Marie Pelé
email: marie.pele@univ-catholille.fr

Gwendoline Thomas

Alaïs Liénard

Nagi Eguchi

Masaki Shimada

Cédric Sueur

I wanna draw like you: Inter-and intra-individual differences in orang-utan drawings

Keywords: Comparative cognition, scribbles, evolutive anthropology, art, aesthetics

Recently discovered, the oldest human abstract drawing is around 73,000 years. Although the origins of drawing behaviour have remained an enigma to this day, light may be shone on the subject through its study among our closest neighbours, the great apes. This study analyses 749 drawings of five female Bornean orang-utans (Pongo pygmaeus) at Tama Zoological Park in Japan. We searched for differences between individuals but also tried to identify possible temporal changes among the drawings of one individual, Molly, who drew almost 1,300 drawings from 2006 to 2016. A classical analysis of the drawings was carried out after collecting quantitative and qualitative variables. Our findings reveal evidence of differences in the drawing style of the five individuals as well as creative changes in Molly's drawing style throughout her lifetime. Individuals differed in terms of the colours used, the space they filled but also the shapes (fan patterns, circles or loops) they drew. Molly drew less and less as she grew older, and we found a significant difference between drawings produced in winter, when orang-utans were kept inside and had less activity, and those produced during other seasons. Our results suggest that the drawing behaviour of these five orangutans is not random and that differences among individuals might reflect differences of styles, states of mind but also motivation to draw. These novel results are a significant contribution to our understanding of how drawing behaviour evolved in hominids.

Introduction

The origins of drawing behaviour in Homo sapiens remain a subject of debate among researchers. The oldest human abstract drawing was recently discovered and is around 73,000 years old [START_REF] Henshilwood | An abstract drawing from the 73,000-yearold levels at Blombos Cave, South Africa[END_REF], and the oldest figurative drawing (depicting a wild pig) is around 45,000 years old [START_REF] Brumm | Oldest cave art found in Sulawesi[END_REF]. Given the fact that changes occur gradually in different species, it is interesting to investigate drawing behaviour in our neighbouring species in order to understand its origin. However, since the human species is limited to sapiens, the closest species for this study are the great apes, with whom we share many characteristics. But do these related species exhibit drawing behaviour? And if so, how? Drawing behaviour has been studied in non-human primate species including chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), orang-utans (Pongo pygmaeus), capuchin monkeys (Cebus apella) and rhesus macaques (Macaca mulatta) (for a review, see [START_REF] Martinet | Drawing in nonhuman primates: What we know and what remains to be investigated[END_REF]. Nowadays, drawing is increasingly proposed as an enrichment activity for captive great apes in zoological parks and research institutes. The monkeys and apes are free to use the material at their disposal, and are not constrained or conditioned to show this behaviour. This provides a perfect opportunity to collect drawings by non-human primates and allows comparative studies between hominids.

This study is based on 1,433 drawings recovered from five orang-utans (Pongo pygmaeus) at Tama Zoological Park in Japan, where caretakers regularly proposed drawing activity to the apes.

Orang-utans (Pongo sp.) are phylogenetically close to humans, with 96.96% of common genetic heritage [START_REF] Schwartz | The evolutionary relationships of man and orang-utans[END_REF]. This makes them ideal candidates to help us understand the origins of drawing. They demonstrate highly developed cognitive abilities [START_REF] Dufour | Calculated reciprocity after all: computation behind token transfers in orang-utans[END_REF]Damerius et al. 2019a) and tool use (Bardo et al., 2017;Mendes et al., 2007;[START_REF] Lethmate | Tool-using skills of orang-utans[END_REF], that are both at least comparable to those observed in chimpanzees. They are also well known for their curiosity (Damerius et al. 2019b). The wrists of orang-utans are more flexible than those of chimpanzees, making it easier for them to draw as they can bend their hands backwards [START_REF] Mackinnon | The behaviour and ecology of wild orang-utans (Pongo pygmaeus)[END_REF]. In our drawings database, the daily number of drawings varies significantly between individuals, showing that they had different levels of motivation to draw. This observation leads us to a first question: Do orang-utans show differences in their drawing/marking behaviour? More specifically, it is interesting to note that one female, called Molly, drew almost 1,300 drawings in her last five years of life. She had the opportunity to draw regularly from her 54th birthday onwards (in 2006, at Tama Zoo). Moreover, the colours and page filling in Molly's drawings might be affected by aspects of her daily life such as the identity of her caretaker and events in her environment [START_REF] Hanazuka | The Effects of the Environment on the Drawings of an Extraordinarily Productive Orangutan (Pongo pygmaeus) Artist[END_REF]. This leads us to a second question: Is there any temporal evolution in her drawing behaviour? Previous studies on captive orang-utans and other primates showed that they will continue to draw even in the absence of rewards [START_REF] Boysen | Simian scribbles: a reappraisal of drawing in the chimpanzee (Pan troglodytes)[END_REF][START_REF] Tanaka | Finger drawing by infant chimpanzees (Pan troglodytes)[END_REF][START_REF] Deloache | How very young children think about animals[END_REF][START_REF] Hanazuka | The Effects of the Environment on the Drawings of an Extraordinarily Productive Orangutan (Pongo pygmaeus) Artist[END_REF]. These findings are consistent with the Gestalt principle found in young children, which links the scribbling activity to a discovery of motor play activity [START_REF] Casti | Reaction to Stimulus Figures in Chimpanzee Drawings[END_REF]. Like in humans, spontaneous drawings indicate an intrinsic interest in exploratory and manipulative play for captive non-human primates [START_REF] Casti | Reaction to Stimulus Figures in Chimpanzee Drawings[END_REF][START_REF] Hanazuka | The Effects of the Environment on the Drawings of an Extraordinarily Productive Orangutan (Pongo pygmaeus) Artist[END_REF]. Moreover, when tracks have already been drawn on the paper, further scribblings are added, suggesting that visible tracks have some kind of reinforcing value [START_REF] Tanaka | Finger drawing by infant chimpanzees (Pan troglodytes)[END_REF]. [START_REF] Schiller | Figural preferences in the drawings of a chimpanzee[END_REF] worked with an 18-year-old female chimpanzee called Alpha, who showed a keen interest in drawing. In order to study figure formation and position, different stimuli (squares or scribbles) were drawn on paper by researchers. In terms of figure production, the results showed that Alpha mainly used two types of strokes: short dashes, and almost parallel broad zigzag strokes (also referred to as the fan pattern). Findings for the placement of figures reveal that when a single figure was positioned off-centre, Alpha drew in the largest open space, producing what Schiller calls a 'sort of balance between her markings and the presented figure'. A similar experiment by [START_REF] Morris | The biology of art: A study of the picture-making behaviour of the great apes and its relationship to human art[END_REF] with a one-year-old male chimpanzee named Congo showed similar results: three-quarters of the 40 free drawings on blank sheets of paper showed marks on spaces that had previously been empty, and half of the drawings featured marks that were concentrated at the centre of the paper. Although these results are only descriptive (like those found for Alpha), they do seem to support Schiller's findings concerning the strong tendency to mark a central figure and to position marks in the blank space opposite an offset figure, as well as the inclination to simply enjoy scribbling. [START_REF] Smith | Systematic study of chimpanzee drawing[END_REF] was the first to use quantitative methods in order to analyse chimpanzee drawings. All his results were consistent with the previously cited findings. Later, [START_REF] Boysen | Simian scribbles: a reappraisal of drawing in the chimpanzee (Pan troglodytes)[END_REF] continued the stimulus-drawing test in chimpanzees by presenting 18 different figures to three chimpanzees. Like [START_REF] Smith | Systematic study of chimpanzee drawing[END_REF], Boyson attained a point where the presence of any stimulus figure on the page elicited more centralised markings than in cases where the sheet of paper was blank. The notion of the centre of a page therefore seems to be an important point to take into consideration when studying the emergence of drawing capacities. Kinematic aspects could be seen as precursors of a graphic representation [START_REF] Levin | Emergence of representation in drawing: The relation between kinematic and referential aspects[END_REF]). Yet drawings by non-human primates include different types of marks, such as the straight lines, curves, loops or hook-like strokes observed in drawings by chimpanzees [START_REF] Tanaka | Finger drawing by infant chimpanzees (Pan troglodytes)[END_REF]. Although chimpanzees only develop the skill to use a mark-making instrument at the age of 20-23 months, these marks can be observed before the age of 13-23 months through the use of touch-screens. In a comparative study, a set of 396 pictures was collected, made up of 40 drawings by chimpanzees, 153 by gorillas, 146 by orang-utans, and 57 non-figurative drawings by children up to and including the age of four [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF]. [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF] noticed that the main features of orang-utans' patterns were diagonals, arcs, and curvilinear designs; those of gorillas contain mainly arcs and open curving lines, and a very high proportion of dots. In contrast, chimpanzees' drawings were mainly characterised by the use of straight and jointed lines. Unlike Tanaka, Zeller noted that orang-utans were the only apes that can use a closed loop or circular pattern [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF]. This is the most difficult pattern to produce, because it requires high levels of motor control. Another important point concerns the use of colours.

Findings in children have shown that their choice of colours reflected their emotional state at that time [START_REF] Crawford | Does children's colour use reflect the emotional content of their drawings?[END_REF], and it seems that this may be the case in individuals of other species, as with the case of the orang-utan called Molly [START_REF] Hanazuka | The Effects of the Environment on the Drawings of an Extraordinarily Productive Orangutan (Pongo pygmaeus) Artist[END_REF]. These results support the hypothesis that there is a choice in the use of colour, the type of strokes and the use of space, and that drawings 'do not result from a totally random scribbles' [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF][START_REF] Martinet | New indices to characterize drawing behavior in humans (Homo sapiens) and chimpanzees (Pan troglodytes)[END_REF]. Despite their nonfigurative nature, drawings produced by apes could therefore provide a great deal of meaningful information.

In this study, we analyse 790 drawings (selected from a total of 1,433 drawings) drawn by five orang-utans (Figure 1) and compare them at an inter-individual level. Indeed, a study of five chimpanzees by [START_REF] Morris | The biology of art: A study of the picture-making behaviour of the great apes and its relationship to human art[END_REF] suggests that inter-individual differences exist in drawing and that personality may have an impact on the way marks are distributed. We analyse drawings at an individual level in order to assess temporal changes in this behaviour. We expect to find preferences pertaining to the use of colours, as well as a trend of centering drawings, a use of curved strokes and circles inside the drawings, and possibly a number of differences in marking behaviour between individuals. We also expect possible temporal changes in the drawings of one individual, Molly, who drew almost 1,300 drawings from 2006 to 2011. To our knowledge, this is the first study analysing such a large number of drawings by orang-utans, and indeed by non-human primates in general.

Material & Methods

Subjects and collection of drawings

One thousand four hundred and thirty-three drawings by five female orang-utans at Tama Zoological Park in Tokyo were collected by the caretakers from 2006 to 2016 (Table 1). We analysed 749 drawings (26/26 drawings by Gypsy, 16/16 by Julie, 32/32 by Yuki, 60/60 by Kiki, and 656/1299 by Molly). White, high-quality paperboard (272 x 242 mm) and 16 different coloured crayons were proposed to the orang-utans as permanently available enrichment. Several paperboards were simultaneously provided to orang-utans to avoid competition, but no conflict was observed given the small number of drawings for four of the five individuals. The drawing activity, in the screened restroom, was not part of a research protocol implemented at the zoo but was for enrichment [START_REF] Sueur | Importance of living environment for the welfare of captive animals: behaviours and enrichment[END_REF]. The drawings were recovered for analysis after the drawing activities, meaning that we had no control over the methodology. Individuals did not provide the same number of drawings each year, suggesting different motivations between apes and over time. We pseudo-randomly selected 656 drawings by Molly to obtain approximately equal numbers for each three-month period from 2006 to 2011 (about 30 drawings for each of the 19 periods; Molly died in 2011).

Ethics note

The orang-utans were housed in social groups in outdoor and indoor enclosures with environmental enrichments. Food and water were supplied ad libitum. The Tama Zoological Park Ethics Board approved this non-invasive behavioural study, which complied with the Code of Ethics for Japanese Association of Zoos and Aquariums.

Data collection

To ensure that our observations were as accurate as possible, we used Gimp 2.10.22 software to apply a 10*10 grid [START_REF] Casti | Reaction to Stimulus Figures in Chimpanzee Drawings[END_REF]) to every drawing. Torn sheets were not analysed due to the risk of missing variables. Two observers (GT and AL) made the measurements with a correlation of 0.964≤

𝑟𝑟 ≤1 for all variables (Figure S1 in Supplementary Material). A third observer (MP) then confirmed the qualitative variable measurements.

Quantitative variables

We first looked for non-exclusive quantitative variables (Figure S2):

(1) the coverage rate, defined as the number of cells containing one or more strokes out of the total number of cells, (2) the overlap rate, defined as the number of cells containing strokes of at least two different colours that overlap, divided by the coverage rate and multiplied by 100, (3) the solid colour rate, defined as the number of drawing grid cells that were covered at rates of 50% or more, divided by 100, and (4) the distance to the centre, defined as the absolute distance between the centre of an ellipse surrounding the design and the centre of the grid. Three other indices were measured via Gimp 2.10.22 software, namely (5) the number of colours used, (6) the mean deviation of the colour spectrum and (7) the standard deviation of the colour spectrum.

Qualitative variables

We also extracted data from every drawing (8) for the predominant colour used by the individual and the drawings shapes, as defined in previous studies [START_REF] Kellogg | Analyzing children's art[END_REF][START_REF] Tanaka | Finger drawing by infant chimpanzees (Pan troglodytes)[END_REF]:

(9) fan patterns, (10) circles, (11) triangles and ( 12) loops (Figure S3). These indices are not exclusive, as several can be found on the same drawing. A fan pattern is a stroke making at least three round trips of angles ≤ 45°. A loop is a curved stroke forming a single distinct angle where it intersects itself.

A circle is a curved stroke intersecting in itself without distinct angles. A triangle is a flat loop with distinct angles.

Statistical analysis

We conducted two types of analyses: a comparative study between individuals and a longitudinal study of data for Molly. For both, we first checked for high correlations between our drawing variables using the chart.Correlation() function of the R package PerformanceAnalytics (Peterson and Carl 2020). Our correlation chart revealed no strong correlation between our quantitative variables (Figure S4). A principal component analysis (PCA) was then carried out with the R package FactoMineR [START_REF] Lê | FactoMineR: A Package for Multivariate Analysis[END_REF]) in order to reduce our 12 variables and group them into various dimensions that were then further interpreted from a biological perspective. A PCA with Varimax rotation was also carried out [START_REF] Kaiser | The varimax criterion for analytic rotation in factor analysis[END_REF] but did not explain more variance than classical PCA did.

For the comparative study between individuals, the coordinates of each drawing in each dimension were used to compare individuals two by two during comparisons of means for each dimension with the functions kruskal_test() and pairwise.wilcox.test() from the R packages coin [START_REF] Hothorn | Implementing a class of permutation tests: The coin package[END_REF] and stats (R Core Team 2020).

For the longitudinal study of data for Molly, we carried out another PCA. Here, we wanted to study the effect of both 3-month periods (N=19 periods) and seasons (N= 4 seasons) on Molly's drawing behaviour. We then applied a multifactorial linear model (LM) for each dimension of our PCA using the function lm() from the R package car [START_REF] Fox | An R Companion to Applied Regression, Third[END_REF]. The potential collinearity between our two predictor/predictive variables was tested by the calculation of the variance inflation factor VIF from the R package car. These diagnostics revealed a VIF of < 1.1 for both predictors, indicating that there was no notable problem of collinearity. p-values for LM were calculated via Monte Carlo sampling with 10,000 permutations, using the function PermTest() of the R package pgirmess [START_REF] Giraudoux | pgirmess: Spatial Analysis and Data Mining for Field Ecologists[END_REF]. Permutation tests for LM were well adjusted for moderate sample size and did not require normal distribution of model residuals [START_REF] Good | Permutation, parametric and bootstrap tests of hypotheses[END_REF].

Pairwise post hoc comparisons for significant LMs were carried out with the function pairwisePermutationTest() from the R package rcompanion [START_REF] Mangiafico | rcompanion: Functions to Support Extension Education Program Evaluation[END_REF]. α levels (0.05) were

Benjamini-Hochberg corrected. Finally, we analysed the main colour used by orang-utans with the function chisq.test() of the R package stats. We only reported only differences where p < 0.05. All statistical analyses were done with R, version 4.0.3 (R Core Team 2020).

Results

Comparative analyses between individuals

On average, the coverage rate of the drawings by the five orang-utans was 50±30%, the overlap rate was 20±30% and the solid colour rate was 10±20%. As regards the average number of colours used and shapes drawn, the five orang-utans used 3.0 ± 1.8 colours and drew 1.8 ± 2.0 fan patterns, 0.0 ± 0.2 circles, 0.2 ± 0.6 triangles, and 0.7 ± 1.2 loops per drawing. The colour spectrum had a mean of 0.8 ± 0.1 and a standard deviation of 0.1 ± 0.1. Finally, strokes were applied 33.7 ± 25.9 mm from the centre, on average. Variations of each metric per individual are presented in Figure S5.

The three dimensions retained in the PCA had an eigenvalue above 1 and described 63.5% of the explained variance of the dataset. Each metric showed a higher loading in one dimension than in the two others (Table 2, Figure 2). Thus, the first dimension of the PCA (eigenvalue = 3.86, variance = 35.2%) was mainly explained by the filling variables: the recovery rate, the overlap rate, the solid colour rate, the number of fan patterns, the number of colours and the distance to the centre. Colour variables applied to the second dimension (eigenvalue = 1.92, variance = 17.5%), namely the standard deviation and the mean of the colour spectrum. Finally, geometrical shapes were associated with the third dimension (eigenvalue = 1.19, variance = 10.8%), i.e. the number of triangles, loops, and circles. The arrow size indicates the correlation with the dimensions.

There were significant differences between the five individuals in the first dimension (Kruskal-Wallis: 𝜒𝜒 2 = 50.31, p < 0.001). Pairwise comparisons revealed that Molly had higher values than Gypsy and Kiki. Kiki also differed from Julie and Yuki (see pairwise statistics in Table S1, Figure 3a). There were significant differences between orang-utans in the second dimension (𝜒𝜒 2 = 150.48 p < 0.001). Pairwise comparisons revealed that Molly had significantly higher values than all other individuals. Julie had lower values than Kiki and Yuki (Table S1, Figure 3b). There were significant differences between individuals in the third dimension (𝜒𝜒 2 2 = 44.51, p < 0.001) with differences between Gypsy and Molly (Table S1, Figure 3c). Statistical analyses for each of the 12 metrics are given in the Supplementary Material section (Table S2- The main colour found in each drawing revealed a non-random and significant difference in the colours used between individuals (Figure S6). For Kiki (𝜒𝜒 2 = 62,647, p < 0.001) and Molly (𝜒𝜒 2 = 289.07, p < 0.001), the main colour in the drawings is green (representing 27.45% and 22.6% of the drawings, respectively). For Gypsy (𝜒𝜒 2 = 41.72, p < 0.001), Julie (𝜒𝜒 2 = 29, p = 0.002) and Yuki (𝜒𝜒 2 = 53.2, p < 0.001), the main colour in the drawings is red (representing 40%, 33.3% and 36.67% of the drawings, respectively).

Longitudinal changes in Molly's drawings

The number of drawings by Molly over the six years is shown in Figure S7. Three dimensions were retained in the PCA. Their sum represented 64.3% of the explained variance of the dataset (dimension 1: eigenvalue = 4.26, variance = 38.7%; dimension 2: eigenvalue = 1.60, variance = 14.5%; dimension 3: eigenvalue = 1.21, variance = 11%). Metrics are found in the same three dimensions for Molly as for the four other individuals (Table S5). The effect of 3-month periods and seasons on the three dimensions of the PCA were calculated. There was a significant effect of both seasons (linear model with permutation test: p < 0.0001) and periods (p < 0.0001) on the first dimension. The filling behaviour of drawings decreased with the periods (linear regression: t-value = -4.7, p<0.001, R² = 0.03, F=22.73, Figure 4a). Pairwise comparisons revealed that winter had significantly lower values than all other seasons (p<0.005, figure 4b). There was no significant effect of the seasons or of the periods on the second (p = 0.162 and p = 0.869 respectively) or on the third dimension (p = 0561 and p = 0.769 respectively). More precisely, Molly's drawings differed according to the seasons and time in terms of the coverage rate (p = 0.002 and p <0.0001 respectively), the overlap rate (p = 0.002 and p <0.0001), the solid colour rate (p <0.0001 and p <0.0001), the distance to the centre (p =0.04 and p =0.0002) and the fan patterns (p = 0.045 and p <0.0001), with the winter globally always being different from other seasons. However the number of colours used does not differ according to time (p=0.09) or the seasons (p=0.273). Although Dimension 2 did not show differences for the time and the period, the mean colorimetric profile showed differences according to the season (p=0.009) with higher values in winter (p<0.03) than other seasons, because Molly filled the paper sheet less in winter, thus making these drawings brighter. Dimension 3 did not show any differences according to time and period, but the number of loops decreased with time (p=0.02, z-score = -2.29) and was higher in summer (p<0.045) compared to other seasons. Other metrics did not show any difference according to time or seasons.

The analysis of the main colour qualitative variable revealed a non-random and a significant difference in the main colour used by Molly in the four seasons (Figure S8): winter (𝛸𝛸 2 = 100.16, p < 0.001), spring (𝛸𝛸 2 = 90,912, p < 0.001), summer (𝛸𝛸 2 = 87,385, p < 0.001), and autumn (𝛸𝛸 2 = 103.27, p < 0.001). Molly preferred green in summer and winter and swapped to pink for spring and autumn.

Purple is used more in spring compared to other seasons.

Discussion

We studied drawings by orang-utans in order to consider their cognitive capacities phylogenetically within the framework of other hominoids like humans or chimpanzees. In particular, we looked for differences in the drawing behaviour of five female orang-utans, one of which was in her last years of life. We found differences between individuals and observed temporal changes in Molly's drawings. The principal component analysis reduced the number of variables to three dimensions that had biological, cognitive meaning, namely the filling aspect, the colour or contrast aspect and the shape aspect.

Most of the time, orang-utans used several colours in their drawings. [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF] found that blue was the most commonly used colour used in the drawings of apes (including orang-utans) and children. The main colour used in our orang-utan's drawings differed between individuals. As orangutans see colours as we do [START_REF] Tigges | On color vision in gibbon and orang-utan[END_REF], this difference could reflect either an aesthetic or an emotional preference [START_REF] Mikellides | Colour psychology: The emotional effects of colour perception[END_REF]. In children, colours are linked to emotion [START_REF] Mikellides | Colour psychology: The emotional effects of colour perception[END_REF], including in their drawing [START_REF] Crawford | Does children's colour use reflect the emotional content of their drawings?[END_REF]). This link with emotion may have an evolutionary origin concerning mate choice and competition [START_REF] Rowland | Human colour in mate choice and competition[END_REF]. Molly, who usually used green and pink colours, mainly chose the colour red when another orang-utan was giving birth [START_REF] Hanazuka | The Effects of the Environment on the Drawings of an Extraordinarily Productive Orangutan (Pongo pygmaeus) Artist[END_REF]. All the individuals drew patterns that were described in Kellogg's children's scribbles classification [START_REF] Kellogg | Analyzing children's art[END_REF]. Like children, orang-utans can draw multiple lines (called fan patterns), loops and circles. Orang-utans have more dexterity than other great apes since they can draw curved lines, unlike other great apes [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF]. Orang-utans have also drawn triangles [START_REF] Vancatova | Creativity and innovative behaviour in primates on the example of picturemaking activity of apes. Creativity and innovative behaviour in primates on the example of picture-making activity of apes[END_REF]. Thus, in a way, orang-utans draw better than other non-human apes. The drawing technique used by orang-utans might also be interesting. Previous studies showed that orang-utans often hold the drawing tool between their fingers. However, one female was regularly observed laying the pastel on the sheet before rolling it with her hand [START_REF] Vancatova | Creativity and innovative behaviour in primates on the example of picturemaking activity of apes. Creativity and innovative behaviour in primates on the example of picture-making activity of apes[END_REF]. This behaviour could explain some large fan patterns observed in orang-utan drawings. In many studies, primates demonstrated an ability to draw fan patterns. We can cite the chimpanzees Congo and Bella, and the capuchin monkey Pablo [START_REF] Morris | The biology of art: A study of the picture-making behaviour of the great apes and its relationship to human art[END_REF]. Drawings by chimpanzees, human children and orang-utans are therefore comparable. This finding may enrich the phylogeny.

Our comparative study revealed differences in drawing behaviour among orang-utans, especially for the filling dimension. Here, it is not simply a question of one individual standing out from other individuals, but several differences observed between the five individuals: the drawings by Molly are the most complex (more amply filled than pictures by other individuals, with the use of more shapes and colours) followed by those drawn by Yuki. Kiki also shows differences to the other individuals with her simple but strongly marked drawing (i.e. one colour used, pressing hard on the pencil). We can attribute these differences to personality, motivation or even to different cognitive abilities between individuals. Studies in human children suggest that these interindividual differences might be due to varying levels of cognitive skill maturation [START_REF] Saito | The Origin of Representational Drawing: A Comparison of Human Children and Chimpanzees[END_REF]) and the different speeds at which children learn to draw [START_REF] Martinet | New indices to characterize drawing behavior in humans (Homo sapiens) and chimpanzees (Pan troglodytes)[END_REF]. According to [START_REF] Willats | Making Sense of Children's Drawings[END_REF], there may be an interrelation between a child's stage in their drawing skill development and their increasing comprehension of their living world. Our orang-utans have different life experiences. While Molly, who did more complex drawings, was born in the wild and had lived in two zoological parks and given birth 4 times, Kiki, who produced fewer complex drawings, was born in captivity and quickly moved to Tama Zoological Park where she gave birth to one baby. Molly had also lost her sight in one eye between 1993 and 1996 (personal communications from Mr. Kurotori, Tama zookeeper). These different experiences could perhaps explain the differences found in the way they drew. We can also mention the age difference between Molly, who was 54 years old at the beginning of her drawing period, and Kiki who was the youngest of the study group at just 10 years old. Kiki's minimalist use of colours and space in the paper was evidence of either her drawing style or a lack of motivation or interest in the drawing activity.

However, Kiki was not the least experienced of the individuals in terms of drawing, even if she was the youngest: she produced 60 drawings whereas 44-year-old orang-utan Julie produced just 16 drawings.

Data for Molly showed a higher mean and a lower standard deviation of the colour spectrum. This indicates a higher diversity in Molly's drawings, many of which are bright, and have lower levels of contrast. Indeed, although Molly did sometimes fill the sheet, particularly with fan patterns -as described for the chimpanzee Congo [START_REF] Morris | The biology of art: A study of the picture-making behaviour of the great apes and its relationship to human art[END_REF]) -many of her drawings were almost empty. Other orang-utans showed smaller patterns, which contrasted with the white background. Moreover, Molly seemed to press less on pencils than other individuals, which explains the lower contrast (lowest standard deviation of the colour spectrum) in comparison to data for other individuals. The drawings of our orang-utans (and especially those drawn by Molly) confirm the findings of [START_REF] Smith | Systematic study of chimpanzee drawing[END_REF], who was the first to report that chimpanzees tended to draw near the centre of the page. [START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF] confirmed this finding for other apes. The inter-individual differences we describe in orangutans drawing in terms of the way individuals draw suggest the existence of different personalities, as first suggested by [START_REF] Morris | The biology of art: A study of the picture-making behaviour of the great apes and its relationship to human art[END_REF]. The wide difference in the number of productions per individual also shows different levels of interest in the activity; this finding confirms previous observations in orangutans but also in chimpanzees [START_REF] Boysen | Simian scribbles: a reappraisal of drawing in the chimpanzee (Pan troglodytes)[END_REF][START_REF] Tanaka | Finger drawing by infant chimpanzees (Pan troglodytes)[END_REF][START_REF] Zeller | What's in a picture?' A comparison of drawings by apes and children[END_REF].

The longitudinal study then demonstrated that Molly's drawings evolved over time and between seasons. She used fewer colours, less space and drew further from the centre as the years progressed. At the end of her life, Molly had fat deposits above her eyes and had to use her hands to lift it out of the way, meaning that she could use her hands less to draw. She also became blind in her left eye, which diminished her visual field. Regarding the difference observed in winter, the results tend to show that Molly's frame of mind changed. The orang-utans in our study group do not go outside in winter, and the weather in Japan is very cold for them. They might become bored, and their motivation may decrease. In this way, our results show that Molly does not systematically use the same main colour in her drawings for all seasons: while she preferred purple as main colour in spring (in 23.4% of her drawings), she used it much less in summer (7.25%), autumn (3.27%) and winter

(5.45%). As Hanazuka and his colleagues (2019) hypothesised for non-human primates, drawings could be a window into the internal state of the orang-utan. Moreover, there are no visitors during winter.

Evidence that Molly draws more loops and changes the colour she uses in summer could be a cue indicating a good mood due to the weather and the presence of more visitors. Further research is needed to assess whether this number of loops is just the reflect of Molly's motivation or have a more cognitive aspect as symbolism (Gardner and Wolf 1987).

However, this study is limited by the enrichment origin of these drawings, which restrict the possibilities of controlled conditions. Several drawings were covered with stains and were dirty, with some shapes overlapping. It is difficult to be precise and objective when measuring variables such as the shapes present in the drawing or the predominant colour, especially when water had blurred the patterns. In the future, we could therefore use touch-screen tablets instead of paper [START_REF] Martinet | New indices to characterize drawing behavior in humans (Homo sapiens) and chimpanzees (Pan troglodytes)[END_REF]. The use of a tablet requires a genuine trust between the researcher and the animal, established from an early age so as not to put the researcher in danger [START_REF] Matsuzawa | The 40th anniversary of the Ai Project: the commemorative gift is a silk scarf painted by Ai the chimpanzee[END_REF]. Drawing on tablets has several advantages such as the extraction of more important information such as temporal data. This method will reveal differences between individuals in terms of order of colours used but also the fractality of the patterns [START_REF] Martinet | New indices to characterize drawing behavior in humans (Homo sapiens) and chimpanzees (Pan troglodytes)[END_REF]. It would also be interesting to study drawings by males to see if these preferences change according to sex, although this may be difficult as males have previously demonstrated less interest in this task [START_REF] Vancatova | Creativity and innovative behaviour in primates on the example of picturemaking activity of apes. Creativity and innovative behaviour in primates on the example of picture-making activity of apes[END_REF]. Another question pertains to the part of the body orang-utans use to draw. Indeed, some apes use their hands while others use their mouth [START_REF] Vancatova | Creativity and innovative behaviour in primates on the example of picturemaking activity of apes. Creativity and innovative behaviour in primates on the example of picture-making activity of apes[END_REF].

Overall, the fact that orang-utans draw freely without constraint shows that they are capable of doing so. It would be interesting to study these individuals further and also to study motheroffspring pairs in order to identify a possible transmission of the motivation or ability to draw. Cultural transmission of habits has been shown for example in chimpanzees [START_REF] Whiten | Cultures in chimpanzees[END_REF], and in orangutans [START_REF] Schaik Cp Van, Ancrenaz | Orangutan Cultures and the Evolution of Material Culture[END_REF]. This type of study would contribute to our understanding of the origins of drawings in humans.

Figure 1 :

 1 Figure 1: Examples of two drawings for each of the five orang-utans.

Figure 2 :

 2 Figure 2: Distribution of data (each point is a drawing) and of variables (each arrow is a variable) respectively for Dimension 1 and Dimension 2 (A., B.) and for Dimension 1 and Dimension 3 (C., D.).

  4, Figure S5). The metrics of Dimension 1 are all different between individuals (𝜒𝜒 2 2 > 11.53, p < 0.02) with data for Molly being globally different from other individuals (more recovery, more overlap, more colours, closer to the centre) and Kiki showing different results to those of Yuki. Indeed Kiki presented the lowest values for recovering, overlapping and colours whilst showing the second highest values after Molly. Although the metrics of the second dimension also revealed interindividual differences (𝜒𝜒 2 2 > 40.08, p < 0.001), the pairwise comparisons yielded more restricted results with only Molly showing very low values and Kiki showing very high values of standard deviation of the colorimetric profile. This indicates that Molly was filling the sheet andshowing less contrast, whilst Kiki's drawing showed high contrast due to the few but strong marks on the paper (see Figure1). Finally, the only difference between individuals in Dimension 3 concerned the number of loops (𝜒𝜒 2 2 = 25.07, p < 0.001), with Molly drawing more loops than Kiki.

Figure 3 :

 3 Figure 3: Boxplots showing the differences between individuals in each of the three drawing dimensions. Individuals having different letters show statistical differences.

Figure 4 :

 4 Figure 4: effect of a) seasons and b) time on Dimension 1 (coverage) of Molly's drawings

Table 1 .

 1 Information on individuals.

	Name	Date of Birth	Birthplace	Number of drawings	Percentage of drawings	Stay at Tama Zoo	Drawing period
	Molly 01/01/1952	Wild	1299	91 %	2005 -2011	2006 -2011
	Gypsy 01/01/1955	Wild	26	2%	1958 -2017	2007 -2014

Table 2 .

 2 Loadings of the metrics on the three PCA dimensions of our dataset (five orang-utans). Bold values indicate the dimension in which each variable is retained.

		Dim.1	Dim.2	Dim.3
	Coverage rate	0.84	0.34	-0.02
	Overlap rate	0.86	0.17	-0.15
	Number of colours	0.72	0.32	-0.26
	Fan patterns	0.76	-0.12	-0.05
	Circles	0.10	0.14	0.50
	Triangles	0.08	0.10	0.73
	Loops	0.33	0.31	0.50
	Colour mean	-0.39	0.79	-0.14
	Std. deviation of colour mean	0.38	-0.82	0.15
	Distance to centre	-0.61	-0.31	-0.02
	Solid colour rate	0.74	-0.37	-0.07

Acknowledgements

We are grateful to the Tama Zoological Park in Japan for the providence of orang-utans drawings. This project has received financial support from the CNRS through the MITI interdisciplinary programs and from a KAKENHI program.

Supplementary material for Interindividual and intraindividual differences in orangutans drawings

Marie Pelé 1 , Gwendoline Thomas 2 , Alaïs Liénard 2 , Shimada Masaki 3 , Cédric Sueur


Alessia Melelli

Darshil U Shah

Gemala Hapsari

Roberta Cortopassi

Sylvie Durand

Olivier Arnould

Vince Placet

Dominique Benazeth

Johnny Beaugrand

Frédéric Jamme

Alain Bourmaud
email: alain.bourmaud@univ-ubs.fr

Lessons on textile history and fibre durability from a 4,000-year-old Egyptian flax yarn

Flax has a long and fascinating history. This plant was domesticated around 8,000 bce 1 in the Fertile Crescent area 2 , first for its seeds and then for its fibres [START_REF] Van Zeist | Evidence for linseed cultivation before 6000 bc[END_REF][START_REF] Herbig | Flax for oil or fibre? Morphometric analysis of flax seeds and new aspects of flax cultivation in late Neolithic wetland settlements in southwest Germany[END_REF] . Although its uses existed long before domestication, residues of flax yarn dated 30,000 years ago have been found in the Caucasus area [START_REF] Kvavadze | 30,000-year-old wild flax fibers[END_REF] . However, Ancient Egypt laid the foundations for the cultivation of flax as a textile fibre crop [START_REF] Heer | Prehistoric culture of flax[END_REF] . Today flax fibres are used in high-value textiles and in natural actuators [START_REF] Le Duigou | Evaluation of force generation mechanisms in natural, passive hydraulic actuators[END_REF] or reinforcements in composite materials [START_REF] Mohanty | Composites from renewable and sustainable resources: challenges and innovations[END_REF] . Flax is therefore a bridge between ages and civilizations. For several decades, the development of non-or micro-destructive analysis techniques has led to numerous works on the conservation of ancient textiles. Non-destructive methods, such as optical microscopy [START_REF] Bruni | Analysis of an archaeological linen cloth: the shroud of Arquata[END_REF] or vibrational techniques [START_REF] Edwards | Preliminary study of the application of Fourier Transform Raman Spectroscopy to the analysis of degraded archaeological linen textiles[END_REF][START_REF] Kavkler | FTIR spectroscopy of biodegraded historical textiles[END_REF] , have been largely used to investigate archaeological textiles, principally to evaluate their degradation mechanisms and state of conservation. Vibrational spectroscopy studies can now benefit from synchrotron radiation [START_REF] Kavkler | Investigation of biodeteriorated historical textiles by conventional and synchrotron radiation FTIR spectroscopy[END_REF] and X-ray diffraction measurement in the archaeometric study of historical textiles [START_REF] Müller | Identification of ancient textile fibres from Khirbet Qumran caves using synchrotron radiation microbeam diffraction[END_REF][START_REF] Herrera | Identification of cellulose fibres belonging to Spanish cultural heritage using synchrotron high resolution X-ray diffraction[END_REF] . Conservation of mechanical performance and the ultrastructural differences between ancient and modern flax varieties have not been examined thus far. Here we examine the morphological, ultrastructural and mechanical characteristics of a yarn from an Egyptian mortuary linen dating from the early Middle Kingdom (Eleventh Dynasty, ca. 2033-1963 bce) and compare them with a modern flax yarn to assess the quality and durability of ancient flax fibres and relate these to their processing methods. Advanced microscopy techniques, such as nano-tomography, multiphoton excitation microscopy and atomic force microscopy were used. Our findings reveal the cultural know-how of this ancient civilization in producing high-fineness fibres, as well as the exceptional durability of flax, which is sometimes questioned, demonstrating their potential as reinforcements in high-technology composites.

The most beautiful fabric pieces of flax date from Ancient Egypt (Fig. 1), their highly preserved state a result of their optimal conservation over millennia in coffins or tombs with remarkably stable moisture and thermal conditions, as well as sheltering from UV light. Flax textiles were particularly prized by the Egyptians because of their comfort and the fineness of their fibres [START_REF] Weiss | The Neolithic southwest Asian founder crops: their biology and archaeobotany[END_REF] . Flax was widely used for clothing (Fig. 1a) and in the fishing sector for work clothes, felucca sails and nets. The funerary uses included mummy strips (Fig. 1b), funeral linen (Fig. 1c,d,g) as well as ornaments (Fig. 1e).

In terms of cultivation, the fertile Nile valley with its light and rich sandy soils was particularly suitable for flax. After growing, the stems were pulled out, as shown in illustrations found in the tomb of Sennedjem (Deir el Medineh, Egypt) and then probably water-retted. Over the past century or so, growth conditions have changed and varietal selection has significantly increased the crop's fibre yields 15 . It is therefore difficult to compare varieties across the ages. Even so, an in-depth study of 407 flax genotypes of different origins has shown that regions that have been the centre of origin of the crop, such as the Mediterranean or Abyssinia, highlight haplotypes that are more unique than the temperate group and are representative of oil-seed plants [START_REF] Sertse | The genetic structure of flax illustrates environmental and anthropogenic selections that gave rise to its eco-geographical adaptation[END_REF] . The flax found in the lake dwelling does not belong to the species now cultivated (Linum usitatissimum L) but to Linum angustifolium [START_REF] Braun | Plants of ancient Egypt[END_REF] , which is not cultivated at the present time. Today's cultivated flax Linum usitatissimum L. is considered as being domesticated from the wild progenitor pale flax Linum angustifolium Huds. Both have phenotypic characters of great heritability and are distinguishable by several characteristics, such as the length and width of petals, size of seeds, colour and shape of the flower, height of plants, but also the number of days until emergence from the soil or flowering [START_REF] Diederichsen | Variation of cultivated flax (Linum usitatissimum L. subsp. usitatissimum) and its wild progenitor pale flax (subsp. angustifolium (Huds.) Thell.)[END_REF] . However, the height of the plants shown on the Egyptian bas-reliefs [START_REF] Kawami | A craft in antiquity[END_REF] and the size of the seeds found during excavation [START_REF] Van Zeist | Evidence for linseed cultivation before 6000 bc[END_REF] suggest that the species cultivated by the Egyptians were morphologically close to those we know today.

Figure 2a,d compare the overall architecture, observed by scanning electron microscopy (SEM), of old (Fig. 1g and Supplementary Fig. 1a) and modern (Supplementary Fig. 1b) flax yarn, respectively. Despite a lower level of twist (about 180 turns per metre (tpm) against 320 tpm for the modern flax), the old flax possesses a similar metric number (about 122 tex or 8.2 km kg -1 ), showing the mastery of the Egyptians in manual spinning. Figure 2b reveals the level of individualization of the fibres. In the flax stem, fibres are aggregated in cohesive bundles made of several tens of fibres, the fibres being more or less divided after retting and extraction stages. The old yarn is mainly made up of elementary flax fibres; the residues of cortical parenchyma and middle lamellae are very few. This demonstrates the effectiveness of the water-retting process utilized at the time as Pliny the Elder explained [START_REF] André | Pline l'Ancien, Histoire naturelle[END_REF] . Water-retting enables homogeneous retting and, when it is well executed, enables the production of very fine fibres. One can notice that the low fibre yield in ancient flax varieties can also lead to easier retting and fibre division [START_REF] Goudenhooft | Varietal selection of flax over time: evolution of plant architecture related to influence on the mechanical properties of fibers[END_REF] . In modern flax fibre extraction processes, stems undergo field retting over several weeks. Dependent on natural weather conditions, this can lead to retting heterogeneity. As a consequence, numerous residues of pectic intermediate lamellae or cortical parenchyma are visible on the modern flax yarn (Fig. 2e,h). Such residues increase roughness of the yarn and are detrimental to the sensation of comfort (for example softness). This contrasts with the reputation of Egyptian flax fabrics, the most beautiful specimens of which were reserved for members of high society. These observations validate the important know-how of ancient Egyptians in textile manufacturing. The scanning electron micrographs (Fig. 2a,b) also reveal the excellent general conservation of the ancient fibrous yarns.

Figure 2g,h present cross-sections of the old and the modern yarn observed in nano-tomography and Fig. 2i illustrates the analysed distribution of elementary fibre diameters for the two materials. The mean diameters were 14.3 ± 3.3 μm for the fibres in the old yarn (n = 523) and 17.6 ± 3.6 μm for the modern yarn (n = 208); both diameter values are consistent with typically reported values on flax fibres [START_REF] Baley | Average tensile properties of French elementary flax fibers[END_REF] . A significant difference in elementary fibre diameters was observed and confirmed by a Student's t-test with P ≤ 0.001. The smaller diameters of old flax may be related to the plant variety, the weather conditions during growth (hydric stress, for example) [START_REF] Chemikosova | The effect of soil drought on the phloem fiber development in long-fiber flax[END_REF] and/or even the sampling area within the stem, with larger diameter fibres being generally located in the middle section of the stem height [START_REF] Bourmaud | Influence of the morphology characters of the stem on the lodging resistance of Marylin flax[END_REF] . The retting method utilized may also explain the differences and scatter in elementary fibre diameters between the old and the modern flax yarn (Fig. 2i). The use of water-retting for the old flax yarn leads to completely separated fibres that are free from surface residues (Fig. 2b). This further demonstrates the skill of ancient Egyptians in obtaining fine yarns and textiles.

Differences are also visible in the size of the lumens, with SEM and tomographic images (Fig. 2c,f-h) showing larger lumens for old flax fibres. While the lumens of modern flax fibres represent only a few percent of the total surface area [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF] , old fibres studied here possess lumens of the order of 30-40%, comparable to wood or coconut fibres [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF] . We hypothesize that the low wall thickness of old flax may be due to a premature halt in the cellulose filling process of the cell walls following the intrusive growth phase [START_REF] Bourmaud | Influence of the morphology characters of the stem on the lodging resistance of Marylin flax[END_REF] . This filling can be interrupted by extreme weather conditions, such as lodging or marked periods of hydric stress [START_REF] Goudenhooft | Study of plant gravitropic response: exploring the influence of lodging and recovery on the mechanical performances of flax fibers[END_REF] .

Flax fibres are characterized by their multilayered structure, their generally polygonal shape and the presence of structural defects known as kink-bands 27 distributed along the fibre length. Notably, the relative quantity and size of these kink-bands are particularly large on old fibres (Figs. 2b and3a,b) in comparison with modern fibres (Fig. 3d,e). The origin of these defects is not well known, but the plant fibre community generally attributes them to mechanical stresses induced during the extraction or processing of the stems, as well as residual stresses that may be released during periods of stem or fibre drying, possibly during the retting stage [START_REF] Hernandez-Estrada | Assessing the susceptibility of hemp fibre to the formation of dislocations during processing[END_REF][START_REF] Hughes | Defects in natural fibres: their origin, characteristics and implications for natural fibre-reinforced composites[END_REF] . The large quantity of kink-bands on old flax fibres may be the result of aggressive decortication, scutching or spinning processes used by the Egyptians following water-retting, but may also be caused by progressive release of internal stresses over the four millennia. In flax fibres, kink-bands modify the aesthetics and regularity of the fibres, and are also considered as zones of weakness, especially when utilized in a fibre-reinforced composite [START_REF] Duc | Polypropylene/natural fibres composites: analysis of fibre dimensions after compounding and observations of fibre rupture by rheo-optics[END_REF] . Kink-bands also make the fibre more susceptible and sensitive to ageing by acting as entry points for microorganisms or moisture to access the inner layers of the cell walls [START_REF] Foulk | Influence of pectinolytic enzymes on retting effectiveness and resultant fiber properties[END_REF] .

We specifically examined the kink-bands through multiphoton microscopy with second harmonic generation (SHG) imaging, which highlighted crystalline cellulose within the plant cell walls. Figure 3c shows discontinuity and disorganization of crystalline cellulose in the kink-band of old flax, which possibly indicate areas of low crystallinity in this region. Both these factors would cause kink-band-rich ancient flax fibres to be more brittle [START_REF] Herrera | Identification of cellulose fibres belonging to Spanish cultural heritage using synchrotron high resolution X-ray diffraction[END_REF] . Indeed, these old fibres have proved to be very fragile during handling, and impossible to isolate without breaking/damaging them for any single-fibre tensile testing. Finally, atomic force microscopy tests in peak force quantitative nano-mechanical (AFM-PF-QNM) mode were conducted on transverse cross-sections of old and modern flax fibres. Such measurements (Fig. 4) allow estimation of the indentation modulus of flax plant cell walls, that is, they do not depend on the relative lumen size and are a useful measure for highlighting mechanical property gradients or heterogeneities within cell walls [START_REF] Eder | Experimental micromechanical characterisation of wood cell walls[END_REF] . Interestingly, we found that the AFM mechanical properties are slightly higher for cell walls of old flax (23.7 ± 0.2 GPa) than those of modern flax (20.3 ± 0.1 GPa); for each batch, 2,500 indentation modulus values were statistically compared and the Student's t-test confirmed that the two sets of moduli are different, with P ≤ 0.001. Values for modern flax are in line with measurements in the literature [START_REF] Bourmaud | Towards the design of high-performance plant fibre composites[END_REF] as measured by nanoindentation (Supplementary Table 1). Measurements by infrared spectroscopy (Supplementary Fig. 1 andTable 2) revealed a lower intensity of peaks attributed to parietal hemicelluloses in old flax. It has been shown that the longitudinal and transverse shear moduli of the fibres [START_REF] Jäger | The relation between indentation modulus, microfibril angle, and elastic properties of wood cell walls[END_REF] and especially the stiffness of the non-cellulosic matrix of the plant cell walls [START_REF] Capron | Mechanical characterization of developing tension wood fibre wall by atomic force microscopy[END_REF] have a major effect on the indentation modulus; our results confirm this important influence of hemicelluloses on the indentation modulus, even though hemicelluloses are the softest component of the cell wall. Higher indentation moduli have previously been recorded on old wood samples and are attributed to a loss of pectins, as well as modification of the ligno-cellulosic cell wall polymers [START_REF] Bader | A nano to macroscale study on structure-mechanics relationships of archaeological oak[END_REF] . The wider literature supports the hypothesis of a significant evolution in ageing sensitive hemicellulosic polymers over 4,000 years old. Differences in crystallinity were also checked through both nuclear magnetic resonance (NMR) and X-Ray diffraction (XRD) measurements; Supplementary Fig. 2 shows that the cellulose crystallinity measured by both NMR (58.0% for the old Egyptian yarn and 56.0% for the modern yarn) and XRD (59.6% for Egyptian yarn and 58.4% for modern yarn) techniques was comparable between the ancient and the modern flax yarns. Moreover, the measured indentation moduli were homogeneous in the fibre sections and show little dispersion, suggesting no ageing gradient across a fibre transverse section. Such quantitative nano-structural measurements, never before conducted on such ancient fibres, reveal the durability of these flax plant cell walls. Even though at the fibre-scale, the kink-bands are regions of pronounced damage, the cell walls themselves exhibited a moderate change in their elastic performance despite their age; only a slight increase in their stiffness, connected to the evolution of their non-cellulosic polysaccharide composition, was demonstrated. Our structural examination of 4,000-year-old Egyptian flax fibres in comparison with modern flax fibres has offered several insights on the textile know-how of the Egyptians, as well as on the temporal evolution of flax fibres. Through water-retting and manual processing, the ancient Egyptians could separate the flax into very fine fibre bundles and in most cases, even into single fibres to make soft and luxurious quality textiles despite fully manual processing. Local nano-mechanical measurements show an increase in cell wall stiffness in old fibres, probably induced by the alteration of non-cellulosic polymers, as cellulose retained a crystallinity close to that of contemporary fibres. In addition, a larger presence of structural defects -stress-concentrating kink-bands with low cellulose crystallinity -is notable on the old, fragile fibres. In future and in current work, we aim to go further by exploring the microfibrils angle values of ancient flax (through single-fibre XRD and SHG), the internal structure of kink-bands (by nano-tomography) and if possible, to gain information on the Linum used by ancient Egyptians through genetic analysis. To improve durability at the fibre scale, producing fibres with low quantities of defects is necessary, in particular if they are to be used as reinforcements of next-generation environmentally friendly composite materials. Intriguingly, the ancient Egyptians had also dabbed their hands in making the first linen/plaster cartonnage biocomposites for death masks, a number of which survive to date (Fig. 1b).

Methods

Materials. Two samples of flax yarns were studied (Supplementary Fig. 3), an ancient and a contemporary sample, referred to as old and modern flax, respectively. The large linen tabby, bordered with a fringe (inv. E 13595, Supplementary Fig. 3a) was given in 1929 by Georges Daressy, former General Secretary of the Antiquity Service in Egypt, to the Louvre Museum. Its provenance is unknown, but this piece of shroud most probably came from a tomb because all the textiles of ancient Egypt were found in cemeteries. These cemeteries were located in the desert to ensure dryness and optimal conservation of the burials. Indeed, in the valley, the annual flooding of the Nile was too risky. Thus, the Egyptian climate of the desert areas, which was exceptionally dry and favourable to the proper conservation of organic materials, made it possible to find many fabrics in excellent condition. The linen was radiocarbon dated in 2009 (Laboratoire de Mesure du Carbone 14, CEA-Saclay, France): it had been collected between 2140 and 1976 bc (with 95.4% probability), during the 9th, 10th or 11th dynasties, a period known as the First Intermediate Period and the beginning of the Middle Egyptian Kingdom. Morphological characteristics of the ancient yarn were calculated from mass measurements and from image analysis. The linear density and twist of this old yarn are 122 tex and 180 tpm, respectively. In addition, a contemporary yarn was used (Supplementary Fig. 3b). It was produced from textile flax (Melina variety) cultivated in 2018 in Normandy (France) by Teillage Saint-Martin company; this flax was dew-retted conventionally over six weeks and then scutched and hackled (Bourmaud et al. 2018 PMS). Then, it was wet spun by Safilin Pionki (Poland), with a linear density and twist of 105 tex and 320 tpm, respectively. SEM observations. For each of the two yarns, a sample of a few millimetres was used. A Jeol JSM 6460LV scanning electron microscope was used to analyse the flax yarns; secondary emission electrons were used and the accelerating voltage was 3.0 kV. The yarn samples were glued to a sample holder using a conductive adhesive and then metallized with a thin layer of gold using an Edwards Scancoat Six device for 180 s.

Multiphoton microscopy. Preparation of samples. An elementary fibre was extracted from the modern flax yarn and mounted on paper support commonly used for tensile tests according to ASTM C1557 and fixed with universal glue. The sample prepared was placed between two coverslips and scanned. In contrast, the preservation state of the Egyptian yarn did not allow the extraction of elementary fibres, which were more brittle, so a whole collective of less than 1 cm was mounted on paper support commonly used for tensile tests but glued in the horizontal direction to use the aperture of 5 mm. The sample prepared was placed between two coverslips and scanned by the multiphoton microscope. The samples were mounted at 90° to the initial laser polarization position. SHG microscopy imaging. SHG imaging was performed with a multiphoton Nikon A1 MP+ microscope (NIKON) equipped with a long working distance 16× (NA 0.80) water immersion objective (NIKON). The system is equipped with a tunable Mai Tai XF mode-locked Ti:sapphire femtosecond laser (SPECTRA PHYSICS) and a half-wave plate (MKS) in front of the laser excitation beam. The half-wave plate was rotated to change the laser polarization angle to reach the maximum intensity SHG signal of both flax yarns (maximum signal reached 2-3°). The excitation wavelength chosen was 810 nm (average power at 1.5 W) to obtain the maximum performance from the equipped filters (SH collected with a bandpass filter at 406/15 nm), and the maximum laser power percentage used was 2% for the Egypt yarn and 5% for the modern yarn to avoid bleaching of the surface. We collected both autofluorescence and SHG signals by GaAsP NDD (gallium arsenide non-descanned) detectors. The scan line average was 16, the scan velocity was fixed at 1 (fps) and the scan size was 512 × 512 pixels. All the measurements were performed at room temperature and dry ambient conditions. X-ray tomography measurements. The yarns' microstructure was characterized using X-ray nano-tomography. Image acquisition was realized on an EasyTom micro/nano-tomograph (RX Solutions). A Lanthanum hexaboride (LaB6) filament was used as cathode with a voltage of 50 keV and a current of 100 µA, leading to a resolution of 0.5 μm. The anode was in beryllium and has a thickness of 0.5 mm. Resolution of the micro-CT images was set to 4.44 μm per pixel.

The imager used was a fluoroscopic high speed imaging sub-system PaxScan 2520DX and the scintillator was produced with a direct deposition of cesium iodide. To obtain optimum measurement contrast, the framerate was set to 0.25 fps. In addition, to minimize the measurement noise, each projection obtained was the result of the averaging of 15 acquisitions. Finally, to obtain the most faithful reconstruction possible, the flax fibres were measured in 1,440 different positions (angles). Yarn centering was carried out using a perforated carbon tube. The tube outside diameter was 1 mm and the inside hole diameter was 0.5 mm. A little bit of glue was used to maintain the yarns. The measured volume was 0.5 mm in diameter over a height of 0.8 mm, with a resolution of 500 nm. To allow maximum beam stability from the start of the measurement, the wire was preheated 3 h before the start of the measurement. In total, each measurement therefore lasted 1).

approximately 27 h. An X-ray radiograph is given in Supplementary Fig. 4a to illustrate the measurement (the yarn is hardly perceptible). The reconstruction was carried out with Xact software using the filtered back-projection method. For noise filtering, the apodization was done using a sine window with a threshold of 75% for the low-pass filter. For the border filter, a Tukey window type was used with a non-filtered area of 46%. For more information on filters and the effects of reconstruction filter on cone beam computed tomography (CBCT) image quality, see ref. [START_REF] Shaw | Cone Beam Computed Tomography[END_REF] . Once the reconstruction had been carried out, the result was stored in the form of a slices stack. An illustration is given in Supplementary Fig. 4b. Finally, the analyses and reconstruction of surfaces were carried out using the VGSTUDIO MAX software as illustrated in Supplementary Figs. 5a,b at two different scales.

Nano-mechanical investigations. Preparation of samples. A subsample of less than 1 cm was cut from the ancient flax yarn (Louvre) and modern flax yarn samples. The two subsamples were put in an oven at 60 °C for 2 h and then embedded in Agar resin (epoxy resin Agar Low Viscosity Resin, Agar Scientific). The blocks prepared were put back in the oven at 60 °C overnight for the final polymerization of the resin, then machined to reduce their cross-section and glued on a 12 mm AFM stainless steel mounting disk. The sample surface was then cut using an ultramicrotome (Ultracut S, Leica Microsystems) equipped with diamond knives (Histo and Ultra AFM, Diatome) to obtain thin sections (about 50 nm thick in the last step) at reduced cutting speed (~1 mm s -1 ) to minimize compression and sample deformation during the cutting process, and thus reduce the sample surface damage and topography (Supplementary Fig. 6).

AFM PF-QNM investigations. A Multimode 8 Atomic Force microscope (Bruker) was equipped with a RTESPA-525 probe (Bruker) with a nominal spring constant of 200 N m -1 and a resonance frequency of 525 kHz. The actual spring constant was calculated with the Sader Method (https://sadermethod.org/). The AFM set-up was calibrated with the relative method using sapphire as hard standard material to calculate the deflection sensitivity and the PF-QNM synchronization distance. A sample of aramid fibres K305 Kevlar Taffetas (305 g m -2 , Sicomin Epoxy Systems) accurately prepared in blocks of Agar resin (Agar Low Viscosity Resin, Agar Scientific), with the surface prepared using the same protocol as that of the flax samples, was previously tested by nanoindentation and then used to calibrate the tip radius (aramid fibre ~24.3 GPa and embedded resin ~5.4 GPa). The range of stiffness of the cantilevers used was between 109 and 161 N m -1 and the tip radius between 15 and 30 nm at the beginning of the measurements. The fast scan axis angle was 90°, the maximum of the peak force setpoint used was 200 nN, the oscillation frequency selected at 2 kHz and the Poisson's ratio used was set to 0 as the tested cell walls are anisotropic, thus the modulus measured is the indentation modulus. The maximum fast scan velocity was selected at 8 µm s -1 and the image resolution set to 512 × 512 pixels. The gain was set in automatic mode.

Two to three different areas of each sample (old and modern, respectively) were measured to obtain a better statistic, but only one representative area for each sample is reported here (Fig. 4 and Supplementary Figs. 6 and7). Supplementary Fig. 6 shows the topography images corresponding to investigated areas of Fig. 4. To obtain the indentation modulus values, the entire surface of the secondary S 2 (or G) wall of each fibre was selected; indentation modulus data were automatically calculated for each point from the force-distance curves with a Derjaguin-Muller-Toporov (DMT) contact model using NanoScope Analysis software (Bruker).

Consequently, for each sample, the indentation moduli calculated were obtained from two or three separate images and from between 80,000 and 140,000 points for each image analysed using Gwyddion free software (http://gwyddion. net/). Supplementary Fig. 7 shows the calculation mask, covering the investigated area for fibres in Fig. 4. For each sample, histograms in Fig. 4 represent the data obtained from all the images analysed; 206,613 and 364,575 AFM force curves were used for old and modern flax indentation modulus calculation, respectively. Statistical analysis. A Student's t-test was performed to quantify the statistical differences in fibre diameters and indentation modulus values between the old and the modern fibres. A P value was calculated for the two cases, with significance level α = 0.05.

Reporting Summary. Further information on research design is available in the Nature Research Reporting Summary linked to this article.

Fig. 1 |

 1 Fig. 1 | Examples of the uses of flax in ancient Egypt. a, Child's vest with dyed blue edges, 800-720 or 700-540 bce. b, Mummy of man with agglomerated and stuccoed flax fabric, 332-30 bce. c, Flax hypocephalus, 305-30 bce. d, Fragment of flax shroud, 1550-1295 bce. e, Hairnet cap, fifth or sixth century ad. f, Unspun flax hank, 1420-1230 bce. g, Mortuary linen, 2140-1976 bce. Objects in c and d are exposed at the British Museum (London, UK); b is exposed and a, e, f and g are in the store room at Le Louvre Museum (Paris, France). All images are from the authors' personal collection. Images of objects in a, e, f and g were obtained with the specific permission of Le Louvre Museum.

Fig. 2 |

 2 Fig. 2 | SEM and nano-tomography images of modern and 4,000-year-old flax. a-h, Overview of yarn (a,d) and fibres within the yarns (b,e); tomographic overview of fibres highlighting the larger lumen size of old flax (c,f) and tomographic yarn cross-section showing the smaller diameter of old flax fibres(g,h). i, Histograms of the distribution of single-fibre diameters for both old and modern flax. For SEM, at least five areas were investigated and the most representative images selected for publication; nano-tomographic images were acquired from a measured volume of 0.5 mm in diameter over a height of 0.8 mm and each projection obtained was the result of the averaging of 15 acquisitions. More images and raw data are available at https://doi. org/10.17863/CAM.72394.

Fig. 3 |

 3 Fig. 3 | Focus on kink-band (defect) regions in the fibres. a-f, SEM images showing differences between kink-band structure and intensity in old (a,b) and modern (d,e) flax. SHG microscopy observations highlight the local disorganization of cellulose macrofibrils in the kink-band region for old flax (c) compared with modern flax (f). In c and f, kink band regions are circled by the dotted white line. For SEM and SHG, at least five areas were investigated and the most representative images selected for publication. More SEM images and raw SHG data are available at https://doi.org/10.17863/CAM.72394.

Fig. 4 |

 4 Fig. 4 | AFM peak force measurements in old and modern flax fibres. a,b, One can observe larger lumen size on old flax fibre (a) and residue of cortical parenchyma on modern flax (b). c,d, Profile of indentation moduli in old (c) and modern (d) flax based on the position on the white line in a and b. e, Distributions of indentation moduli, which are in good agreement with the preliminary nanoindentation tests performed (Supplementary Table1).

Acknowledgements

V.P. and G.H. sincerely thank P. Malécot and the MIFHySTO research platform (FEMTO-ST, UTINAM and ICB institutes) at Université Bourgogne Franche-Comté (UBFC) for the technical and scientific support provided for nano-tomography experiments; X. Falourd and L. Foucat (INRAE, BIBS platform) for NMR investigations. We thank the INTERREG IV Cross Channel programme for funding this work through the FLOWER project (grant no. 23); SOLEIL Synchrotron for funding the 99180266 and 99200015 in-house proposals; and the EIPHI Graduate school (contract "ANR-17-EURE-0002").

Data availability

The data that support the plots within this paper and the findings of this work are available from the corresponding author and at the following address: https://doi. org/10.17863/CAM.72394. Source data are provided with this paper.

Code availability

The open-source and commercial software used for data analysis are referenced in the Methods section.

Author contributions

Competing interests

The authors declare no competing interests.


H Beaugendre
email: heloise.beaugendre@math.u-bordeaux1.fr

A Ern
email: ern@cermics.enpc.fr

S Huberson
email: serge.huberson@univ-poitiers.fr

Combined finite element -particles discretisation for simulation of transport-dispersion in porous media

Introduction

Combining finite element together with particle methods provide one of the best compromise for solving transport problem in porous media. Saturated or non-saturated flows are determined by boundary condition and the media permeability. 4 For real terrain, permeability can consist in various almost constant and imbricated zones with complex shapes. Thus, it is of some interest that the boundary between two adjacent zones coincides with a natural mesh interface and that each element is entirely contains in one such zone. Beside this, solving transport equation by means of particle methods offers two distinctive advantages. The method is unconditionally stable when applied to a pure convective equation, and it does not contain any numerical diffusion if the particle trajectories are correctly computed. Therefore the combination of finite elements and particle method appears to be a straightforward application of the principle : "the right method at the right place".

Although the previous statement provide a consistent basis to build a numerical model, there still remain some options in the choice of the two components themself. To start with, it has long been recognised that the computed flow must satisfied as much as possible the divergence free condition; this can be achieved by selecting a non-conforming or mixed method. Second, there exist many way to design particles methods for the convective part as well as for the dispersion term. For the first one, a so-called streamline method can be used as an alternative to the more classical time integration. For the last one, there is a profusion of model including random walk, particle mesh, particle strength exchange and dispersion velocity among others. Our purpose is to compare some of these different strategies in order to provide as clear as possible criteria to be used when designing a solver. Three different points will be successively addressed hereafter : the finite element scheme, the particle trajectories computation and the dispersion simulation.

Finite element flow computation

The first point was adressed by considering two finite element schemes to approximate the flow, the usual conforming scheme and a non-conforming scheme [START_REF] Beaugendre | Finite volume box scheme for a certain class of nonlinear conservative laws in mixed form[END_REF]). The latter is quite similar to the more usual mixed hybrid finite element method: it uses one degree of freedom per mesh face and produces a discrete flow field with continuous normal component. The difference with the mixed hybrid finite element approach is that the present scheme can be interpreted as a finite volume box scheme where the mean of governing equations is considered elementwise. Figure 1 shows an example of computed potential flow for the lense test-case.

The streamlines method

The trajectories computation was based on the flow computed with the previous finite element method. This is a two steps procedure. First the location of the particle on the finite element mesh have to be determined, second the trajectories of the particle across the resulting element has to be computed. The first step was achieved by superimposing a regular cartesian grid to the finite element mesh. The cartesian grid cells are selected as small as possible so that a large number of cells cover one single element. Therefore, a particle contains in one grid cell is usually contained by no more than one element. The trajectory computation was performed by using two alternative procedures. The first one consists in a numerical time integration of the differential equation dX/dt = U as usual in particles method. The second consist in using the polynomial form of the velocity field on each element to compute the local streamlines and then the particle trajectory across this element. Associated to this calculation is a time interval corresponding to the particle sojourn within this element also called flight-time. The whole procedure constitutes the streamline method [START_REF] Matringe | Robust streamline tracing for the simulation of porous media flow on general triangular and quadrilateral grids[END_REF]).

The resulting method was applied to different test-cases. On figure 2, we present the lense test case streamlines computed from the potential obtained with the conforming and non conforming method. Three different meshes were used in order to point out the convergence of the two method. It was observed that the non-conforming scheme always provide the best streamlines pattern.

The second test case -figure 4 corresponds to a similar configuration, but uses a fully unstructured mesh. It can be observed from the computed streamlines compared to that of figure 2 that the method still works in this case.

Dispersion

The dispersion simulation can be based on many different methods. In the present work, two methods were particularly investigated : the diffusion velocity method and the particle strength exchange (PSE) method. The first one was selected because it yields modified streamlines, keeping constant the weight associated to each particles weither the second keep the streamlines unchanged and only modifies the particles weight. The first method was assumed to be in agreement with the streamline method concept. It consists in an algebraic manipulation of the original convection-dispersion equation in order to obtain a pure transport equation where the velocity consist in two parts, the original velocity component computed with finite elements and a Fig. 7. velocity dispersion : selection of eight particles modified trajectories. dashlines : original streamlines, continuous lines : modified streamlines dispersion component which is proportional to the ratio of the gradient by local value of the transported quantity. The method was found to work well in a previous study [START_REF] Beaudoin | Simulation of anisotropic diffusion by means of a diffusion velocity method[END_REF] and can be easily combined with any of the two procedures used for the computation of the trajectories. The implementation of the PSE method reduces to the addition of the computation of the particles weight at each time step. It has to be noticed that both method necessitates to have a common time-stepping for all the particle which was not the case when only streamlines were computed.

Eventually, it was found that the PSE metod was the best candidate for extending the streamline method to the case of dispersive flows. The possibility to display initialy the particles along pre-computed streamlines enables to reduce the additional computational work to the particle weights. The application of the velocity diffusion do not permit to keep the same streamlines set for all the computation and, therefore, was found much more CPU-time consuming. It can be obviously guessed that the same drawback definitely plague the application of the Monte-Carlo simulation of dispersion.

Fig. 1 .

 1 Fig. 1. a) Schematic representation of the domain for the lense test case; b) Flow potential distribution.

Fig. 2 .

 2 Fig. 2. Comparison of the computed trajectories with non-conservative (left) and conservative (right) flow fields. From top to bottom, finer meshes are used.

Fig. 3 .

 3 Fig. 3. a) Schematic representation of the domain for the multi-conductivity test case; b) Flow potential distribution.

Fig. 4 .

 4 Fig. 4. Computed trajectories for the non-conservative (left) and conservative (right) flow fields.

Fig. 5 .

 5 Fig. 5. PSE dispersion. The square indicate the initial and final location of the particle set

This work was partially supported by GDR MOMAS-CNRS