Europe: Structure, function and symbolism[END_REF][START_REF] Kalhous | Church Organisation as a Bearer of New Culture and Innovations and Potential Support of Central Power[END_REF][START_REF] Macháček | Great Moravian central places and their practical function, social significance and symbolic meaning[END_REF]). The skeletal material analysed in this study comes from the settlement agglomeration at Mikulčice (Czechia, N 48°48'15.9", E 17°05'08.5", [START_REF] Kuna | Mikulčice. Archeologický atlas ČR[END_REF], which is believed to be one of the prominent power centres of the Great Moravian Empire [START_REF] Poláček | The Mikulčice-Valy Stronghold and Great Moravia[END_REF]. It attained a degree of urbanization unprecedented in the region, with a high concentration of ecclesiastical buildings. The settlement complex consisted of a fortified acropolis or "castle" and a bailey, surrounded by unfortified suburbs. Ongoing archaeological research begun in the 1950s has uncovered more 2500 graves both in the suburbs and at the acropolis, including presumably dynastic graves in the interior of the main churches as well as a number of richly equipped graves, suggesting the presence of the true elites of Great Moravian society [START_REF] Poláček | Great Moravia, the Power Centre at Mikulčice and the Issue of the Socioeconomic Structure[END_REF]). The adult and peripherally the subadult diets of the Mikulčice population have previously been explored [START_REF] Halffman | Stable isotope evidence for diet in early medieval Great Moravia (Czech Republic)[END_REF][START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF][START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF]), providing a good framework for comparison with current results and documenting the notable input of millet in the Great Moravian diet.

The dataset included 46 adults and 32 subadults, of which 25 died during M1 formation, while 7 died after the completion of the M1 root. There were 41 males and 37 females. In terms of socio-economic status, 38 individuals were classified as elite, the rest (n = 40) as non-elite. The age-at death distribution of the adult dataset was affected by dental wear, so individuals younger than 40 years were strongly prevalent. In adults, sex estimation was based primarily on the morphology [START_REF] Brůžek | A method for visual determination of sex, using the human hip bone[END_REF][START_REF] Phenice | A newly developed visual method of sexing the os pubis[END_REF]) and metrics of the innominate bones [START_REF] Brůžek | Validation and reliability of the sex estimation of the human os coxae using freely available DSP2 software for bioarchaeology and forensic anthropology[END_REF][START_REF] Murail | DSP: A tool for probabilistic sex diagnosis using worldwide variability in hip-bone measurements[END_REF]. Where these were absent or poorly present, an evaluation of the morphological traits of the skull was used [START_REF] Walker | Sexing skulls using discriminant function analysis of visually assessed traits[END_REF]. Age-at death estimation was based on the evaluation of senescent changes of the auricular surface [START_REF] Schmitt | Une nouvelle méthode pour estimer l'âge au décès des adultes à partir de la surface sacro-pelvienne iliaque[END_REF], pubic symphysis [START_REF] Schmitt | Une nouvelle méthode pour discriminer les individus décédés avant ou après 40 ans à partir de la symphyse pubienne[END_REF]) and the acetabulum (Calce 2012). In young adults, indicators of skeletal maturation of iliac crests and clavicles were used [START_REF] Scheuer | Juvenile Osteology: A Laboratory and Field Manual (1st edition)[END_REF]). For subadults, sex was estimated with the help of sex chromosome-linked isoforms of the peptide amelogenin from human tooth enamel. The modification of the method by [START_REF] Stewart | Sex determination of human remains from peptides in tooth enamel[END_REF] was used, employing a minimally destructive acid etching procedure and subsequent nano liquid chromatography tandem mass spectrometry. Further details of the method and the sex estimates for the subadults can be found in the Online Resource 1. The reliable sex estimation in the subadults, still not common in bio-archaeological practice, greatly increases the testimonial power of the subadult skeletons and helps to avoid the interpretative biases resulting from unknown sex ratios in particular age classes or socio-economic groups. Age-at-death for subadults was assessed using dental development [START_REF] Smith | Standards of human tooth formation and dental age assessment[END_REF].

The character of grave goods was used for the categorization of socio-economic status. Elites were considered to be individuals from well-equipped graves containing gold, luxury jewellery and textiles, belt strap-ends or cutlery, gilded buttons, metal weapons, spurs, or iron coffin accessories. Individuals from graves with objects of daily use, such as knives, ceramics, glass buttons or beads, simple jewellery or from graves lacking any grave goods, were considered to be non-elite.

The criterion for inclusion into the dataset was the preservation of at least one first permanent molar (M1) and a low degree of dental wear, admitting max. stage E according to [START_REF] Lovejoy | Dental wear in the Libben population: Its functional pattern and role in the determination of adult skeletal age at death[END_REF]. Samples for stable isotope analysis were taken preferentially from the lower M1, but in cases of absence, damage (e.g. by dental caries) or notable dental wear, the upper M1 was sampled. After removing adhered dirt, the M1 was halved along the mesio-distal (lower M1) or vestibulo-palatinal (upper M1) axis, with half of the tooth preserved for further analyses. Enamel was removed from one half of the M1 using dental burs and saws. Collagen was extracted using Method 2 described by Beaumont et al. (2013) in the modification by van der Haas et al. (2018). The demineralized dentine was sectioned into 10 horizontal sections (in the case of fully formed teeth) reflecting dental developmental stages [START_REF] Smith | Standards of human tooth formation and dental age assessment[END_REF]. As far as possible the calculations of the approximate ages for each section considered the distinct rate of dentine secretion at various stages of tooth development, being calculated separately for each particular tooth segment [START_REF] Czermak | A guide for an anatomically sensitive dentine microsampling and age-alignment approach for human teeth isotopic sequences[END_REF]): the crown (Cri-Crc, divided into 5 slices), the superior half of the root (Crc-R1/2, divided into 2 slices), the inferior half of the root (R1/2-A1/2, divided into 2 slices) and the closing apex (A1/2-Ac, one slice). According to the Smith (1991) developmental scheme, the M1 crown forms approximately from birth, with the age at crown completion being 2.2±0.5 years. The fully formed tooth crowns were cut in 5 horizontal increments, each representing 1/5th of 2.2 years. Analogously, calculations were made for the M1 root.

All the sample preparations were carried out at the Department of Anthropology, National Museum, Prague, CZ. EA-IMRS (Elemental Analysis -Isotope Ratio Mass Spectrometry) was performed at Iso-Analytical, Crewe, UK. Stable carbon and nitrogen isotopic compositions were calibrated relative to the VPDB and AIR scales using IAEA-CH-6 and IAEA-N-1 inter-laboratory comparison standards. Measurement uncertainty was monitored using in-house standards: IA-R068 (soy protein, δ13CV-PDB = -25.22 ‰, δ15NAIR = 0.99 ‰), IA-R038 (L-alanine, δ13CV-PDB = -24.99 ‰, δ15NAIR = -0.65 ‰), IA-R069 (tuna protein, δ13CV-PDB = -18.88 ‰, δ15NAIR = 11.60 ‰) and a mixture of IAEA-C7 (oxalic acid, δ13CV-PDB = -14.48 ‰) and IA-R046 (ammonium sulphate, δ15NAIR = 22.04 ‰). Precision was determined to be ± 0.14‰ for both δ13C and δ15N values based on repeated measurements of calibration standards, check standards, and sample replicates. Accuracy or systematic error was determined to be ± 0.07 for δ13C and ± 0.11 for δ15N values based on the difference between the observed and known δ values of the check standards and the long-term standard deviations of these check standards. The total analytical uncertainty as defined by Szpak et al. ( 2017) was estimated to be ± 0.16 ‰ for δ13C values and ± 0.18 for δ15N values.

To describe the carbon and nitrogen isotopic profiles with respect to dietary and physiological changes, several parameters were chosen: for δ15N values, we noted the presence of the initial peak in isotopic values as primary evidence for breastfeeding. We calculated the maximal isotopic offset (Δ15Nmax) to describe the height of this peak and noted the ages at i) the first notable decrease from the peak value and ii) the smoothing of the isotopic profile to describe its extension (Figure 2A). In both cases 0.4 ‰ (a double of the analytical error at two standard deviations) was considered to be a significant isotopic change. For δ13C values, we also calculated the maximal isotopic offset (Δ13Cmax). We then evaluated the shape of the carbon isotopic profile in relation to that of nitrogen during the initial peak described above. Four basic types of carbon isotopic profiles were defined: i: positive covariance between δ15N and δ13C values, ii: negative covariance when δ13C values are low during the time of peaking δ15N and subsequently increase along with decreasing δ15N values, iii: Flat carbon profile, and finally iv: positive covariance between δ15N and δ13C values in most of the slices, but with an initial decrease in δ13C values preceding the shift in δ15N values (Figure 2B).

To describe the isotopic values after the period of breastfeeding, we calculated the average postweaning isotopic value (i.e. the average value from all the slices developed after smoothing the nitrogen isotopic curve as described above). This was to describe childhood diet in the period following weaning, and to compare it with previously published dietary info from later life periods [START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF][START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF]. For each individual, we also searched for two potential patterns described in previous isotopic studies of dentine incremental profiles: i) the presence of the so-called 'postweaning dip' in nitrogen isotopic values, which is usually linked to the special character of the postweaning diet, with low input from animal products (Tsutaya and Yoneda 2015), and ii) cases of notable shifts in both carbon and nitrogen isotopic values showing a pattern of a negative covariance, usually attributed to biological stress (Beaumont and Montgomery 2016).

Statistical analyses were examined in R software version 3.3.3 (R Core Team 2017). The effect of age, sex, and socio-economic status on the values of nitrogen and carbon post-weaning averages, as well as age at first decrease and age at final smoothing, were modelled using linear models and tested with ANOVA. The values of age at first decrease and age at final smoothing, which were classified into 3 and 6 levels respectively (i.e. an ordinal scale), were treated as numerical variables in the analyses, because there were relatively high numbers of levels with only a few observations per level.

As maximal offset of nitrogen, maximal offset of carbon, age of weaning, and age at first decrease correlated with each other (with a maximal correlation coefficient of 0.31 between the the maximal offset of nitrogen and maximal offset of carbon), the joint effect of age, sex, survival, and socioeconomic status was modeled with RDA analysis using the library ''vegan'' [START_REF] Oksanen | Community Ecology Package. R package version 2.2-1[END_REF]. The effect of the predictors was tested with an ANOVA-like permutation test with the function "anova.cca". The incidence of distinct carbon isotopic profiles in particular age-groups was compared by Fisher exact test.

Results

Complete isotopic results are given in full in the Online Resource 2. Several increments (N = 6) did not yield enough collagen for analysis, this concerns exclusively apical sections including only a small portion of dentine from dentine horns. All the analysed samples met the criteria for good collagen preservation. Individuals analysed in this study exhibit highly varied δ15N and δ13C profiles, which are shown for each individual in the Online Resource 2.

Nitrogen isotopic profiles

Almost all the individuals show some sort of decrease in δ15N values during the earliest life period, which could be viewed as a reflection of breastfeeding and subsequent weaning. However, notable variation exists in both the timing and magnitude of these shifts. Based on the study by [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF], an isotopic offset of 2-3 ‰ could be seen as primary evidence of weaning from full breastfeeding under healthy circumstances. Shifts beyond these margins (in both directions) suggest the combined impact of the trophic level effect and biological stress, and/or the other factors listed above. In this study, the average Δ15Nmax was slightly higher than the recommended range (3.1 ± 0.8 ‰). In 37 (of the 78) individuals, Δ15Nmax was above 3 ‰, with a maximum of 5.1 ‰. In nine individuals, Δ15Nmax was below 2 ‰ with six of them not showing a typical isotopic curve shape attributable to weaning: five individuals, (Graves 112-VI, 462, 673, 1058, 1171) show very low δ15N in the earliest dentine slice following by a delayed peak in δ15N (Figure 3). No. 207 shows a specific nitrogen isotopic profile with low δ15N in the earliest dentine slice followed by the two subsequent peaks in δ15N of smaller extent (Figure 3).

When comparing the Δ15N between the defined population subgroups, there were no statistically significant differences between age classes, sexes or socio-economic groups (Table 1, Figure 4). In the case of age, the result was close to the 0.05 % level of significance, with non-survivors showing surprisingly lower nitrogen isotopic offsets that older individuals.

Concerning the start and end points of the above described isotopic shift attributable to weaning, in most of the individuals (n = 37) δ15N values decreased significantly in the second slice, representing isotopic values at the age of 5-11 months. In 30 individuals the first decrease was observed in the third slice (representing isotopic values from approx. 11-16 months), while in five individuals, δ15N values did not decrease before 16-21 months (4th slice). In detailed comparison, females showed the first isotopic decrease at a later age than males (Table 1). Age-at-death and socio-economic status had no statistically significant relationship to this parameter (Figure 4). The age at the final smoothing of the isotopic curve showed much higher variation, with the youngest individuals (n = 7) being 11-16 months old (slice 3). In the majority of individuals the final smoothing was relatively uniformly dispersed between the age classes of 16-21 months (slice 4, n = 20), 21 months to 2.2 years (slice 5, n = 25) and 2.2-3.7 years (slice 6, n = 18). Finally, in two individuals the final smoothing occurred as late as at 3.7-5.3 years. Surprisingly, individuals, who died during the first decade of life showed an earlier age at final smoothing than older individuals (Table 1). Also, in elites, the final smoothing occurred on average earlier than in non-elites. The sex of the individual had no important impact at this parameter (Figure 4).

According to the RDA analysis, which included all four factors (maximal nitrogen and carbon offsets, age at first decrease, age at final smoothing), the RDA1 axis shows that adults and males display a higher maximal offset, but non-survivors a higher age at first decrease. RDA1 however explains only 7 % of the variability and this result is not statistically significant (ANOVA-like permutation test, F = 1.45, p = 0.848).

Carbon isotopic profile and the covariance between carbon and nitrogen isotopic values

The maximal carbon isotopic offset (Δ13Cmax) ranged between 0.3 and 3.6 ‰, with an average value of 1.6 ± 0.8 ‰, which is substantially higher than the trophic level effect of exclusive breastfeeding [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]).

Concerning the shape of the carbon isotopic profile and the relationship to nitrogen isotopic values, most of individuals (n = 43) follow the pattern of positive covariance between δ15N and δ13C values at least in the initial portion of the isotopic profile, i.e. in the period of supposed breastfeeding and weaning (Profile i). Instances where carbon and nitrogen isotope values negatively covary during this period, with decreasing δ15N values along with increasing δ13C values, are relatively scarce (n = 7, Profile ii). A flat carbon profile (Profile iii) occurs in nine cases. Lastly, profile iv, where carbon and nitrogen isotopic values covary positively for most of the time, but the decrease in δ13C values precedes the decrease in δ15N values, occurs in eleven cases. There are eight cases (graves nos. 112/VI, 207, 182, 462, 625, 673, 727, 1171) in which the carbon isotopic profile does not corresponds with any of the types described above. In 112/VI and 673 carbon isotopic values covary positively with δ15N, but do not follow the typical weaning scenario (see Figure 3). In 462 and 1171 atypical carbon profiles occur along with atypical nitrogen profiles, but neither positive nor negative covariance between δ15N and δ13C values is present. In 182, 625 and 727, unexpected shifts in δ13C values occur along the nitrogen isotopic peak typical for breastfeeding. Finally, in individual 207 carbon isotopic values covary negatively with nitrogen isotopic values during both episodes of increased δ15N values (Figure 3).

The type of the carbon isotopic curve differed significantly between individuals, who died before M1 completion and others, with a flat carbon isotopic profile being more common in non-survivors (seven of the nine cases). It was not possible to assess the potential relation to sex or socio-economic status or to include this variable in the multifactorial analysis due to the dominance of profile I, and the subsequently low number of cases in some categories.

Post-weaning isotopic values

The average nitrogen isotopic values from all the slices developed after the smoothing of the nitrogen isotopic curve ranged between 9.1 and 12.5‰ with a mean of 11.2 ± 0.8 ‰. The average carbon isotopic values ranged between -19.1 and -16.3 ‰ with a mean of -17.6 ± 0.6 ‰. Seven individuals died too early to identify the point of the final smoothing of the nitrogen isotopic curve and thus were not included in this analysis. Among adults (n = 44) we were able to compare these "post-weaning averages" with previously published adult isotopic values from bone collagen (Jílková et al. 2019, [START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF]). Childhood nitrogen isotopic values were similar to those from adulthood (mean = 11.0 ± 0.9 ‰, p = 0.297), while carbon isotopic values in dentine samples were significantly higher than those from bone (mean = -18.0 ± 0.5 ‰, p ˂ 0.001, Figure 5). In the case of nitrogen, post-weaning averages differed significantly between socio-economic classes (Table 1, Figure 5), with elites showing higher δ15N values (mean = 11.5 ± 0.6 ‰) than non-elites (mean = 10.9 ± 0.8‰). Sex too appeared to have a significant impact on δ15N with males (mean = 11.4 ± 0.7‰) showing higher δ15N values than females (mean = 11.0 ± 0.9 ‰). Multifactorial analysis, however, suggests that this probably results from the distinct distribution of both sexes into socio-economic classes (ANOVA, p = 0.001 for socioeconomic status and 0.139 for sex). Age-at-death had no significant impact on post-weaning δ15N. None of the studied factors show a significant impact on post-weaning carbon isotopic values (Table 1).

Episodes of negative covariance between δ15N and δ13C values during the post-weaning period were observed in twelve cases, including two of the evaluated 18 non-survivors. As stated above, the seven individuals who died too early to identify the exact end point of the isotopic shift associated with weaning were not included in these counts.

Fisher exact test showed no significant difference in the incidence of negative covariance between those who died during M1 formation and others (p = 0.490). The number of cases with opposing covariance was too low to effectuate any deeper statistical comparison concerning differences between sexes or socio-economic classes.

In a number of individuals, however, we observed a mild increase in δ15N in later childhood following the period of depletion in 15N during the post-weaning period. The presence of this event, known as the "post-weaning dip" (Tsutaya and Yoneda 2015) was evaluated only in individuals who died after M1 completion, thus presenting a complete isotopic profile. A post-weaning dip was observed in 25 of the 46 individuals. There were no differences in the incidence of post-weaning dip between sexes (Fisher exact test, p = 0.758) or between socio-economic groups (p = 0.773).

Discussions

Nitrogen isotopic values during the period of supposed breastfeeding

In this study, we have considered three potential indicators of physiological stress. Firstly, a peak in nitrogen isotopic values during the period of infancy higher than 3 ‰ [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]) may suggest physiological stress during full breastfeeding or around the introduction of the first dietary supplements (which form a relatively small proportion of the diet and are thus still isotopically invisible). This pattern was present in 37 individuals (47 %). This percentage is relatively high, taking into account the known information on the background levels of stress acting on the Mikulčice subadult population. In general, the expansion of Great Moravia corresponded to a favourable era of mild and stable weather, which, together with other favourable natural conditions in South Moravia, enabled high productivity on cultivated land and thus enabled strong population growth (Hladík 2020). On the other hand, Mikulčice's inhabitants surely had to face the negative aspects of urbanization, such as poor sanitation, parasitic infestations, an elevated risk of infection, or dependency on food supplies from the hinterland (Walter and DeWitte 2017). However, the degree of urbanization never exceeded a relatively extensive, proto-urban formation. Thus, though life in a newly-established centre clearly brought problems and challenges previously unknown in a rural and tribal community, the level of stress most probably did not reach that observed in the purely urban formations of the High Middle Ages. This is supported by the results of osteological analysis, according to which the prevalence of non-specific stress indicators in Mikulčice infants and young children was comparable to those from the rural hinterland [START_REF] Kaupová | Urban and rural infant-feeding practices and health in early medieval Central Europe (9th-10th Century, Czech Republic)[END_REF]).

Traditionally, the first introduction of dietary supplementation is viewed as a highly risky period in respect to infection, as the child's immune system encounters a range of new pathogens [START_REF] Lamberti | Breastfeeding and the risk for diarrhea morbidity and mortality[END_REF]). It has however been proven that the immunological buffering of breastfeeding still acts during complementary feeding [START_REF] Kendall | The "weanling's dilemma" revisited: Evolving bodies of evidence and the problem of infant paleodietary interpretation[END_REF]).

Moreover, isotopic data from later childhood suggest that the observed pattern could be augmented by the character of post-weaning food. As a ''post-weaning dip'' in nitrogen isotopic values was observed in more than half of the individuals, the lower proportion of animal products consumed during early childhood may boost the Δ15Nmax of these individuals above the 3 ‰ margin.

Secondly, the delayed starting point of the isotopic decrease in δ15N values could suggest biological stress during the early phase of weaning. This parameter, however, has a less precisely defined range than the previous one. According to current medical recommendations, an insufficiency of breastmilk to meet an infant's dietary needs in terms of both micronutrients (e.g. iron) and calories may threaten after just six months of age [START_REF] Fewtrell | Optimal duration of exclusive breastfeeding: What is the evidence to support current recommendations?[END_REF], Pérez-Escamilla et al. 2019). Although the portions of supplementary foods could be negligible at the beginning, it is hardly believable that a child could prosper under a regime of exclusive breastfeeding after 1 year of age. In our sample, such a delayed start of the decline in nitrogen isotopic values, with the first decrease observed at 1.3-1.8 years, was present in five individuals (6 %).

Finally, physiological stress in the later phases of weaning would be expressed by a slower decline and the delayed final smoothing of the nitrogen isotopic profile. However, due to the extreme variation reported in the duration of breastfeeding, which could be over 6 years [START_REF] Dettwyler | When to wean: Biological versus cultural perspectives[END_REF][START_REF] Fildes | The culture and biology of breastfeeding: An historical review of Western Europe[END_REF][START_REF] Piovanetti | Breastfeeding beyond 12 months: An historical perspective[END_REF], it is not possible identify a point on the nitrogen isotopic curve (before the final smoothing) beyond which isotopic enrichment cannot be due to partial breastfeeding.

Looking at the data from a dietary point of view, the first decrease in δ15N values was observed in most of the individuals at approx. 5-10 months. This suggest a practice of weaning roughly in accordance with current medical recommendations (WHO 2009), implying exclusive breastfeeding for 6 months.

Occasionally, infants could have received a notable amount of dietary supplements even earlier, as suggested by the presence of individuals with maximal nitrogen isotopic offset lower than 2 ‰ (n = 3), in these cases, however, other explanations such as change in maternal diet or the use of a wet nurse, cannot be excluded. Due to the competing impact of physiological stress, the detection of the latest age for the first dietary supplements is impossible, and won't be attempted in this paper.

The earliest observed age-at-smoothing of the nitrogen isotopic curve suggests that the first children could well have been fully weaned before 11 months, which is much earlier than the current medical recommendation of partial breastfeeding for 2 years (WHO 2009). This was observed in seven individuals, which is a minor but not negligible proportion of the population. Even if our estimates on age-at-complete weaning are considered to be rather maximalistic (as the presence of biological stress could mimic longer breastfeeding), all except two individuals appeared to have been weaned before 2.2 years. This appears to be relatively early in the perspective of historical populations [START_REF] Dettwyler | When to wean: Biological versus cultural perspectives[END_REF][START_REF] Fildes | The culture and biology of breastfeeding: An historical review of Western Europe[END_REF][START_REF] Thorvaldsen | Was there a European breastfeeding pattern?[END_REF]), but it must be kept in mind that minor supplements of breast milk below 10% of the dietary input would be unobservable isotopically [START_REF] King | Let's talk about stress, baby! Infant-feeding practices and stress in the ancient Atacama desert, Northern Chile[END_REF]. It should also be mentioned here that, due to the chosen sampling strategy, the sixth slice corresponds to relatively long period between 2.2 and 3.75 years. This could further help to blur the potential minor consumption of breastmilk for some time after 2.2 years.

Finally, even with the incremental sampling methods, time averaging inevitably occurs as multiple incremental boundaries are crossed [START_REF] Czermak | A guide for an anatomically sensitive dentine microsampling and age-alignment approach for human teeth isotopic sequences[END_REF]). Also, a new dietary source may be introduced well in advance of a detectable isotopic shift. Further, there is some intra-individual variation in teeth development (e.g. [START_REF] Smith | Standards of human tooth formation and dental age assessment[END_REF]). This means that the ages noted in this study as the periods of notable dietary/or physiological change are only rough estimates and should be used rather for comparative purposes.

At the individual level, six cases showed a nitrogen isotopic pattern not compatible with the breastfeeding and weaning scenario. All of these showed low nitrogen isotopic values in the first slice, which can be interpreted as a failure of breastfeeding. However, these originally low δ15N values are in all cases followed by one or two subsequent peaks in δ15N values extending between 0.6 and 3 ‰, which are difficult to interpret. Episodes of physiological stress had to be extremely common in the case of bottle-fed babies consuming non-sterilized food. , report isotopic shifts associated with malnutrition and/or disease higher than 2 ‰, but four of these six cases show a nitrogen isotopic peak ranging between 2-3 ‰ (see Figure 3, for example). Although an isotopic shift of this extent could well correspond to exclusive breastfeeding [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]), this explanation is highly improbable, as the first slice provides the dietary information from approx. 5 months of life. Though there could be variation in the growth pattern of the teeth [START_REF] Smith | Standards of human tooth formation and dental age assessment[END_REF], it is extremely unlikely, that an infant could retain the ability to effectively suckle after several weeks (or more probably months) of artificial feeding. This suggests the role of some unknown factor (or a combination of several factors) affecting the isotopic values of these children in early childhood. In one case only (No. 207, age-at-death = 8-10 years, Figure 3) the extent of the nitrogen isotopic shifts (together with the presence of negative covariance with carbon isotopic values) offers a typical image of repeated episodes of physiological stress (Beaumont and Montgomery 2016, Craig-Atkins et al. 2018), which however, was the child able to overcome. This, as well as the age of two of the afore-mentioned individuals (112-VI and 673), who survived till 40-50, and 20-30 years respectively, contradicts the general view that in the past there was no choice other than breastfeeding to ensure the survival of the child [START_REF] Dettwyler | When to wean: Biological versus cultural perspectives[END_REF].

Intra-population comparison of early life δ15N

An intra-population comparison of early life experiences does not suggest that individuals who died during the first decade of life experienced a greater level of environmental stress during early childhood. None of the three parameters listed above was higher in non-survivors. In fact, the age at complete smoothing of the nitrogen isotopic curve was lower in non-survivors, as was the maximal nitrogen isotopic offset (though here, the difference was on the borderline of statistical significance).

As lower Δ15Nmax could result from an earlier start to the weaning process (before 5 months), the observed pattern may theoretically demonstrate lower parental investment [START_REF] Quinlan | Human parental effort and environmental risk[END_REF], impacting the life expectancy of a child.

The delayed onset of the nitrogen isotopic decrease observed in females in comparison to males is probably caused by the higher prevalence of individuals with a first decrease observed as late as in the fourth slice, with four of the five cases being female. As noted above, such a pattern is probably linked with physiological stress, but considering the low number of cases, it is unclear whether this can be seen as a reflection of a higher level of stress during the weaning process, imposed systematically on female children e.g. by the lower quality of complementary food.

The earlier occurrence of final smoothing observed in elites in comparison to non-elites has at least two equally valid explanations: the first relates to the higher incidence of stress during the weaning process in non-elites, the second to the earlier weaning of elite children. The latter may well be linked to the ongoing Christianization of the Great Moravian population. Although there is no direct written testimony from the Great Moravian context, certain rules concerning family life and childcare probably existed [START_REF] Thorvaldsen | Was there a European breastfeeding pattern?[END_REF]): as a unique example, the 9th century document "The Responses of Pope Nicholas I to the Questions of the Bulgars" recommends sexual abstinence during the entire period of breastfeeding [START_REF] Bartoňková | Magnae moraviae fontes historici IV[END_REF]). As the pressure to abide by Christian rules was probably higher in elite groups, this could have led at least some elite women in Mikulčice to shorten the breastfeeding period. To estimate the strength of this argument, however, is beyond the scope of this paper.

In any case, as RDA analysis including all four explicative factors gave non-significant results, explaining only a small portion of the data variation, the results of uni-factorial analysis suggesting a relationship between some parameters and age-at-death, sex or social status must be considered with caution.

Carbon isotopic values during the period of supposed breastfeeding

The presence of negative covariance, usually regarded as an indication of physiological stress, in the initial part of the isotopic profile was relatively rare in this dataset, being observed in only seven cases. This however does not necessarily mean that the level of physiological stress during infancy and early childhood was low. As stated above, the results of experimental studies suggest that the isotopic changes in a carbon body pool in relation to physiological stress may vary (Canterbury et In fact, taking into account both the potential roles of stress and millet consumption, it is quite surprising that most of the individuals (n = 43) show a positive covariance between carbon and nitrogen isotopic values, as can be observed during the process of breastfeeding and weaning under healthy circumstances and with a stable and isotopically analogous diet for both mother and child. This could mean that millet played a special role in the diet of infants or potentially also their breastfeeding mothers. As will be explained in detail bellow, carbon isotopic values from the post-weaning period suggest, that the childhood diet included more millet than that of adults. The persistence of the carbon isotopic peak in these circumstances is thus quite surprising. If an infant was being weaned onto resources with a higher millet proportion than those in the maternal diet, we would expect a rise in δ13C values, "cancelling out" the trophic level shift due to weaning. The result would be either a flat carbon isotopic profile (as observed in nine cases in this study) or the occurrence of negative covariance mimicking the stress pattern described above [START_REF] King | Let's talk about stress, baby! Infant-feeding practices and stress in the ancient Atacama desert, Northern Chile[END_REF]). As this is not true in a majority of the sample, it means that either millet was part of a special diet preferred for some reason by lactating women, or was introduced as the very first dietary supplement. Consumed in the form of porridge [START_REF] Adamson | Food in medieval times[END_REF]), millet has the ideal consistency for first dietary supplementation, it is also easily digestible, and in comparison to other cereals has a higher nutritional value with a high content of vitamins, minerals and antioxidants [START_REF] Aurelia | Use of millet grain in weaning pigs diet: Effects on performance and health status[END_REF][START_REF] Kulp | Handbook of cereal science and technology, revised and expanded[END_REF][START_REF] Weber | Millets and their role in early agriculture[END_REF]. While this information was not known to Great Moravian people, the consumption of millet could have reduced the risk of both malnutrition and disease in their children during the period of complementary feeding and -from the immunological point of view -the extremely risky time of the complete cessation of breastfeeding [START_REF] Kendall | The "weanling's dilemma" revisited: Evolving bodies of evidence and the problem of infant paleodietary interpretation[END_REF]. At the same time, due to a low protein content in comparison to breastmilk, millet-based dietary supplements have the potential to impact carbon isotopic values (as carbohydrates could be partially used to build the carbon skeletons of non-essential amino-acids), while breast milk remains the main source of nitrogen [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]. Another point suggesting a special role for millet impacting the isotopic values from infancy (either directly or indirectly via maternal diet) is the extent of maximal carbon isotopic offset occurring during the supposed breastfeeding period, which in the vast majority of individuals (n = 55) surpassed the maximum of 1 ‰ which could be attributable to the trophic level effect of breastfeeding [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]). In 16 individuals Δ13Cmax was even higher than 2 ‰.

In cases showing profile iv, where carbon and nitrogen covary positively but the decrease in δ13C precedes the decrease in δ15N (eleven cases), as well as in eight cases not corresponding with any of the outlined scenarios, a number of equally plausible explanations may be offered and thus, we will not attempt any interpretation here.

Incidence of distinct carbon isotopic profiles in particular age groups

The substantially higher prevalence of flat carbon isotopic profiles observed in those who died before M1 completion has no simple explanation. The absence of breastfeeding as a factor impacting further survival would be a valid cause [START_REF] Craig-Atkins | The role of infant life histories in the construction of identities in death: An incremental isotope study of dietary and physiological status among children afforded differential burial[END_REF]), but all the non-survivors with a flat carbon isotopic profile show a nitrogen isotopic peak within or very close to the 2-3 ‰ range observed in fully breastfed babies [START_REF] Fuller | Detection of breastfeeding and weaning in modern human infants with carbon and nitrogen stable isotope ratios[END_REF]). As discussed above, such a high nitrogen isotopic offset is unlikely to have been caused solely by physiological stress. A dietary explanation might be the weaning of the child onto resources with a higher millet proportion than in the maternal diet. However this, like other explanations arising from dietary factors, does not explain, why this scenario was more prevalent in non-survivors.

Carbon and nitrogen isotopic values in the post-weaning period [START_REF] Poláček | Great Moravia, the Power Centre at Mikulčice and the Issue of the Socioeconomic Structure[END_REF]). Thus, the combined results of the two studies suggest that while the higher input of millet in a child's diet was a widespread practice among Mikulčice inhabitants, the access of children to animal products may have differed in particular parts of Mikulčice.

Intra-population variation in post-weaning isotopic values

The absence of any systemic differences in post-weaning averages between non-survivors vs individuals who died after M1 completion suggests that the quality of the post-weaning diet did not impact the chance of survival, and that there was no higher incidence of long-term physiological stress in nonsurvivors. However, it must be noted that severe and long-lasting physiological stress would have had to act to significantly impact the post-weaning values, averaging the isotopic signal from several years. The distinct nitrogen isotopic values of elite vs non-elite graves attest that the socio-economic differences in diet documented isotopically in later life periods [START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF][START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF] were already present during childhood.

Exploring the latter part of the isotopic profile in greater detail, the episodes of negative covariance, generally viewed as evidence of physiological stress, had no higher prevalence in non-survivors. Cases of the so called "post-weaning dipp", i.e. the notable increase in δ15N in later childhood, which is commonly observed in agricultural populations and is generally viewed as testimony to the lower input of animal products in the period immediately following weaning (Tsutaya and Yoneda 2015), were much more common than episodes of opposing covariance (in more than half of cases). While this could also be viewed as a reflection of anabolic activity linked to mid-growth spurt [START_REF] Kendall | The "weanling's dilemma" revisited: Evolving bodies of evidence and the problem of infant paleodietary interpretation[END_REF]), but the knowledge on the role of growth velocity is still limited, with current experimental studies not providing convincing evidence of the assertion of this factor on nitrogen isotopic values (Reitsema and Muir 2015, Waters-Rist and Katzenberg 2010). Further, the presence of the "post-weaning" dip appeared to be linked to the subsistence strategy [START_REF] Tsutaya | Reconstruction of breastfeeding and weaning practices using stable isotope and trace element analyses: A review[END_REF]. In a Romano-British population sample, for which we have written records on the perception of childhood, the timing of the dip coincides with the change in social status of children [START_REF] Cocozza | Investigating infant feeding strategies at Roman Bainesse through Bayesian modelling of incremental dentine isotopic data[END_REF]). Both of these findings further enhance the first explanation.

And finally, as emerges from the above discussion on the testimonial power of carbon isotopic values in respect to stress, it appears evident that neither can the presence of physiological stress be excluded in these individuals.

Limitations of the study

The interpretative difficulties due to equifinality [START_REF] King | Let's talk about stress, baby! Infant-feeding practices and stress in the ancient Atacama desert, Northern Chile[END_REF]), unavoidable by current methods, have been repeatedly stressed over the course of the discussion, so need no recapitulation here.

As a much more important point, we would like to stress the limited knowledge on the action of physiological stress in the subadult organism. This could be due to the potentially competing influence of negative and positive nitrogen balance (Waters-Rist and Katzenberg 2010). Also, a reduction in growth, rather than tissue catabolism, may be the primary response of subadults to dietary stress [START_REF] Ambrose | Controlled diet and climate experiments on nitrogen isotope ratios of rats[END_REF]). Thus, more experimental studies on subadult organisms are greatly needed to confirm that the pattern of negative covariance between nitrogen and carbon isotopic values is the real response of subadult body to physiological stress.

Conclusion

Alhough the current data -mainly maximal nitrogen isotopic offsets commonly increased over 3‰suggest that physiological stress might have been relatively common during early childhood, there are no indications that the occurrence of such physiological stress in this life phase affected life expectancy in the long term, or that the stress level in early life differed substantially between sexes or socioeconomic classes.

The isotopic data also suggest that in the majority of the studied sample, a notable proportion of supplementary food was introduced between 5-10 months, i.e. roughly in the interval recommended by current medical advice. The first children in our dataset were fully weaned before 10 months of age.

The latest age at the final smoothing of the nitrogen isotopic curve suggesting weaning was as late as 3.75-5.3 years, but this was observed in 2 individuals only. All the other individuals showed final smoothing of the nitrogen isotopic curve at 2.2-3.75 years. With respect to age-at-weaning, this has to be viewed rather as the upper range of the possible estimate, as the impact of physiological stress cannot be excluded in these seemingly late weaned children. Also, it has to be kept in mind that minor supplementation by breastmilk would not be observable by isotopic analysis. Six individuals do not show the nitrogen isotopic shift attributable to breastfeeding at all. Typical nitrogen isotopic profiles for particular population groups (median values). To illustrate the shape of the isotopic curve while eliminating the confounding factor of underlying dietary variation among children and their lactating mothers, isotopic offsets from the highest δ 15N value within the isotopic profile were used to create these graphs. a: divided according to age-at-death, b: divided according to sex, c: divided according to socio-economic status

  al. 2017, Demmelmair et al. 2006, Kendall et al. 2021, Lamberti et al. 2011, McDade 2005, Palou and Picó 2009). Moreover, as a bio-socio-cultural phenomenon, infant and young child feeding practices are influenced by a number of cultural, religious, economic and also environmental factors (Fildes 2017, Quinlan 2007, Thorvaldsen 2008, Tomori et al. 2017, Yovsi and Keller 2003).

  al. 2012, Kaupová et al. 2014), from different parts of bone (Waters-Rist et al. 2011), and finally from serial sections of dental tissues, with the number of serial samples increasing along with development in mass spectrometry reducing the sample size (Beaumont et al. 2013, Eerkens et al. 2011, Fuller et al. 2003, Howcroft et al. 2012).

  al. 2018, Beaumont and Montgomery 2016, Craig-Atkins et al. 2018). An increasing number of studies (Canterbury et al. 2020, D'Ortenzio et al. 2015, Drtikolová Kaupová et al. 2021, Fuller et al. 2005, Katzenberg and Lovell 1999), however, have found no change in δ13C values during the stress episodes affecting δ15N values. There are even studies (Drtikolová Kaupová et al. in press, Neuberger et al. 2013) reporting positive covariance between δ13C and δ15N values during such stress episodes.

Figure 2 .Figure 3 .Figure 4 .

 234 Figure 2. a: Theoretical weaning model with expected incremental nitrogen isotope data for an individual being weaned onto a diet similar to that of the mother. Note that 0.4 ‰was considered a cut-off value of significant isotopic change when defining the points marked on the isotopic curve, b: four categories of incremental carbon isotope data in relation to nitrogen isotopic data present in the studied dataset: i: positive covariance between δ 15N and δ 13C values, ii: negative covariance when δ 13C values are low during the time of peaking δ 15N values and subsequently increase along with decreasing δ 15N values. iii: flat carbon profile and finally iv: positive covariance between δ 15N and δ 13C values in most of the slices, but initial decrease in δ 13C values preceding the shift in δ 15N values

Figure 5 .

 5 Figure 5. Nitrogen and carbon post-weaning averages (defined as an average value from all the dentine slices formed after the termination of the initial decline in δ15N): in comparison with bone isotopic values of the same individuals (A, B), compared between socio-economic classes (C, D)

Table 1 .

 1 Relationship between defined parameters of both carbon and nitrogen isotopic curve and the explicative variables (p -values a )

				δ15N			δ13C
		Δ15Nmax	Age at first decrease	Age at final smoothig	Post-weaning average	Type of the profile	Post-weaning average
	Age	0.064	0.897	0.007	0.222	0.008	0.096
	Sex	0.835	0.008	0.411	0.038	x	0.097
	Socio-
	economic	0.617	0.550	0.030	˂0.001	x	0.058
	status

a results significant at 0.05 level are in bold

  Neither experimental studies focused on the effect of physiological stress (D'Ortenzio et al. 2015, Drtikolová Kaupová et al. 2021, Fuller et al. 2005, Katzenberg and Lovell 1999, Mekota et al. 2006) nor observations of subadult famine victims (Beaumont and Montgomery 2016)

  al. 2020, D'Ortenzio et al. 2015, Drtikolová Kaupová et al. 2021, Fuller et al. 2005, Neuberger et al. 2013). This inconsistency can theoretically be caused by intra-individual differences in the amount of body fat and the phase of malnutrition. At first, a decrease in δ13C values may reflect the use of carbon from energy sources to synthesize non-essential amino acids, but later body protein catabolism may predominate and cause an increase in δ13C values (Drtikolová Kaupová et al. in press, King et al. 2018, Salesse et al. 2019). Moreover, even if we accept negative covariance between carbon and nitrogen isotopic values as being the primary response of a subadult organism to biological stress, this concept could be valid only a in purely terrestrial C3 plant-based ecosystem, where isotopically distinct food groups such as fish (either freshwater or marine) or C4 plants were not accessible in substantial quantities. This was clearly not the case for the Great Moravian population[START_REF] Halffman | Stable isotope evidence for diet in early medieval Great Moravia (Czech Republic)[END_REF][START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF][START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF]).

  The intra-individual comparison of the post-weaning isotopic averages vs bone isotopic values confirms the earlier results of[START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF], reporting a higher importance of millet in childhood diet. While Jílková's team focused on the later period of childhood (approx. 8-10 years as a portion of the M2 root was sampled), this study documents the importance of millet in earlier phases of life. The span of the post-weaning period actually varies between 1.3-9.4 and 3.75-9.4 years, based at the age on the final smoothing of the isotopic curve.On the other hand, the proportion of animal products consumed by individuals included in this study was stable throughout life, opposing the results from[START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF], who observed a lower proportion of animal products in the subadult diet of individuals buried around Mikulčice 6th church. It needs to be stated, however, that both studies focused on different population subgroups buried within the Mikulčice settlement agglomeration. While the 6th church is located in the suburb of Mikulčice, this study includes mainly individuals buried at the fortified Acropolis i.e. within a supposed residential area of the highest elites of Great Moravian society

  Most of the experimental studies on physiological stress carried out on humans concern adult individuals only (D'Ortenzio et al. 2015, Drtikolová Kaupová et al. 2021, Fuller et al. 2005, Katzenberg and Lovell 1999, Mekota et al. 2006, Tea et al. 2021). The only exception is the work by Beaumont and Montgomery (2016), using incremental sampling of dentine tissue from developing teeth, this study is however based on an archaeological population, albeit from the well-documented context of a 19th century famine event. Moreover, it has to be kept in mind that the isotopic reflection of famine, where starvation is the primary cause of death (though it surely acted in synergy with a number of diseases, e.g. Solomons 2007), may well differ from the less extreme consequences, where the pathophysiological origin of tissue catabolism may prevail.Moreover, the limited number of experimental studies based on subadult individuals from different animal species suggest that a meaningful level of nutritional stress need not necessarily induce an isotopic response in either δ15N or δ13C[START_REF] Ambrose | Controlled diet and climate experiments on nitrogen isotope ratios of rats[END_REF][START_REF] Kempster | Do stable isotopes reflect nutritional stress? Results from a laboratory experiment on song sparrows[END_REF][START_REF] Williams | Effects of nutritional restriction on nitrogen and carbon stable isotopes in growing seabirds[END_REF]).

Acknowledgements

We would like to thank the following institutions for their financial support: the Czech Science Foundation (Grant number: 19-13265S), and the Ministry of Culture of the Czech Republic (Grant number: DKRVO 2019-2023/7.I.d, 00023272). Alastair Millar helped to revise the English.

Funding and competing interests

This study was supported financially by Czech Science Foundation (Project Id: 19-13265S) and Ministry of Culture of the Czech Republic (DKRVO 2019-2023/7.I.d, 00023272). The authors have no conflicts of interest to declare that are relevant to the content of this article.

Carbon isotopic values from infancy and early childhood suggest that millet probably had an important role at the very beginning of the weaning process, and/or in the diet of lactating women. The isotopic data from later childhood, together with the data from a previous study [START_REF] Jílková | Early medieval diet in childhood and adulthood and its reflection in the dental health of a Central European population (Mikulčice, 9th-10th centuries, Czech Republic)[END_REF], suggest that a higher input of millet was characteristic of childhood idet during the first decade of life.

Nitrogen isotopic values from the post-weaning period suggest that though a small drop in the dietary proportion of animal products may occur in the period immediately following weaning, the average proportion of animal products consumed during childhood is comparable to that consumed in adulthood. The socio-economically motivated differences in dietary behaviour previously observed in adults [START_REF] Kaupová | Diet in transitory society: Isotopic analysis of medieval population of Central Europe (ninth-eleventh century AD, Czech Republic)[END_REF]) were already present during childhood.

In future, a comparison of the set of potential indicators of physiological stress listed above with other population samples with distinct documented levels of environmental stress will be carried out. Such comparisons may bring more knowledge about the level of environmental stress that the subadult population of Mikulčice had to face.