Anna Flamand
email: anna.flamand@inrae.fr

Cheyenne Zellenka

Juliette Mos

Audrey Starczan

Aurélien Polak

Odile Petit

Neigh-bours: why every young horse needs good friends. A pilot study during the breaking-in period

Keywords: welfare, social, equus caballus, housing, horse-human relationship, training

published or not. The documents may come

Introduction

Sociality plays an important role in the horse's life (Equus caballus): in wild conditions, horses live in stable social groups where daily activities are shared and practiced together [START_REF] Feh | Relationships and communication in socially natural horse herds[END_REF]. The most common social structure is the familial herd composed of one or two stallions, a few mares and their young [START_REF] Waring | Horse behavior[END_REF]. More transitional herd structures can be joined by young horses aged between 1 and 4 years old when they leave their initial family group [START_REF] Boyd | Behavioural ecology of feral horses[END_REF]. These groups are composed of males and females or exclusively of males, and individuals can remain in them for several years [START_REF] Heitor | Dominance relationships and patterns of aggression in a bachelor group of Sorraia horses (Equus caballus)[END_REF]. In bachelor groups, the age range of the stallions can vary [START_REF] Briefer Freymond | Pattern of Social Interactions after Group Integration: A Possibility to Keep Stallions in Group[END_REF]. Young males will acquire social codes with other juveniles and older conspecifics, and will particularly learn to express ritual and aggressive behaviors, especially through playful fights [START_REF] Briefer Freymond | Pattern of Social Interactions after Group Integration: A Possibility to Keep Stallions in Group[END_REF][START_REF] Søndergaard | [END_REF]. The young horse's development is highly influenced by social relationships [START_REF] Søndergaard | [END_REF] and strong affinities can emerge between individuals of the same age: playing and sharing activities in proximity of others strengthens this bond and contributes to social skill development [START_REF] Waring | Horse behavior[END_REF]. In domestic conditions, group living could be limited to the early stages of life. After weaning at around 6-9 months, foals are separated from their mother and are commonly housed in groups of individuals the same age [START_REF] Hartmann | Keeping horses in groups: A review[END_REF][START_REF] Waran | The effects of weaning on the domestic horse (Equus caballus)[END_REF]. Although this approach allows the development of strong bonds, the absence of adults can lead to more inter-individual aggressiveness [START_REF] Bourjade | Adult-Young Ratio, a Major Factor Regulating Social Behaviour of Young: A Horse Study[END_REF][START_REF] Jansson | Behaviour of colts in pasture conditions[END_REF]. In most European countries, the horse is permanently housed in an individual box [START_REF] Bachmann | Haltung und Nutzung von Pferden in der Schweiz: Eine repräsentative Erfassung des Status quo[END_REF][START_REF] Popescu | Welfare quality of breeding horses under different housing conditions[END_REF][START_REF] Søndergaard | A survey of housing and management conditions for horses in Denmark[END_REF] from the age of 2-3 years, i.e. generally from the breaking-in period onwards. This change is drastic and stressful [START_REF] Harewood | Behavioral and physiological responses to stabling in naive horses[END_REF]. Many authors have reported the harmful consequences of the individual stall on health problems such as respiratory problems or colic [START_REF] Cohen | Factors predisposing to colic[END_REF][START_REF] Yngvesson | Health and Body Conditions of Riding School Horses Housed in Groups or Kept in Conventional Tie-Stall/Box Housing[END_REF] and on behaviour, leading to the appearance of disorders [START_REF] Fureix | Towards an Ethological Animal Model of Depression? A Study on Horses[END_REF][START_REF] Fureix | Partners with Bad Temper: Reject or Cure? A Study of Chronic Pain and Aggression in Horses[END_REF][START_REF] Ruet | Housing Horses in Individual Boxes Is a Challenge with Regard to Welfare[END_REF] from a very young age [START_REF] Waters | Factors influencing the development of stereotypic and redirected behaviours in young horses: findings of a four year prospective epidemiological study[END_REF]. Indeed, Visser et al. (2008b) found that more than two thirds of young horses developed stereotypies during this initial transition from group to individual housing, with 22% of individuals performing cribbing, 22% box walking and 44% weaving, which is often associated to the lack of social contact [START_REF] Benhajali | Reproductive status and stereotypies in breeding mares: A brief report[END_REF][START_REF] Mcafee | The use of mirrors for the control of stereotypic weaving behaviour in the stabled horse[END_REF]. Many other risk factors are also linked to the appearance of stereotypical behaviours, including restricted free movement [START_REF] Lesimple | Free movement: A key for welfare improvement in sport horses?[END_REF] or a low-fibre diet [START_REF] Cooper | Stereotypic behaviour in the stabled horse: causes, effects and prevention without compromising horse welfare[END_REF]. Interestingly, in this study comparing horses stabled individually or in pairs, none of the paired horses developed stereotypical behaviours. This suggests that a poor social environment strongly influences the development of these behaviours. Restrictions related to confinement in individual stalls, i.e. limited space without physical social contact, can also lead to aggression towards humans, causing dangerous situations [START_REF] Fureix | Partners with Bad Temper: Reject or Cure? A Study of Chronic Pain and Aggression in Horses[END_REF][START_REF] Mcgreevy | Equine behavior: a guide for veterinarians and equine scientists[END_REF] and a deteriorated relationship with humans [START_REF] Ruet | Horses Could Perceive Riding Differently Depending on the Way They Express Poor Welfare in the Stable[END_REF]. Breaking-in per se is a particularly stressful period for the young horse in conventional practices that involve daily training by humans following a period during which the horse had experienced very little constraint [START_REF] Kędzierski | Physiological response to the first saddling and first mounting of horses: comparison of two sympathetic training methods[END_REF][START_REF] Rivera | Behavioral and physiological responses of horses to initial training: the comparison between pastured versus stalled horses[END_REF][START_REF] Waran | Training methods and horse welfare in[END_REF]). These interactions, traditionally based on negative reinforcement or even punishment, can be a negative experience for the horse [START_REF] Hall | Is There Evidence of Learned Helplessness in Horses[END_REF][START_REF] Mcgreevy | The advent of equitation science[END_REF]. The relationship between the horse and the human is built up through repeated interactions, and the valence of these repeated interactions, whether positive or negative, can have a profound and lasting effect on the developing relationship (see review [START_REF] Hausberger | A review of the human-horse relationship[END_REF]. The improvement of housing conditions, especially through social enrichment, has been shown to have a positive influence on the behaviour of young horses towards humans. Indeed, [START_REF] Søndergaard | Group housing exerts a positive effect on the behaviour of young horses during training[END_REF] have demonstrated that horses housed in groups expressed fewer aggressive behaviours towards humans during training than horses housed in individual stalls. Furthermore, horses housed in conditions that allow them to interact are easier to handle than those that are socially isolated [START_REF] Yarnell | Domesticated horses differ in their behavioural and physiological responses to isolated and group housing[END_REF].

These studies have tested different housing methods comparing group-housed individuals to isolated individuals. The present study seeks to compare the welfare and also cooperative and conflict behaviours of young horses during the breaking-in period. The horses are housed individually, but half of the stock have access to a social partner for two hours per day. This paradigm avoids the confounding effects of a group living in pastures and the subsequent combination of social life and ranging freely in an open environment. In our study, the only parameter that varied was the daily access to a social partner for half of the individuals. In this context, we predict that maintaining social interactions should promote the well-being of young horses and increase their receptiveness to interactions with humans.

Material and methods

Study site

The study took place at the stud farm of Rosières-aux-Salines, France (48° 35′ 47″ N, 6° 20′ 15″ E).

The site covers 12 hectares with 60 stalls, two indoor arenas, three outdoor sand arenas and other facilities for horses at rest, at training and for breeding.

Animals

We studied 12 young horses (females: n = 6, males: n = 6) aged 24-36 months (in average 26 ± 5 months) of Selle Français (n = 4) and Thoroughbred (n = 8) breeds. All animals came from different farms and had been group-housed in pasture without any experience of the individual stable before the study began.

Procedure

The 12 horses were equally and semi-randomly divided into two conditions: the Social Condition horses (n = 6, three females and three males; three Thoroughbred and three Selle Français) and the Isolated Condition horses (n = 6, three females, three males, five Thoroughbred and one Selle Français). All horses were housed in individual stalls but the horses in the Social Condition were allowed to interact with a partner in a "social box" (Fig. 1) for two consecutive hours/day and five times per week, while the isolated horses had no opportunity for social interaction. As the horses had arrived in the facility just two days before breaking-in began, we decided to assign pairs randomly and only assign individuals who did not know each other before the experiment in order to ensure that each individual created a new relationship with its social partner, which remained the same throughout the experimental period. Each resulting pair was composed of a female and a male. The study was conducted from November 2, 2019 to February 17, 2020 as three one-month replicate periods due to the organisation of breaking-in sessions. As a result, four horses were studied per month with two individuals per condition.

Housing conditions

All horses were housed in 4*3 meter stalls with solid walls and a grid on the door that allowed only visual contact with the horses in the opposite stalls. The horses in the Social Condition had no visual contact with their partner, and their physical and visual interactions only occurred during encounters in the social box. In each period, the four horses were housed in the same aisle of the same stable and were randomly assigned to one of the four stalls (whilst a horse of the other condition was always installed in the opposite stall). During each replicate, eight unstudied horses were also housed in the same stable but in a different aisle and had no physical or visual interactions with the horses of the study. All horses were fed pellets twice a day (morning and evening) and had free access to hay and water. Straw bedding was provided daily and cleaned out once a week. Horses were allowed individual free movement in the indoor riding arena for less than one hour per week. These regular tasks were carried out by the same two grooms for each replicate.

Social box

The horses in the Social Condition met in pairs in the social box, a device allowing contact between two horses in a secure manner. The social box was composed of two adjacent 4*3 stalls which could communicate via a 2 m-high partition with two central 25 cm-wide vertical openings, allowing each individual to pass its head and neck to the shoulders through to the other stall to interact with the horse in the adjacent stall. The central part of the partition and each side of the two openings were padded with foam covered with a rubber mat, ensuring the complete safety of the horses. The remainder of the partition was composed of pine boards (Fig. 1). This device was located in another stable between the riding arena where the training sessions took place and the stable where the studied horses were housed. Each pair was handled by the same two professional riders (randomly assigned for each encounter) during the experiment and for each replicate.

Training sessions

The training sessions took place in an indoor arena measuring 50 x 20 m with a sand floor. Each period was carried out over 4 weeks with five sessions each, with a total of 20 sessions per horse. Five riders (four women and one man) accustomed to training young horses were involved and one female expert supervised all of the sessions. During the first week, the horses were lunged at all three gaits with an increasing amount of equipment (roller, then bridle and bit, then saddle). From the second week, the horses were ridden with standard equipment (single bridle, simple bit, mixed saddle) by a rider and held with the lanyard by the expert in breaking-in. During the third week, the horse was expected to be ridden alone by a rider without being held. At the end of the fourth week, the objective was that each horse could be ridden calmly and independently at all three gaits (see details in Table 6 of the Supplementary Material).

Data collection

Behavioural data

Individual stall

The behaviour of individuals was recorded every 5 min for two one-hour sessions held each day at different times (between 7:30 and 18:30) and outside of meal times. To take account of the potential effect of training for both conditions, an equal number of observation sessions in the individual stalls were carried out before and after each session. Data were collected by an experienced observer (AF, 5 years of experience in horse behaviour) using the scan sampling method [START_REF] Altmann | Observational Study of Behavior: Sampling Methods[END_REF] and following the protocol used by [START_REF] Ruet | Housing Horses in Individual Boxes Is a Challenge with Regard to Welfare[END_REF]. All the behaviours expressed by horses were recorded and classified according to functional categories mostly adapted from the ethogram of [START_REF] Lesimple | Stall architecture influences horses' behaviour and the prevalence and type of stereotypies[END_REF][START_REF] Mcdonnell | The equid ethogram: a practical field guide to horse behavior[END_REF], [START_REF] Ruet | Horses Could Perceive Riding Differently Depending on the Way They Express Poor Welfare in the Stable[END_REF] and [START_REF] Yarnell | Domesticated horses differ in their behavioural and physiological responses to isolated and group housing[END_REF]. The study sought to understand whether there was a change in the distribution of daily activities of horses with social contact compared with that of horses that were socially isolated. The list of behaviours is shown below in Table 1. Since the overall distribution of activities was of interest, certain categories were grouped together, such as elimination and comfort behaviour, under the heading 'maintenance' or 'rest', which includes various behaviours related to rest (e.g. standing, and sternal or lateral rest). We also chose to differentiate observation from vigilance, a behaviour that the horse adopts when it detects a potential threat and which is characterised by a specific posture and a different level of body tension (see "vigilance" in the ethogram Table 1). An "abnormal behaviours" category was created including common indicators of a compromised state of welfare [START_REF] Ruet | Housing Horses in Individual Boxes Is a Challenge with Regard to Welfare[END_REF]. The latter can be expressed by a variety of behaviours depending on the horse concerned (agressivity towards humans [START_REF] Normando | Variables affecting the prevalence of behavioural problems in horses. Can riding style and other management factors be significant?[END_REF], apathy [START_REF] Fureix | Towards an Ethological Animal Model of Depression? A Study on Horses[END_REF], stereotypies [START_REF] Waters | Factors influencing the development of stereotypic and redirected behaviours in young horses: findings of a four year prospective epidemiological study[END_REF] or other signs of troubles [START_REF] Lesimple | Stall architecture influences horses' behaviour and the prevalence and type of stereotypies[END_REF]). Some of these behaviours expressed in the stall seem to result from multiple unsatisfied fundamental needs [START_REF] Popescu | Welfare quality of breeding horses under different housing conditions[END_REF][START_REF] Sarrafchi | Equine stereotypic behaviors: Causation, occurrence, and prevention[END_REF]) rather than being related to a unique causation. We considered the different forms of expression of poor welfare as a whole to investigate the effect of social contact on all these behaviours, without focusing on the detailed expression of each one. The hypothesis was that satisfying the need for social contact would be beneficial to the general mental state of the horses and potentially reduce the appearance or expression of behavioural disorders.

Ear position during foraging was also scored according to the three modalities described in Table 2.

Ear position is a useful indicator of welfare, as it has been shown that the time spent with ears facing backwards during foraging or when eating hay is related to an impaired state of welfare [START_REF] Lesimple | Free movement: A key for welfare improvement in sport horses?[END_REF]. We therefore investigated whether the possibility of social contact for horses housed in stalls could have an influence on their emotional state in the individual stall.

Training

Observation of the horses during the training sessions alternated randomly between the morning and the afternoon each day. For each daily session, the order of the horses was randomly selected. The same procedure was repeated for each replicate. Each session (average session length = 26 minutes 47 seconds) was filmed by one experimenter using a GoPro camera (GoPro Hero9) held in a GoPro harness in order to record all behaviours. Recording was carried out from behind a fence to ensure the safety of the experimenter. The video recordings were then analysed through continuous focal sampling using Boris software [START_REF] Friard | BORIS: A free, versatile open-source event-logging software for video/audio coding and live observations[END_REF]. To obtain an overview of the effects of social contact on the behaviour and emotional state of horses at training, we decided to collect a certain number of behavioural variables (postures, ear positions and isolated behaviours). We also analysed the latency to respond to a rider command, as a measure of the cooperation of the horse (Visser et al. 2008a).

We collected data relating to overall body tensionreferred to hereafter as attitudeand described it on a 4-level scale adapted from [START_REF] Hall | Assessing ridden horse behavior: Professional judgment and physiological measures[END_REF] (Table 3). Additional details were incorporated into the original scale concerning head carriage, as the latter is a relevant indicator for assessing the tension of horses during training [START_REF] Hall | Assessment of ridden horse behavior[END_REF]. In order to have a finer measure of emotional state, we observed the time spent in each ear position to investigate whether social contact had an effect on the distribution of the three different positions (forward, asymmetric, backward) (Table 2). The main hypotheses were that horses with social contact would spend more time in a relaxed attitude and less time with their ears back than socially isolated horses. The time spent in each attitude and for the different ear positions was recorded as durations.

The ethogram of ridden horse behaviours was adapted mostly from König von Borstel and Glißman (2014) and [START_REF] Rivera | Behavioral and physiological responses of horses to initial training: the comparison between pastured versus stalled horses[END_REF] (Table 4). It includes stress behaviours such as defecation or vocalisations [START_REF] Wilk | Relationship between behavior and cardiac response to round pen training[END_REF] and common defence behaviours, also called conflict behaviours. The latter are defined as "responses exhibited by animals that experience difficulty coping with mental or physical discomfort, and most often demonstrated as some form of resistance to handling or training cues and/or equipment" [START_REF] Górecka-Bruzda | Conflict behavior in elite show jumping and dressage horses[END_REF]. Data relating to these behaviours were collected in order to assess whether the possibility of social contact for the Social Condition had an effect on the frequency of expression of these behaviours during the training of the horse.

Statistical analysis

All data from the individual stalls and training sessions were analysed using generalized linear mixed models (GLMM) with the "glmmTMB" package [START_REF] Brooks | glmmTMB balances speed and flexibility among packages for zero-inflated generalized linear mixed modeling[END_REF]. For each model, we used the variance inflation factor (VIF) with the "check_collinearity" function of the "performance" package to check for collinearity of the selected predictors [START_REF] Lüdecke | performance: An R Package for Assessment, Comparison and Testing of Statistical Models[END_REF], retaining all predictors with values below 3 [START_REF] Zuur | Mixed effects models and extensions in ecology with R[END_REF]. We also tested the uniformity and dispersion of residuals, the homoscedasticity and possible zero-inflation with model diagnostics of the "DHARMa" package [START_REF] Hartig | DHARMa: residual diagnostics for hierarchical regression models[END_REF]. We selected the best-fitting model using the "dredge" function of the "MuMIn" package [START_REF] Bartoń | Mumin: Multi-model inference[END_REF]. We chose the best explanatory model from the two AICc units that have the lowest value, based on the corrected Akaike information criterion [START_REF] Burnham | Model selection and Multimodel Inference. A Practical Information-Theoretic Approach[END_REF].

Due to the small sample size, the factors included in the model were limited to reduce statistical instability and the risk of overfitting. Consequently, several factors were considered similar between conditions (age, sex and replicate) and were excluded from the models. Moreover, the effect of the rider was tested independently to evaluate whether this factor needed to be integrated to the models.

The number of commands made by each rider was analysed. There were no significant differences between the riders for the commands (Kruskal-Wallis test, χ2 = 4, df = 4, p = 0,406). Based on these findings, rider effect was not included as a factor in the analysis.

The condition (Social or Isolated), the target variable, the interaction between these two predictors and the pair to which each individual belongs in the Social Condition were included as fixed factors in all analyses. For all proportions data, given that the response variable was a continuous proportion, we fitted GLMMs with an ordered beta family and a logit link function. Whenever the interaction of the condition and the target variable was significant, post-hoc comparisons were conducted by calculating estimated marginal means using the "emmeans" package [START_REF] Lenth | emmeans: estimated marginal means, aka least-squares means[END_REF]. To account for the repeatability of the measurements on individuals according to the observation days, individuals were set as random intercept terms and days were nested within individuals.

All analyses were performed in R 4.0.2 (R Core Team, 2023) and significance was set at p < 0.05.

Data analysis in individual stalls

Distribution of the behavioural categories

We compared the distribution of behavioural categories between the Social and Isolated conditions.

We calculated the frequency as a proportion (i.e. number of scans for each category out of the total number of scans) per individual and per daily session as a response variable in the GLMM analysis.

The "vocalisationwhinnying" category was not retained in the analyses because this behaviour was seldom observed (only two observations were made out of the total number of observations).

Ear positions during foraging

We compared the distribution of ear positions while foraging according to three modalities (forward, asymmetrical, backward) between the Social or Isolated Conditions. The response variable was the ratio of each ear position to the total number of ear positions observed.

Data analysis during training

Attitude

We used the proportion of the duration of each attitude to the duration of the session as a response variable.

Ear position

Ear position during sessions was categorised in the same way as for the ear position in individual stalls. We used the proportion of the duration of each ear position to the duration of the session as a response variable.

Conflict behaviours

As the duration of sessions varied slightly each day, we calculated the number of behaviours per hour to allow comparison on the same time basis. In order to analyse frequency, we fitted a GLMM assuming a Poisson error distribution with a log link.

Cooperation

Given that the response variable was expressed as a continuous variable (latency to respond to each rider command), we fitted a GLMM using a Gaussian distribution and an "identity" link function.

Ethics Statement

This experiment complies the ethical policy of the International Society for Applied Ethology and has been approved by the ethical committee of Val de Loire (N° CE19 -2022-0601 -1).

Results

Behaviour in individual stalls

Distribution of the behavioural categories

The ratio of behavioral categories was compared between the two conditions (Social and Isolated).

The best-fitting model included the behavioural category, the condition and the interaction between behavioural category and condition (see Supplementary Material, Table 7 and Table 8 for model selection and predicted means ± SE). Post-hoc comparisons of the interaction showed that there were significant differences in the distribution of behavioral categories between the two conditions: horses in the Social Condition expressed fewer abnormal behaviors (emm contrast: β ± SE = -0.429±0.124, z=-3.46, p<0.001) and were immobile more often (emm contrast: β ± SE = 0.329±0.106, z-=3.11, p<0.01) than horses in the Isolated Condition. For the other behavioural categories, no significant difference was found (Supplementary Material, Table 9).

Ear positions during foraging

The ratio of the three ear positions was compared between the two conditions (Social or Isolated) and then within each condition. The best-fitting model included ear position, condition and the interaction between ear position and condition (see Supplementary Material for model selection, Table 10). Posthoc tests showed that there was no significant difference between the Social Condition and the Isolated Condition for the three positions of the ears (emm contrast: asymmetrical position : β ± SE= -0.108 ±0.059, z=-1.82 p=0.068; backward position : β ± SE= -0.020±0.071, z= -0.273, p=0.785; forward position : β ± SE= 0.100±0.058, z=1.69, p=0.092). For the Social Condition, there is a significant difference in the frequency of the three ear positions: horses have their ears facing forward more often than in asymmetric positions (emm contrast: β ± SE= -0.550 ± 0.060, z=-9.32, p<0.001), in asymmetric positions more often than facing backward (emm contrast: β ± SE= 0.613 ± 0.066, z=9.28, p<0.001) and face forward more often than they do backward (emm contrast: β ± SE= -1.16 ± 0.066, z=-17.70, p<0.001). The same result was found for horses in the Isolated Condition (emm contrast: forward vs. asymmetric: β ± SE= -0.344 ± 0.059, z=-5.85, p<0.001; asymmetric vs. backward: 0.701± 0.066, z=10.69, p<0.001; forward vs. backward: -1.05± 0.066, z=-15.90, p<0.001).

Behaviours during training

Attitude

The proportion of the four types of attitude was compared between the two conditions (Social or Isolated). The best-fitting model included the types of attitude, condition and the interaction between types of attitude and condition (see Supplementary Material, Table 11). Post-hoc analysis revealed that horses in the Social Condition spent significantly more time in a relaxed attitude (emm contrast: β ± SE= 0.532±0.142, z-=3.76, p<0.01; Fig. 2) and less time in a tense attitude (emm contrast: β ± SE= -0.546±0.141, z=-3.89, p<0.001) than the horses in the Isolated Condition. There was no significant difference between the two conditions for the attentive attitude (emm contrast: β ± SE= 0.158±0. 163, z=0.971, p=0.332) and the extremely tense attitude (emm contrast: β ± SE= -0.425 ±0.241, z=-1.76, p=0.079).

Ear position

The ratio of the three ear positions was compared between the two conditions (Social or Isolated), then the distribution of positions was analysed for each condition. The best explanatory model included condition and ear position and the interaction of these two variables (see Supplementary Material, Table 12). However, post-hoc analysis revealed no significant effect of the condition on ear positions (emm contrast: asymmetric position: β ± SE= -0.134±0.093, z=-1.444, p=0.149; backward position: β ± SE= 0.093±0.115, z=0.805, p=0.421; forward position: β ± SE= 0.171±0.102, z=1.68, p=0.092). The analysis within each condition showed that horses in the Social Condition spent significantly more time with their ears in an asymmetric position than facing forward (β ± SE= -1.78 ±0.090, z=-19.88, p<0.0001) or backward (β ± SE= -2.42 ±0.098, z=-24.77, p<0.001), and they also spent more time with their ears facing forward than facing backward (β ± SE= -0.640±0.099, z=-6.46, p<0.001). The exact same distribution was found in isolated horses (emm contrast: asymmetric position vs forward position: β ± SE= -2.09±0.107, z=-19.44, p<0.001; asymmetric position vs backward position: β ± SE= -2.65±0.116, z=-22.86, p<0.001; forward position vs backward position: β ± SE= -0.560±0.118, z=-4.74, p<0.001).

Conflict behaviours

The best-fitting model included the behaviour, the condition and the interaction of both terms (see Supplementary Material, Table 13). Horses in the Social condition defecated significantly less than horses in the Isolated Condition (emm contrast: β ± SE= -0.871±0.215, z=-4.05, p<0.001) and they also tended to grind their teeth less than horses in the Isolated Condition (emm contrast: β ± SE= -2.00±1.08, z=-1.85, p=0.065). On the other hand, horses in the Social Condition expressed significantly more tail switching (emm contrast: β ± SE= 0.529±0.247, z=2.14, p=0.033) and head tossing (emm contrast: β ± SE= 0.500 ±0.224, z=2.23, p=0.026) than those in the Isolated Condition.

There are no significant differences for other behaviours (Table 5).

Cooperation

As the condition was the only fixed effect in this analysis, the best-fitting model was also one of the two with the lowest AICc value (Supplementary Material, Table 14). The response latency for a rider command was not affected by the condition (Social or Isolated) (GLMM: β ± SE = 0.085 ± 0.284, z=0.297, p=0.766).

Discussion

Our results show that horses that have access to a social partner during the breaking-in period expressed fewer abnormal behaviours and stayed still more often in the individual stall than isolated horses. Moreover, during training sessions, they spent more time in a relaxed attitude and less time showing a tense attitude, and defecated less often but expressed more head tossing and tail switching than horses in the Isolated Condition. We did not find any difference in ear positions and cooperation between the two groups of horses.

Behaviours in individual stalls

Horses in the Social Condition expressed abnormal behaviours less frequently than horses in the Isolated Condition. Similar results have been found across studies that compare the behaviours of young horses in individual stalls with those of counterparts in a socially enriched condition [START_REF] Heleski | Influence of housing on weanling horse behavior and subsequent welfare[END_REF][START_REF] Hoffman | Preweaning diet and stall weaning method influences on stress response in foals[END_REF]Visser et al. 2008b) or counterparts in environments that provide more global enrichment [START_REF] Lansade | Behavioral and Transcriptomic Fingerprints of an Enriched Environment in Horses (Equus caballus)[END_REF]. Thus, our results suggest that reducing the time spent in social isolation could reduce the development of these abnormal behaviours. These behaviours are recognised as reliable indicators of poor welfare linked to past or present unfavourable housing conditions [START_REF] Hall | Is There Evidence of Learned Helplessness in Horses[END_REF]. In young animals with no previous experience of restrictive housing, this low frequency of abnormal behaviours expressed by the individuals in the Social Condition therefore indicates better welfare in comparison to horses in the Isolated Condition due to current social enrichment. Indeed, being able to interact, to express the species' behaviours and to be more stimulated through this social environment may reduce the need for coping strategies [START_REF] Søndergaard | The effects of social environment on the behaviour of young horses[END_REF]. Furthermore, social interactions in young horses (including play) contribute to their overall development, particularly in terms of cognitive development, by increasing their capacity for behavioural adaptability and flexibility [START_REF] Rivera | Behavioral and physiological responses of horses to initial training: the comparison between pastured versus stalled horses[END_REF]. This cognitive development could enable horses that have access to social contacts to better adapt to the challenges of their environment.

The percentage of time that the horses in our experiment spent standing still (Social Condition: 9,3%; Isolated Condition: 6,9%) is lower than that of feral horses or group housing horses, which spent 15 to 50% of their time-budget standing [START_REF] Budiansky | The nature of horses: exploring equine evolution, intelligence, and behavior[END_REF][START_REF] Duncan | Time-budgets of Camargue horses II. Time-budgets of adult horses and weaned sub-adults[END_REF]. However, the latter seems to indicate that spending time standing still is a behaviour that is also expressed in conditions closer to natural contexts and is therefore not specific to the confinement of the stall. The lower value of immobility found for isolated horses may be explained the higher levels of stress or frustration experienced by the latter, as illustrated by the frequency of abnormal behaviours they expressed, compared to the horses in the Social Condition. According to some authors, abnormal behaviours such as stereotypies could be a means for the horse of attempting to adapt to a challenging environment [START_REF] Arena | Assessment of horses' welfare: Behavioral, hormonal, and husbandry aspects[END_REF][START_REF] Budzyńska | Stress Reactivity and Coping in Horse Adaptation to Environment[END_REF]. [START_REF] Cooper | Behavioural adaptation in the domestic horse: potential role of apparently abnormal responses including stereotypic behaviour[END_REF] refer to how the horse seeks control over its environment, where stereotypies could reduce arousal, and regulate stress or certain physiological functions. However, actual adaptation is unlikely to be effective, given the negative consequences these behaviours can have on health [START_REF] Cabib | Parallel strain-dependent susceptibility to environmentally-induced stereotypies and stress-induced behavioral sensitization in mice[END_REF][START_REF] Krueger | Basic Needs in Horses?-A Literature Review[END_REF][START_REF] Nicol | Study of crib-biting and gastric inflammation and ulceration in young horses[END_REF][START_REF] Nisticò | Behavioral and electrocortical effects of beta-endorphin after intracerebral infusion in rats[END_REF][START_REF] Powell | A rodent model of spontaneous stereotypy: initial characterization of developmental, environmental, and neurobiological factors[END_REF]. Horses can display a wide range of abnormal behaviours in response to an unsuitable environment causing chronic stress [START_REF] Ruet | Housing Horses in Individual Boxes Is a Challenge with Regard to Welfare[END_REF]. Horses are less active in stalls than in pasture due to the lack of space and stimulation [START_REF] Chaplin | Effect of housing conditions on activity and lying behaviour of horses[END_REF]. Consequently, the appearance of stereotypic abnormal behaviours could be related to the horses' motivation to be active, and this physical and social restriction could lead to frustration or boredom [START_REF] Cooper | Stereotypic behaviour in the stabled horse: causes, effects and prevention without compromising horse welfare[END_REF]. Moreover, resting time did not differ between the horses of the two conditions in our study. In other studies conducted on the influence of the social environment on young horses after weaning [START_REF] Søndergaard | [END_REF] and at breaking-in (Visser et al., 2008b), isolated horses spent more time lying down than those housed in groups during the first days, but differences between the two groups disappeared after two weeks. This finding suggests that social interaction, while stimulating for the horse, does not increase tiredness or the need for rest.

Behaviour during training

During training sessions, the level of tension that we recorded according to a four-level grade of intensity showed that horses that had access to social contact were less tense than those who were isolated. The horse's posture is an indicator that is often used as a score in addition to other measures to assess the horse's stress level in a specific situation [START_REF] Hall | Assessing ridden horse behavior: Professional judgment and physiological measures[END_REF][START_REF] Polito | A Pilot Study on Yearlings' Reactions to Handling in Relation to the Training Method[END_REF][START_REF] Wulf | Sex differences in the response of yearling horses to handling by unfamiliar humans[END_REF][START_REF] Yarnell | Domesticated horses differ in their behavioural and physiological responses to isolated and group housing[END_REF]. Indeed, a tense posture with the neck raised is linked to a higher eye temperature, indicating a stress response [START_REF] Hall | Assessing ridden horse behavior: Professional judgment and physiological measures[END_REF]. We used this indicator because it seemed relevant during breaking-in, where the horse is not constrained by harnesses imposing a certain posture or head carriage, as the case may be with adult horses [START_REF] Smiet | Effect of different head and neck positions on behaviour, heart rate variability and cortisol levels in lunged Royal Dutch Sport horses[END_REF]. The rider's requests are limited to basic changes of directions or transitions with little pressure on the horse's mouth and head, which made it possible to observe the horse's free neck carriage during interaction with the human and use this observation as an indicator. [START_REF] Visser | A comparison of sympathetic and conventional training methods on responses to initial horse training[END_REF] used this body tension to compare different breaking-in methods and showed a positive correlation with an elevated heart rate. In our study, this tense posture was expressed for longer periods by horses in the Isolated Condition, possibly indicating higher stress levels during training sessions. In addition, these horses also defecated more often than horses in the Social Condition, another stress-related indicator confirming this stressful state [START_REF] Hall | The role of the ethogram in equitation science[END_REF]. Conversely, horses that had access to social contact adopted a low-head posture for longer periods than those in the Isolated Condition, indicating a state of relaxation. The low-head, relaxed posture is known to lower arterial blood pressure and is quite similar to the resting or grazing postures assumed naturally by horses [START_REF] Warren-Smith | The use of head lowering in horses as a method of inducing calmness[END_REF]; this seems to indicate that training is apparently a less negative experience for horses in Social Condition, with a more serene attitude.

However, horses in the Social Condition expressed head tossing and tail switching more often than horses in the Isolated Condition. Conflict behaviours are common during breaking-in, which is by nature a stressful event for the animal [START_REF] Rivera | Behavioral and physiological responses of horses to initial training: the comparison between pastured versus stalled horses[END_REF]. The aim of breaking-in, regardless of the method used, is to train the horse to accept carrying a human and equipment on its back and allow the rider to decide on its gait and direction. The horse subsequently loses control, which can lead to the expression of defensive behaviours. The change in relationship with the human and the difficulty in adapting to this new relationship may be another important factor in the expression of these conflict behaviours [START_REF] Sankey | Reinforcement as a mediator of the perception of humans by horses (Equus caballus)[END_REF]. The latter can disappear as training progresses, due to habituation but also to what is known as acquired resignation: the horse learns that whatever its response, it will not be able to escape the constraint and discomfort [START_REF] Hall | Is There Evidence of Learned Helplessness in Horses[END_REF]. The loss of control of horses in the Isolated Condition, both through their housing conditions and training with humans, may have led to a rapid extinction of resistance behaviours in comparison to those in the Social Condition, which resist occasionally, i.e. in response to a rider's command. The control an animal has on its environment is an important and frequently discussed feature [START_REF] Cooper | Stereotypic behaviour in the stabled horse: causes, effects and prevention without compromising horse welfare[END_REF][START_REF] Schork | Personality, abnormal behaviour, and health: An evaluation of the welfare of police horses[END_REF].

Allowing animals to retain some control over their environment and the choices they can make, like interacting with each other as they do in social groups, may reduce anxiety [START_REF] Hall | Is There Evidence of Learned Helplessness in Horses[END_REF]. The social device used in this study enabled horses in the Social Condition to regain some form of choice of interaction with a conspecific without human intervention. This social break is particularly important during breaking-in, when the horse is quite brutally deprived of free movement, interaction and expression. These daily moments of regaining social control can lead to an improvement in welfare, which could also explain the general attitude observed at training. Additionally, this social interaction could explain why these resistance behaviours persist: the opportunity to express social behaviours on a daily basis could have strongly preserved the horses' capacity to express themselves in other contexts. However, we cannot ignore the possibility that these results may partly reflect a more general effect due to other factors related to the entire procedure. Although the two stables were very close and handling time was reduced, horses in the Social Condition had a little more experience with humans than those in the Isolated Condition and spent time in a different environment than their usual stall, which may have an impact.

Conclusion

Individual stabling and breaking-in are particularly stressful events for the young horse. They can cause behavioural disorders in individual stalls and dangerous defensive behaviours in human-horse interactions. In the stall, the prevalence of abnormal behaviour in both social and isolated horses raises questions about the ongoing use of this type of housing. The individual stall, which results in social isolation and confinement, leads to a distinct change in the activity budget of the horse and depletes its environment compared with outdoor housing [START_REF] Cooper | Stereotypic behaviour in the stabled horse: causes, effects and prevention without compromising horse welfare[END_REF]. We suggest that the enriched context (i.e. social interactions) offered to the horses in the Social Condition of our study had a positive impact on their first experience of training. The social box we used for this study is a very simple and secure solution. Using such facilities could reduce abnormal behaviours in stalls, allow horses to be more expressive, improve overall welfare and thus contribute to a better human-horse relationship. More generally, this easy-to-install device will promote the welfare of horses in equestrian facilities and will be in line with changes in society regarding respect for our domestic companions. Tables Table 1: Repetitive licking: licking of the same object in its environment (except the trough) [START_REF] Sénèque | Could posture reflect welfare state? A study using geometric morphometrics in riding school horses[END_REF] Repetitive biting: biting of the same object in its environment (except the trough) [START_REF] Sénèque | Could posture reflect welfare state? A study using geometric morphometrics in riding school horses[END_REF] Wood chewing: Chewing and/or ingesting wooden objects such as fences or stall construction materials [START_REF] Mcdonnell | The equid ethogram: a practical field guide to horse behavior[END_REF] Clapping of lips: repetitive movement of the lips [START_REF] Ruet | Horses Could Perceive Riding Differently Depending on the Way They Express Poor Welfare in the Stable[END_REF] Tongue movements outside of the mouth [START_REF] Ruet | Housing Horses in Individual Boxes Is a Challenge with Regard to Welfare[END_REF] Wall kicking: Deliberate extension of the hind leg to contact barriers, using one or both hind legs alternately or simultaneously [START_REF] Mcdonnell | The equid ethogram: a practical field guide to horse behavior[END_REF] • Vigilance: stands alert, neck raised and nostrils dilated, eyes wide opened and ears held stiffly without any perceptible perturbation in the environment

Figures

Foraging

The horse ingests hay or straw, with or without movement.

Investigation

The horse smells its environment (litter, wall or door), with its nostrils in contact with the surface, sometimes moving its head and neck within or towards the area of interest. [START_REF] Heleski | Influence of housing on weanling horse behavior and subsequent welfare[END_REF]

Figure 1 :

 1 Figure 1: Pictures of the social box: empty (on the left) and during a social interaction (on the right)

  Ethogram of the behavioural categories collected in the individual one of the following behaviours :• Aggressiveness toward humans: pins its ears back, threatens with its head and/or mouth or bites the grid when the observer walks in front of its stall • Apathy: stands still with the neck at the same height as the back, fixed ears, a fixed gaze with opened eyes, and unresponsive to the environment (Fureix et al. 2012) • Signs of agitation without any perceptible perturbation : trots, gallops, snores, blows, tosses its head, paws repeatedly in contexts other than investigation or foraging • Stereotypic and abnormal repetitive behaviours : Weaving: Abbreviated pacing involving rhythmic, repeated side-to-side shifting of the weight on the forelegs (McDonnell, 2003) Box walking: repetitive tracing a route within the stable (Lesimple et al. 2020) Cribbing/Wind sucking: grasping of surface with incisors while arching the neck, repeated rhythmically in bouts (McDonnell, 2003) Head-shaking: Repetitive vertical head movements Ruet et al. 2020 Sénèque et al. 2019 Lesimple et al. 2020

Acknowledgment

This project was supported by the French Horse and Riding Institute (IFCE), France (Project "Box social"). This funding source had no role in study design, data collection or analysis, nor did they take part in preparation or decisions during the submission of the manuscript. We are grateful to Pierre Fontaine, Daniel Lagnaux, all the stud staff, Killian Martin for his advices on statistical methods and the trainees for their help during the experiment. We would also like to thank Joanna Lignot, professional English copy editor, for editing the manuscript.

Position

Description [START_REF] Ruet | Horses Could Perceive Riding Differently Depending on the Way They Express Poor Welfare in the Stable[END_REF] Behaviours collected in other publications Ears forward Both ears are oriented forward.

When recorded from behind, the inside of the auricle of both ears is completely invisible.

Locomotion

The horse walks from one point to another. Visser et al. 2008b[START_REF] Cooper | The effect of increasing visual horizons on stereotypic weaving: implications for the social housing of stabled horses[END_REF] Maintenance

The horse performs elimination behaviours (urinates, defecates) or comfort behaviours (rolls, shakes, scratches on a surface or grooms itself).

Lesimple et al.

Visser et al. 2008b

Observation

The horse scans its surroundings.

Lesimple et al.

Resting

The horse stands at rest (head slightly lowered, eyes partly or nearly closed, lips relaxed and often bearing weight on three legs) or lies down with its head up or with its legs and head outstretched [START_REF] Mcdonnell | The equid ethogram: a practical field guide to horse behavior[END_REF].

Cooper et al. 2000

Lesimple et al. Visser et al. 2008b[START_REF] Yarnell | Domesticated horses differ in their behavioural and physiological responses to isolated and group housing[END_REF] Standing still The horse is awake and stays immobile. [START_REF] Cooper | The effect of increasing visual horizons on stereotypic weaving: implications for the social housing of stabled horses[END_REF] Vocalisation -

Whinnying

The horse produces a loud, prolonged call, typically of 1 to 3 seconds, beginning high pitched and ending lower pitched. The head is elevated and the mouth slightly opened during the whinny (Mc Donnell, 2003).

Head tossing

The horse attempts to move the head in a quick forwardupward motion [START_REF] Minero | Behaviour and heart rate of therapeutic riding horses interacting with patients[END_REF] that is usually restricted by the reins held by the rider [START_REF] Waring | Horse behavior[END_REF].

König von Borstel &

Glißman 2014

Rearing

The horse suddenly changes its posture so that it stands only on its hindlegs [START_REF] Mcgreevy | Defining the terms and processes associated with equitation[END_REF].

Dyson & Pollard 2020

Snoring

The horse produces a short raspy inhalation sound [START_REF] Waring | Horse behavior[END_REF].