François Druelle
email: françoisdruelle@mnhn.fr

Jonathan Özçelebi

François Marchal

Gilles Berillon

Development of bipedal walking in olive baboons, Papio anubis : A kinematic analysis

Keywords: Balance, Bipedalism, Coordination, Evolution, Ontogeny

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INTRODUCTION

Humans can be defined as habitually bipedal walkers and runners (e.g., [START_REF] Bramble | Endurance running and the evolution of Homo[END_REF][START_REF] Harcourt-Smith | The origins of bipedal locomotion[END_REF][START_REF] Holowka | Rethinking the evolution of the human foot: insights from experimental research[END_REF]. As only two feet contact the ground during their locomotion, keeping balance is a challenge, and as loading is only shared between the two legs, musculoskeletal stress is high. During development, children gradually acquire independent walking (e.g., [START_REF] Assaiante | Development of locomotor balance control in healthy children[END_REF][START_REF] Hallemans | Pressure distribution patterns under the feet of new walkers: the first two months of independent walking[END_REF][START_REF] Ledebt | Acquisition of upper body stability during walking in toddlers[END_REF][START_REF] Van Dam | A cross-sectional study about the relationship between morphology and step-time parameters in children between 15 and 36 months[END_REF][START_REF] Van Dam | A cross-sectional study about the relationship between morphology and kinematic parameters in children between 15 and 36 months[END_REF] and their walking skills improve by repeatedly experiencing this behaviour [START_REF] Adolph | How do you learn to walk? Thousands of steps and dozens of falls per day[END_REF]. In contrast to humans, extant non-human primates are not habitual bipeds. They commonly develop a set of locomotor skills that can be summarized into their positional repertoire (including postural and locomotor modes; [START_REF] Hunt | Standardized descriptions of primate locomotor and postural modes[END_REF]. The locomotor skills of extant non-human primates commonly take somewhere between months and years to mature (e.g., [START_REF] Doran | The ontogeny of chimpanzee and pygmy chimpanzee locomotor behavior: a case study of paedomorphism and its behavioral correlates[END_REF][START_REF] Doran | Ontogeny of locomotion in mountain gorillas and chimpanzees[END_REF]Druelle, Aerts, & Berillon, 2016a;[START_REF] Dunbar | Development of Posture and Locomotion in Free-ranging Primates[END_REF][START_REF] Wells | Ontogeny of locomotion in rhesus macaques (Macaca mulatta): II. Postural and locomotor behavior and habitat use in a free-ranging colony[END_REF]. The developmental trajectories related to locomotion are species-specific and there is a broad diversity of positional behaviors with various degrees of specialization in adults (see [START_REF] Hunt | Why are there apes? Evidence for the co-evolution of ape and monkey ecomorphology[END_REF], for a review). Overall, occasional bipedal walking represents a common part of the natural positional repertoire of non-human primates [START_REF] Druelle | Bipedalism in non-human primates: a comparative review of behavioural and experimental explorations on catarrhines[END_REF]. Bipedalism can be considered a non-erratic locomotor behaviour that is commonly observed in various contexts for short bouts (e.g., [START_REF] Carvalho | Chimpanzee carrying behaviour and the origins of human bipedality[END_REF]Druelle, Aerts, & Berillon, 2016b;[START_REF] Rose | Bipedal behavior of olive baboons (Papio anubis) and its relevance to an understanding of the evolution of human bipedalism[END_REF][START_REF] Stanford | Arboreal bipedalism in wild chimpanzees: Implications for the evolution of hominid posture and locomotion[END_REF][START_REF] Videan | Bipedality in chimpanzee (Pan troglodytes) and bonobo (Pan paniscus): Testing hypotheses on the evolution of bipedalism[END_REF][START_REF] Wrangham | Bipedal locomotion as a feeding adaptation in gelada baboons, and its implications for hominid evolution[END_REF]. It is suggested that bipedal walking is, in some respects, "stereotyped", i.e., similar postural features are shared between species (e.g., [START_REF] Demes | Three-dimensional kinematics of capuchin monkey bipedalism[END_REF][START_REF] Demes | Center of mass mechanics of chimpanzee bipedal walking[END_REF][START_REF] Pontzer | Bipedal and quadrupedal locomotion in chimpanzees[END_REF]. These features are the "bent-hip, bent-knee" posture, a significant protraction and reduced retraction of the hindlimbs during the movement.

Although bipedalism is occasionally performed in non-human primates, we previously hypothesized that the motor development of the main locomotor mode (for instance quadrupedal walking) may lead to an improvement of the secondary locomotor modes, such as bipedal walking (Druelle, Aerts, & Berillon, 2017a). This hypothesis is based on evidence that the rhythmic motor patterns related to locomotion are likely to share common neural control networks (e.g., [START_REF] Balter | Neural coupling between the arms and legs during rhythmic locomotorlike cycling movement[END_REF][START_REF] Dominici | Locomotor Primitives in Newborn Babies and Their Development[END_REF][START_REF] Zehr | The quadrupedal nature of human bipedal locomotion[END_REF]. In other words, multiple motor components can interact to mature different movement output simultaneously [START_REF] Dominici | Locomotor Primitives in Newborn Babies and Their Development[END_REF][START_REF] Ivanenko | Development of independent walking in toddlers[END_REF]. Our previous results on the early locomotor development of olive baboons support this prediction because we observed that the coordination pattern during bipedal locomotion significantly improves in infant baboons (between ~6 months old and ~2 years old), while they mostly experience quadrupedal walking and only very little bipedal walking (Druelle et al., 2017a). In addition, although adult baboons are generally less inclined to use bipedalism than infants, we have observed that when adult baboons use bipedalism, they tend to engage in it for periods of longer duration than infants do (Druelle et al., 2016b). To our knowledge, only one other research team has explored the development of occasional bipedal walking in a non-human primate species. Kimura and colleagues have provided significant work on the bipedal locomotion of chimpanzees and how it changes during development (e.g., [START_REF] Kimura | Voluntary bipedal walking of infant chimpanzees[END_REF][START_REF] Kimura | Centre of Gravity of the Body during the Ontogeny of Chimpanzee Bipedal Walking[END_REF][START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF]. They have shown that there is an improvement in the regularity of bipedal gaits in chimpanzees during the first 3 years of life [START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF]. This occurs during the acquisition of their quadrupedal gait [START_REF] Doran | The ontogeny of chimpanzee and pygmy chimpanzee locomotor behavior: a case study of paedomorphism and its behavioral correlates[END_REF][START_REF] Sarringhaus | Locomotor and postural development of wild chimpanzees[END_REF]. Furthermore, [START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF] observed that infant chimpanzees exhibit different postures than adults, such as larger extension and range of motion of hip joints. Compared to humans, adult chimpanzees exhibit a relatively large variance, short braking duration and small energetic recovery capacities during bipedal walking (2-45%). Nevertheless, the studies on chimpanzees and baboons both point to an improvement of the bipedal walking skills in living non-human primates during development, possibly including significant changes in the kinematics and in the gait pattern. Yet, at the adult stage, the efficiency of bipedal walking in nonhuman primates is relatively low [START_REF] Demes | Center of mass mechanics of chimpanzee bipedal walking[END_REF][START_REF] Nakatsukasa | Energetic costs of bipedal and quadrupedal walking in Japanese macaques[END_REF][START_REF] Nakatsukasa | Energy expenditure of bipedal walking is higher than that of quadrupedal walking in Japanese macaques[END_REF][START_REF] Pontzer | Bipedal and quadrupedal locomotion in chimpanzees[END_REF]. From an evolutionary perspective, the question of the developmental processes of the locomotor skills remains almost entirely unexplored in hominins, certainly because juvenile fossil materials are extremely rare in early hominins (Alemseged et al., 2006). To date, it is still unclear how young hominin individuals learned to walk bipedally (but see [START_REF] Tardieu | Ontogeny of the knee joint in humans, great apes and fossil hominids: pelvi-femoral relationships during postnatal growth in humans[END_REF][START_REF] Tardieu | Development of the human hind limb and its importance for the evolution of bipedalism[END_REF]. Increasing our knowledge on the ontogeny of occasional bipedal walking in non-human primates could provide new insights into the developmental trajectory of this behavior in hominins.

Baboons are clearly specialized for quadrupedal walking, yet, they are also able to suddenly raise their centre of mass relatively high above the hips and to walk on two feet on a reduced area of support [START_REF] D'août | Locomotor Versatility in Primates: Baboons Shifting to Bipedal and Tripedal Gait[END_REF]. Such a transition is obviously a complex task for the neural control system [START_REF] Nakajima | Integration of Upright Posture and Bipedal Locomotion in Non-Human Primates[END_REF][START_REF] Mori | Higher nervous control of quadrupedal vs bipedal locomotion in non-human primates; Common and specific properties[END_REF]. Furthermore, the bipedal posture is inherently mechanically unstable compared to quadrupedalism, but the ability to maintain balance on two legs needs to be solved by the same system. Motor actions (using muscle torques) should be able to correct the perturbations encountered in this unstable posture. Gait maturity should also affect the capacity of the system to do so. As a result, in a primate species like the baboon that only occasionally uses bipedal walking, this locomotor mode should inherently represent higher demands at a younger age; even in a human that is an adapted biped, toddlers need to walk bipedally for long periods of time and they cover significant distances before being able to walk efficiently and without falling [START_REF] Adolph | Walking infants adapt locomotion to changing body dimensions[END_REF][START_REF] Adolph | How do you learn to walk? Thousands of steps and dozens of falls per day[END_REF][START_REF] Hallemans | Mechanical energy in toddler gait A trade-off between economy and stability[END_REF][START_REF] Ledebt | Acquisition of upper body stability during walking in toddlers[END_REF]. Berillon andcollaborators (2010, 2011) conducted a kinematic study of bipedal walking by analysing 10 strides in an ontogenetic sample of 10 olive baboons (Papio anubis).

Overall, they described a general gait pattern that is common to young (~6 months) and adult individuals. The bipedal gait is a compliant gait with a semiplantigrade foot posture during stance phase, a trunk that is slightly tilted forward and immobile forelimbs that are kept forward in a parasagittal plane. Beside this general pattern, they also pointed out a few variations, especially in the youngest individuals that could be related to development and maturation. Specifically, in the youngest individuals they noticed more variation in the trunk angle and a more extended foot at touchdown.

Here, we provide new information on the sagittal plane gait kinematics of bipedal walking, and its relationship to speed in a large ontogenetic sample of olive baboons, clearly separated in three ageclasses from early infancy to adulthood. The aim of this study is to assess whether the kinematics change during the development of bipedal walking in a non-adapted biped species in order to estimate whether the baboons improve the way they walk bipedally. We hypothesize that the average kinematic profile and the scaled spatio-temporal parameters should vary between infant, adolescent, and adult baboons. Given the differences in both gait maturity and body proportions between the age-classes studied, we also hypothesize that speed should have a different influence on gait kinematics between these groups. Considering speed variation offers us a window to observe different control strategies, if any, in the pattern of inter-segmental coordination. By observing the developmental process of bipedal walking in baboons, we aim to test whether a non-adapted biped species changes the way it walks bipedally during development resulting in improvement of its bipedal walking skills. Second, in the evolutionary context of bipedal locomotion in hominins, studying the development of bipedalism in non-human primates and comparing it to humans offers an opportunity for us to propose new hypotheses on the pattern of locomotor learning and development in hominins. We will discuss the implications of our results for the postural movement strategies and balance control, as well as in the evolutionary context of the bipedal development in juvenile hominins.

Material and Methods

Study site and subjects

We sampled 24 individuals, Papio anubis, housed at the Primatology Station of the CNRS (Rousset-sur-Arc, France; 17 individuals are newly sampled, and 7 individuals are from [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]. This research center houses and breeds several colonies of baboons in large enclosures. The individuals studied are all part of the same colony of ca. 60 individuals. In the context of the present study, we have worked on the platform for biomechanics (Motion analysis of Primates, MAP) that is permanently installed in an outdoor enclosure and includes various devices, such as a multi-camera recording setup, force plates and EMG-equipment (for details see [START_REF] Berillon | In What Manner Do Quadrupedal Primates Walk on Two Legs? Preliminary Results on Olive Baboons (Papio anubis)[END_REF]Druelle, Supiot, Meulemans, Schouteden, Molina-Vila, Rimbaud, Aerts, & Berillon, 2021). The present work is part of a large ongoing project contributing to our knowledge about walking skills in non-human primates (e.g., [START_REF] Anvari | Kinematics and spatiotemporal parameters of infant-carrying in olive baboons[END_REF][START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]Druelle et al., 2017;Druelle et al., 2021). The collection of the new data was performed during opportunistic recordings on the technical platform between December of 2010 and August of 2012. At the time of data collection, the colony was composed of male and female infants, adolescents and adults. An ontogenetic sample of 17 individuals aged from 6 months to 7.4 years old was selected. According to previous studies on the growth pattern of olive baboons, this species exhibits important and rapid morphological changes during the first 2 years of age (Druelle et al., 2017;[START_REF] Leigh | Growth and Development of Baboons[END_REF]. The morphological changes gradually slow down until approximately 6 years of age when the growth stops (females can be considered adult at 5 years old and males can be considered adult at 6.5 years old; Druelle et al., 2017). Between 3.5 and 4.5 years of age, baboons become sexually mature. Furthermore, the maturation of the neuromotor control appears to be largely complete at 3 years of age in baboons, as is also the case in other cercopithecoid species (e.g., Druelle et al., 2017;[START_REF] Rose | Positional behaviour of olive baboons (Papio anubis) and its relationship to maintenance and social activities[END_REF]Vilensky & Gankiewicz, 1990;[START_REF] Zeininger | Ontogenetic changes in limb postures and their impact on effective limb length in baboons (Papio cynocephalus)[END_REF][START_REF] Dunbar | Development of Posture and Locomotion in Free-ranging Primates[END_REF].

In this study, we selected 3 well separated age-classes in order to avoid any potential overlap in morphology (morphotype) that could arise from interindividual differences in the timing of maturation. As a result, the first age class included 7 individuals that are aged from 6 months to 1.5 years old (i.e., infants), plus 2 other individuals that are from the same age class in [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]number 854 and 632). The second age class included 6 individuals that are aged from 3 to 4 years old (adolescents), plus 4 other individuals that are from the same age class in [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]number 411, 406, 568 and 604). The last age class included 5 individuals that are older than 5 years old (adults), plus 1 other individual that is from the same age class in [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]number 606); note that the lower number of adult individuals in our sample reflects the fact that they use bipedalism less frequently than the younger ones (see [START_REF] Rose | Bipedal behavior of olive baboons (Papio anubis) and its relevance to an understanding of the evolution of human bipedalism[END_REF][START_REF] Druelle | Bipedal Behaviour in Olive Baboons: Infants versus Adults in a Captive Environment[END_REF]. Table 1 provides individual characteristics of the full sample. All the procedures that are described in this study were approved by the Regional Ethical Committee for animal experimentation of the Midi-Pyrénées Region (Letter MP/01/15/02/2008).

Data collection

The bipedal behaviour of the olive baboons was recorded using a motion analysis system. The animals were easily video captured (see [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF] for a general description of the setup); here we used an updated system with high-definition digital video camera running at 200 fps in 1280 x 800 pixel resolution (Baumer HXC13). The camera was laterally placed perpendicular to the long axis of a walkway (positioned inside the enclosure) to accurately record the main sagittal spatiotemporal and joints kinematics in a large field of view. Calibration of the field was performed using reference points on the platform. The camera was connected via a CameraLink cable to a workstation (Norpix) that enabled us to monitor and adjust various parameters such as the exposure time, the contrasts, the recording period, etc. Streampix 4.7 software (Norpix) was used to control these parameters. We collected our data opportunistically. The animals were able to walk spontaneously on the straight and horizontal calibrated walkway, during their daily activities. A lot of good sequences were collected when the baboons were carrying food items. However, given the small proportion of spontaneous bipedal walking behaviour in baboons in general (Druelle et al., 2017;[START_REF] Rose | Positional behaviour of olive baboons (Papio anubis) and its relationship to maintenance and social activities[END_REF], we also positioned a mirror at the end of the walkway as we observed that it "stimulated" their bipedal behaviour. It increased the likelihood of recording bipedal bouts on the walkway (Table A in supplementary material describes the behavioural context of the bipedal sequences included in the current analysis). We first selected from sight appropriate bipedal sequences during which the individuals were walking steadily, along the platform, to the right or the left direction. For the sequences included in the analysis, we first calculated the following spatiotemporal parameters: the duty factor (proportion of stance phase relative to cycle duration), the stride duration (SD) and frequency (SF, i.e. the inverse of the stride duration), the stride length (SL) and the average speed (v, based on the stride duration and stride length). As the individuals vary greatly in size, we corrected for this effect by scaling the spatiotemporal parameters and the speed by following the common scaling procedures [START_REF] Hof | Scaling gait data to body size[END_REF]:

𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝑠𝑝𝑒𝑒𝑑 = 𝑣 √𝑔. 𝑙 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝑠𝑡𝑟𝑖𝑑𝑒 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 = 𝑆𝐹 √𝑔/𝑙 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝑠𝑡𝑟𝑖𝑑𝑒 𝑑𝑢𝑟𝑎𝑡𝑖𝑜𝑛 = 𝑆𝐷 √𝑙/𝑔 𝐷𝑖𝑚𝑒𝑛𝑠𝑖𝑜𝑛𝑙𝑒𝑠𝑠 𝑠𝑡𝑟𝑖𝑑𝑒 𝑙𝑒𝑛𝑔𝑡ℎ = 𝑆𝐿 𝑙
Where l is the length of the shank length and g is the gravitational acceleration (9.81m.s -2 ). Consistent with previous studies on primate locomotion, we used the shank length as our scaling parameter (e.g., [START_REF] Aerts | Spatio-temporal gait characteristics of the hind-limb cycles during voluntary bipedal and quadrupedal walking in bonobos (Pan paniscus)[END_REF][START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF][START_REF] Vereecke | Speed modulation in hylobatid bipedalism: A kinematic analysis[END_REF]. However, to enable us to make comparisons with the studies that used leg length as the scaling parameter (e.g., [START_REF] Hof | Scaling gait data to body size[END_REF], we also provide the results when we used this (i.e. length of shank and thigh together).

After recording, the digitization of the body was performed manually frame-by-frame by JÖ using the Matlab DLTdv8 application developed by the T. Hedrick lab. Overall, 19 markers corresponding to anatomical references were positioned on the baboons (see Figure 1 and Table B in supplementary material). A bipedal stride was defined as the period between one touch-down of the reference hindlimb (i.e., the one closer to the camera) to the following touch-down of the same hind limb, thus including one stance phase and one swing phase. In total, we analyzed 40 strides separated by age class to which we added 7 strides coming from [START_REF] Berillon | Bipedal versus Quadrupedal Hind Limb and Foot Kinematics in a Captive Sample of Papio anubis: Setup and Preliminary Results[END_REF]: 23 strides among 9 infants, 15 strides among 10 adolescents, 9 strides among 5 adults.

Data analysis

Some data points were difficult to digitize properly during the full strides. This was mainly the case for the limbs positioned further from the camera because of the front limbs' movements. To correct this, we applied a general filter for missing data on our database using a piecewise linear interpolation method. The data were then filtered per stride, using a fourth order low-pass Butterworth filter with a cut-off frequency of 10Hz. We used basic trigonometry between vectors to calculate six angles in the coordinate sagittal plane per frame. The different joint angles calculated on the baboons walking bipedally are the following: the ankle, the knee, the hip and the shoulder (Figure 1). The segment angles calculated are the following: the shank, the thigh, and the trunk (Figure 1). We then applied a cubic spline interpolation over a time base with 100 points in order to stride-normalized the individual bipedal cycles for each angle. Thanks to this treatment, we were able to average the multiple strides per age-class, known as ensemble averages, and we could compare the strides between groups. Each angle was also characterized per stride by using two values: the mean (i.e. its central tendency) and the amplitude (i.e. range-of-motion, calculated as the difference between the maximum value and the minimum value).

To assess the out-phase movement of the forelimb relative to the hindlimb, we calculated the percentage of congruity (i.e., the phase relationship) per stride between the shoulder angle and the hip angle. We calculated this index as follows:

𝐴𝑛𝑔𝑙𝑒 𝑐𝑜𝑛𝑔𝑟𝑢𝑖𝑡𝑦 = ∑ [(𝐻𝑖𝑝 ̂𝑛+1 -𝐻𝑖𝑝 ̂𝑛)(𝑆ℎ𝑜𝑢𝑙𝑑𝑒𝑟 ̂𝑛+1 -𝑆ℎ𝑜𝑢𝑙𝑑𝑒𝑟 ̂𝑛) > 0 𝑛-1 1 ] 𝑛 -1 × 100
Where n is the number of values (i.e., 100 data points after stride-normalization, see above). It measures the proportion of the gait cycle during which both angles change in phase. If the hip angle always extends when the forelimb goes forward and always flexes when the forelimb goes backward, the angle congruity will equal 100%, suggesting a counter balancing movement of fore-and hindlimbs as observed in humans (e.g., [START_REF] Dietz | Neuronal coordination of arm and leg movements during human locomotion[END_REF][START_REF] Goudriaan | Arm swing in human walking: What is their drive?[END_REF][START_REF] Killeen | Increasing cognitive load attenuates right arm swing in healthy human walking[END_REF][START_REF] Van De Walle | Age-related changes in arm motion during typical gait[END_REF].

Statistics

After applying the size correction, all data were log10-transformed before analysis to ensure normality and homoscedasticity assumptions were met (this is controlled using Shapiro-Wilk tests and histograms). We applied mixed effects models in order to control for the effects of individuals. We first tested whether there is an association between the spatiotemporal parameters (i.e., dimensionless stride frequency, dimensionless stride length and duty factor; dependent variables) and the different age-classes when taking the speed effect (fixed effects) into account. To control for the interindividual variation, we included the individual effect as a random effect in the models. We used the function lmer (from lme4 package) for fitting these models (the restricted maximum likelihood method was used). We used the Satterthwaite's approximation method implemented in the lmerTest package [START_REF] Kuznetsova | lmerTest package: tests in linear mixed effects models[END_REF] to assess the significance of the different fixed effect variables of the models (i.e., speed and age-class). We applied the same statistical procedure on the mean and amplitude of the joint and segment angles across strides, as well as on the % of divergence between the shoulder and the hip angles, thus keeping the age-classes and the dimensionless speed as fixed effects. All the statistical analyses were performed using RStudio (v1.2.5001) and the significance level was set at P<0.05.

Results

The spatiotemporal parameters

Table 2 shows the spatiotemporal parameters averaged per age-class and their associated statistical results. There is no significant difference in speed and in dimensionless speed between the age-classes.

The speed is significantly related to all the spatiotemporal parameters, i.e., stride frequency, stride duration, stride length and duty factor. There is a significant age effect on all the spatiotemporal parameters.

When removing the effect of size using the shank length as the scaling factor, there is no difference across age for the stride duration, stride frequency and stride length, and there is no interaction effect for these variables. There is a significant age-class effect on the duty factor and the interaction between speed and age-class is also significant (F=7.43, P=0.004; F=6.29, P=0.005, respectively; Fig. 2).

When applying the same tests in taking the leg length as the scaling parameter, the results are similar.

Kinematics

Figure 3 and 4 show the patterns of the segment and joint angles, respectively, averaged per age-class during bipedal walking and include the ±95% Confidence Interval (CI). The changes in joint and segment angles that occur during bipedal walking are very similar between the age-classes. With regard to the central tendency of the angles, we found three significant results. There is a general speed effect on the mean value of the ankle (F=11.22, P=0.002) and on the mean value of the thigh (F=4.89, P=0.034), and there is an age effect for the mean value of the shank (F=6.17, P=0.017; Figure 5). Table 3 summarizes the average joint and segment angles per age-class at specific instants of the bipedal gait cycle.

With regard to the range-of-motion of the angles, there is an age effect on the hip amplitude (F=3.58, P=0.0499) and on the thigh amplitude (F=4.14, P=0.030). There is a significant effect of age and of the interaction between speed and age in terms of the amplitude of the knee (F=6.87, P=0.007; F=3.69, P=0.037; respectively). There is a significant effect of speed on the shank amplitude (F=7.83, P=0.009;

Figure 6). Table 4 summarizes the statistical results obtained for the mean and amplitude of the joint and segment angles.

Movement of the forelimb relative to the hind limb

There is a no age effect and no effect of speed on the angle congruity between hip and shoulder extension, i.e. at higher speed the congruity is not significantly higher (F=2.88, P=0.10). The congruity values do not indicate a clear pattern of out-phase movements of the forelimbs relative to the hind limbs (infants: 56±16%, adolescents: 53±11%, adults: 57±7%). There is a significant age effect on the shoulder amplitude (F=4.98, P=0.02), toward larger amplitudes in infants compared to in adults and adolescents.

Discussion

Ontogenetic changes in kinematics

We first hypothesized that the average kinematic profile and the scaled spatio-temporal parameters should vary between infant, adolescent, and adult baboons. Overall, our results depict an average profile of the kinematic parameters of the bipedal walking pattern that does not substantially change across age categories (see Fig. 3 and4). However, interesting differences in the stance phase parameter (duty factor) and in segment angles can be depicted. When removing the effect of size, only subtle differences can be found between juveniles and older (adolescent and adult) baboons.

Specifically, we observed that the stride frequency and stride length do not change during development. Infant, adolescent and adult baboons modulate their bipedal gait by altering both their stride frequency and stride length. However, the stance phase is relatively longer in young baboons, and it is not affected by speed (the within-group correlation is not significant for infants; see Fig. 2), suggesting higher balance control requirements in young, immature, individuals. A relatively longer stance phase should indeed enhance stability and control and provides longer double limb support phases as well. Furthermore, the shank angle is lower in infants (~5° lower), mainly because it keeps a more vertical posture at touch-down (see Table 3). The infants also show a larger amplitude in the hip joint which is mainly due to the larger amplitude of the thigh segment, as the trunk pitch amplitude is not significantly different across the age classes. The higher amplitude in the hip joint has also been reported in young chimpanzees but it is not the case in young children [START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF][START_REF] Ivanenko | Development of independent walking in toddlers[END_REF]. In chimpanzees, this may also be related to large movement of the thigh as the maximum forward inclination of the trunk does not vary with age, yet no information is provided on the movement amplitude of the trunk in immature chimpanzees. On contrary, in humans there is a significant hip flexion at an early stage that becomes gradually more extended. Interestingly, the amplitude of thigh movements has been shown to be relatively high in toddlers with respect to that of shank and foot movements [START_REF] Ivanenko | Development of independent walking in toddlers[END_REF].

Age-related influence of speed on kinematics

We also hypothesized that the modulation of the bipedal gait with speed is different between infants and older baboons. The mean angle value of the ankle joint in adults and adolescents gradually increases with speed, i.e., higher ankle extension, but this is not the case in infants (see Fig. 5). On contrary, the mean angle value of the thigh segment in infants gradually decreases with speed, i.e., the limb is brought less forward, but this is not the case in adults and adolescents (see Fig. 5). In addition, an important difference between infants and older baboons concerns the amplitude of the knee joint with speed. While the speed significantly affects the knee amplitude in infants towards smaller amplitudes at higher speed, the reverse pattern is observed in adults. This can be directly related to the way in which infants and older baboons modulate their bipedal gait with speed. Adults and adolescents appear to adjust their kinematics predominantly at the knee and ankle joint ("lower leg strategy"), thus increasing stride length by bringing the lower leg more forward. Infants generally exhibit higher movement amplitude of the thigh segment than older baboons. In this way, it mainly contributes to their stride length and long relative stance phase. When they walk faster, they appear to use a sort of "blocking strategy" by reducing the protraction of the thigh and the movement amplitude of the knee joint, thus keeping a long duty factor while increasing stride frequency. The kinematic variations observed in the present baboon sample may highlight different (neuromotor) control strategies related to maturity (muscle strength and coordination) and morphology (body proportions; see below).

Solving the problem of balance control: the influence of body mass distribution

Multiple options for motor control action are available to move the body effectively and to correct undesired movements [START_REF] Winter | Human balance and posture control during standing and walking[END_REF][START_REF] Winter | Biomechanics and motor control of human movement[END_REF]. Walking is a dynamic process where segmental accelerations and decelerations about joints impact the rate of change of the total angular momentum.

Kinematics adjustments can, therefore, alter the change of the total angular momentum and in this way affect the stability of the body. In a bipedal posture, the balance control is inherently more challenging as the upper body needs to be stabilized above the hip. The baboons, although considered as non-adapted bipeds, are able to solve the balance control problems from an early age, as they are able to walk bipedally without falling. However, the kinematic variations observed in the present study are likely to highlight differences in the motor control strategies between infants and older baboons.

On the one hand, the observation of the longer duty factor, the higher movement amplitude of the thigh and its control with speed in infants suggest an increased control of the walking pattern that is focused on the upper leg. On the other hand, the older baboons are able to smoothly adapt their movement with speed, by reducing the stance phase and increasing both stride length and stride frequency through variations of the amplitude of movement of the lower leg (knee and ankle joints), while the upper part of the leg is not affected by speed. These differences could simply arise from the differences in morphology (e.g., body proportions in toddlers; see [START_REF] Van Dam | A cross-sectional study about the relationship between morphology and step-time parameters in children between 15 and 36 months[END_REF][START_REF] Van Dam | A cross-sectional study about the relationship between morphology and kinematic parameters in children between 15 and 36 months[END_REF]. In infant primates (including humans) the body centre of mass is positioned relatively closer to the head than in adolescents and adults (e.g., in non-human primates: Druelle, Aerts, D'Août, Moulin, & Berillon, 2017b;[START_REF] Grand | Body weight: Its relation to tissue composition, segment distribution, and motor function II. Development of Macaca mulatta[END_REF][START_REF] Turnquist | Ontogeny of locomotion in rhesus macaques (Macaca mulatta): I. Early postnatal ontogeny of the musculoskeletal system[END_REF]e.g., in humans: Palmer, 1944).

Maintaining a relatively high centre of mass in a "bent-hip, bent-knee" posture inherently increases the gravitational moment about the hips, thus making it more challenging for the hip extensor muscles to stabilize the upper body (e.g., [START_REF] Preuschoft | Mechanisms for the acquisition of habitual bipedality: are there biomechanical reasons for the acquisition of upright bipedal posture[END_REF][START_REF] Winter | Biomechanics and motor control of human movement[END_REF]. The angle of permissible sway of the trunk must be kept small because too many oscillations of a trunk in which the centre of mass is cranially positioned would considerably increase the risk of losing balance [START_REF] Winter | Biomechanics and motor control of human movement[END_REF]. For example, in bipedally-trained macaques, trunk sway is very limited (~5°; Hirasaki, Ogihara, Hamada, Kumakura, [START_REF] Nakatsukasa | Energetic costs of bipedal and quadrupedal walking in Japanese macaques[END_REF]. In our study, the baboons also keep a trunk relatively stable, from an amplitude of ~8° on average in infants to an amplitude of ~5° on average in adults, and it is not impacted by the speed of locomotion. Infant baboons are, therefore, able to stabilize their upper body during bipedal locomotion at an early age, but their balance strategy to do so is likely to be different than the one observed in older baboons. Interestingly, we observed that the amplitude of the shoulder joint angle is relatively low in baboons (~16° for the full ontogenetic sample, versus ~25° in humans; [START_REF] Killeen | Increasing cognitive load attenuates right arm swing in healthy human walking[END_REF] but varies significantly across age categories (~21° in average in infants, ~12° in adolescents and adults). This might emphasize the search for active corrective actions from the forelimbs in infants, but further data are required to assess this aspect. In general, there is almost no information about how the forelimbs move during bipedal walking in non-human primates (but see [START_REF] Tardieu | New method of three-dimensional analysis of bipedal locomotion for the study of displacements of the body and body-parts centers of mass in man and non-human primates: Evolutionary framework[END_REF]). It appears that their movement is limited in cercopithecoids compared to hominoids.

Limited movement of the forelimbs has been shown to not compromise gait stability in humans [START_REF] Bruijn | The effects of arm swing on human gait stability[END_REF]. With regard to the out-phase movement pattern, there is no variation across the age classes in our sample, and the speed does not affect the counter balancing movement of the upper arm. The fact that the movement amplitude is low and not affected by speed has been suggested to correspond to a low level of muscular activity in humans [START_REF] Goudriaan | Arm swing in human walking: What is their drive?[END_REF]. However, further research including the muscular activity in non-human primates is required to better understand the dynamics of arm swinging during occasional bipedal walking in these species.

Comparisons with humans

In humans, the duration of the single-limb stance, the relative double support phase, the stride frequency and the stride length have been shown to be important determinants of a mature gait (e.g., [START_REF] Grieve | The Relationships Between Length of Stride, Step Frequency, Time of Swing and Speed of Walking for Children and Adults[END_REF]Ledebt & Bril, 2020;[START_REF] Sutherland | The development of mature gait[END_REF]. In toddlers, a stable posture is achieved relatively rapidly, only a few weeks after the onset of independent walking (e.g., Ledebt & Bril, 2020;[START_REF] Schedler | Age and sex differences in human balance performance from 6-18 years of age: A systematic review and meta-analysis[END_REF]. Although this is not yet an efficient walking pattern, it represents a real improvement of the gait parameters (Ledebt & Bril, 2020). Young children have also been shown to exhibit long periods of coactivation of agonist and antagonist muscles during gait that have been related to immature balance control strategies [START_REF] Berger | Developmental aspects of equilibrium control during stance: a kinematic and EMG study[END_REF]Okamoto & Kumamoto, 2001;[START_REF] Woollacott | Changes in posture control across the life spanâ€"a systems approach[END_REF]. During this early period of walking the ankle muscles in humans are relatively weak [START_REF] Sutherland | The development of mature gait[END_REF]. It is also only after years of practice that the fully efficient gait pattern arises in humans with adult-like joint angle patterns, reciprocal arm swinging and a mature gait [START_REF] Chester | A comparison of kinetic gait parameters for 3-13 year olds[END_REF][START_REF] Hallemans | Mechanical energy in toddler gait A trade-off between economy and stability[END_REF][START_REF] Sutherland | The development of mature gait[END_REF][START_REF] Sutherland | The development of mature gait[END_REF][START_REF] Van De Walle | Age-related changes in arm motion during typical gait[END_REF]. In baboons, the strategy described in young individuals (see above) reflects a more immature control for walking bipedally. The duty factor is relatively fixed and long, the stride length is likely to be controlled at the hip and the frequency varies mainly by reducing the movement amplitude at the knee joint. It is thus possible that this strategy is also related to higher coactivation of agonist and antagonist muscles in the thigh. However, it requires testing by comparing the muscle activity of young and adult baboons. Interestingly, our results on baboons may echo the developmental sequence of the inter-segmental coordination described in the ontogeny of human locomotion (see [START_REF] Assaiante | An ontogenetic model for the sensorimotor organization of balance control in humans[END_REF][START_REF] Assaiante | Development of locomotor balance control in healthy children[END_REF]. Hip stabilization has been suggested to be the reference frame at the onset of bipedal locomotion in humans. At this stage, important co-activation patterns are observed in hip, knee and ankle muscles, hence, anticipatory postural adjustments include the whole body [START_REF] Assaiante | Development of locomotor balance control in healthy children[END_REF][START_REF] Berger | Developmental aspects of equilibrium control during stance: a kinematic and EMG study[END_REF][START_REF] Woollacott | Changes in posture control across the life spanâ€"a systems approach[END_REF]. This strategy has been called the "reduced damping strategy" (or the "blocking strategy") and it is an efficient and simple strategy that means that the number of degrees of freedom to be controlled can be minimized [START_REF] Assaiante | Development of locomotor balance control in healthy children[END_REF]. In older children and adults, the activation level is lower in the hip and knee muscles, but higher at the ankle joint. Hence, the control becomes selective and relies on articulated operations that are focused on the lower leg. This developmental pattern may be suggested, in some respects, in baboons as well. In answer to the question "do baboons improve the way they walk bipedally with age?" it is difficult to reply at this stage of research, but it is obvious that the control strategy for walking bipedally changes with age. The ultimate goals of the musculoskeletal system are certainly to solve the control problems related to equilibrium and to be energetically efficient. The present study shows that the balance control problems are solved at an early age in baboons, yet, it is achieved with a different strategy than in adults. Moreover, a potential improvement, energetically speaking, requires further research. In chimpanzees, young individuals and adults appear to exhibit low levels of energy recovery, while adolescents (5 to 8 years of age) can show high levels of energy recovery at low speed (dimensionless speed ~0.3-0.4; [START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF]. But it is not clear to which anatomo-functional feature this is related.

Evolutionary perspectives

Describing the way bipedal skills develop during ontogeny in non-adapted bipeds enables the development of new evolutionary hypotheses about the way bipedal locomotion might have developed in pre-adapted bipeds and early hominins (see [START_REF] Kimura | Development of Bipedal Walking in Humans and Chimpanzees : A Comparative Study[END_REF]. Learning to walk bipedally, i.e., solving the balance control problems, does not require walking bipedally for long period of time and distances, and appears to be proceeded early in life. The experience of (quadrupedal) walking and the onset of independent locomotion appear to be sufficient enough to be able to walk bipedally (Druelle et al., 2017). Hence, we suggest a similar process in early hominins.

However, being energetically efficient represents another challenge and may require extensive experience of walking bipedally [START_REF] Hallemans | Mechanical energy in toddler gait A trade-off between economy and stability[END_REF]. Previous studies tend to show that bipedal walking is not more energetically demanding than quadrupedal walking in chimpanzee (Sockol, Raichlen, & Pontzer, 2007;[START_REF] Pontzer | Bipedal and quadrupedal locomotion in chimpanzees[END_REF]Taylor & Rowntree, 1973); nevertheless, the individuals studied were regularly interacting with (bipedal) humans as they were privately owned, or trained animal actors for movies and entertainment purposes and they might have a significant experience of walking bipedally from an early age. In an evolutionary perspective, as soon as early hominins started to walk bipedally on the ground, balance was certainly not an important problem to solve, but efficient inter-segmental coordination strategies were required for walking efficiently over longer periods. Further comparative ontogenetic data such as the ones presented here on baboons are required to tackle the question of the developmental processes of bipedal walking skills in early hominins. The present study provides a relevant comparative framework for investigating this matter.

Conclusion

In this paper, kinematic and spatiotemporal parameters of bipedal gait were extracted in an ontogenetic sample of 24 baboon individuals. We investigated whether the bipedal gait pattern varies across age categories. Although the general average walking pattern is remarkably similar during ontogeny, the results show interesting age-related differences in the way baboons adjust their hind limb movements with speed. Walking bipedally for non-human primates, i.e., solving the balance control problem, does not require years of practice. The capacity to stabilise the upper body in a bipedal posture is functional very early in baboons, but the selected kinematic strategies related to speed vary with age. Our results suggest a change in the way they control and balance their posture.

A more controlled bipedal posture is observed in infants ("blocking strategy"), while the adolescents and adults modulate their bipedal gait more smoothly by adjusting knee and ankle joints toward larger movement amplitude with speed. The postural adjustments thus rely on neuromotor operations that are focused on the lower limb in adolescent and adult baboons, while it is focused on the upper leg in infants. This may arise from the interaction between the ontogenetic changes in body proportions and the maturity of the musculoskeletal system. (B) Positioning of the anatomical landmarks (the description of their osteological correspondence can be found in Table B, in supplementary material) and joint and segment angles. Segment angles (in red) are measured relative to the horizontal vector oriented in the direction of movement. Joint angles (in blue) are measured between the segments connected at the joint. For the ankle, the joint angle was measured between the shank (from the patella to the lateral malleolus) and the foot segment as a whole (from the heel to the tip of the toes). [Illustration: Céline Piret]. Following the results of the mixed effects models, a black linear regression is indicated when there is a general effect of speed only, coloured linear regressions associated with each age-class are indicated when there is both, a general interaction effect and a significant within group correlation (duty factor: infants: r=-0.30, P=0.08; adolescents: r=-0.84, P<0.0001; adults: r=-0.84 , P=0.0051). Figure 5. Relationships between the central tendency (mean value per stride) of the joint and segment angles with speed. The mixed effects models show that there is a significant effect of speed on the ankle joint and on the thigh segment, and there is a tendency toward a significant interaction effect between the age-classes and the speed for these angles. We applied a within group Pearson correlation for these angles (mean ankle: infants: r=0. Figure 6. Relationships between the range-of-motion (amplitude spanned per stride) of the joint and segment angles with speed. The mixed effects models show that there is an age class effect for the hip and thigh amplitude. There is a general significant speed effect for the shank amplitude. There is a significant age effect and an interaction effect for the knee amplitude. We applied a within group Pearson correlation for the knee angle (infants: r=-0.47, P=0.012; adolescents: r=0.34, P=0.11; adults: r=0.78, P=0.0099). Boxplots are included to visualize the age class effect for the hip, knee and thigh amplitude, and a star is provided to indicate the general age effect.

Figure 1 .

 1 Figure 1. Images and measurements of olive baboons walking bipedally. (A) Zoomed views of three olive baboons from the lateral camera. Each individual belongs to one of the three age-classes defined. (B) Positioning of the anatomical landmarks (the description of their osteological correspondence can be found in TableB, in supplementary material) and joint and segment angles. Segment angles (in red) are measured relative to the horizontal vector oriented in the direction of movement. Joint angles (in blue) are measured between the segments connected at the joint. For the ankle, the joint angle was measured between the shank (from the patella to the lateral malleolus) and the foot segment as a whole (from the heel to the tip of the toes). [Illustration: Céline Piret].

Figure 2 .

 2 Figure 2. Spatiotemporal characteristics of bipedal walking and their relationships with speed.Following the results of the mixed effects models, a black linear regression is indicated when there is a general effect of speed only, coloured linear regressions associated with each age-class are indicated when there is both, a general interaction effect and a significant within group correlation (duty factor: infants: r=-0.30, P=0.08; adolescents: r=-0.84, P<0.0001; adults: r=-0.84 , P=0.0051).

Figure 3 .

 3 Figure 3. Mean normative sagittal joint angles (hip, knee and ankle) for each age class and the respective ±95% CI. The dashed vertical line indicates the average time transition between the stance and the swing phase, and the grey surrounding surface indicates the standard deviation. The grey dashed vertical lines indicate the min-max values.

Figure 4 .

 4 Figure 4. Mean normative sagittal segment angles (trunk, thigh and shank) for each age class and the respective ±95% CI. The black dashed vertical line indicates the average time transition between the stance and the swing phase, and the grey surrounding surface indicates the standard deviation. The grey dashed vertical lines indicate the min-max values.

  Figure 1

  The significant results of the correlation tests are indicated with the coloured linear regressions. There is also an age class effect for the mean shank angle, but no relationship with speed. A boxplot is included to visualize the age class effect for the shank and a star is provided to indicate a general age effect.

27, P=0.11; adolescents: r=0.73, P=0.0013; adults: r=0.75, P=0.0052; mean thigh: infants: r=-0.35, P=0.046; adolescents: r=0.22, P=0.22; adults: r=0.53, P=0.08)

.

TABLE 1 .

 1 Characteristics of the three age classes

		Age class 1 (infant)	Age class 2 (adolescent)	Age class 3 (adult)
		Mean	min-max	Mean	min-max	Mean	min-max
	Age (year)	0.93	0.5-1.58	3.58	3.14-4.1	6.68	5.39-7.4
	Body mass (kg)	3.3	2.3-5.4	10.22	7.3-12.5	13.9	10.9-15.5
	N (individuals)	9		10		5
	Sex*	4f, 5m		7f, 3m		5f
	Number of
	strides	23		15		9

*f: female; m: male

TABLE 2 .

 2 Spatio-temporal parameters of steady bipedal walking per age-class

		Infant (n=21)	Adolescent (n=11)	Adult (n=8)	Speed effect	Age effect	Interaction effect
		Mean	SD	Mean	SD	Mean	SD	F value	P value	F value	P value	F value	P value
	Cycle duration (s)	0.57	0.13	0.60	0.09	0.58	0.12	117.55	<0.0001	14.39	0.0002	2.42	ns
	Stride frequency (1/s)	1.93	0.50	1.74	0.26	1.83	0.28	115.42	<0.0001	14.52	0.0001	2.48	ns
	Stride length (m)	0.44	0.07	0.65	0.09	0.72	0.08	27.33	<0.0001	14.30	0.0002	2.42	ns
	Speed (m.s -1 )	0.86	0.27	1.14	0.30	1.36	0.30	N/A	N/A	2.05	ns	N/A	N/A
	Dimensionless values (shank length = scaling factor)
	Duty factor	0.71	0.03	0.67	0.04	0.65	0.03	33.82	<0.0001	7.43	0.004	6.29	0.005
	Cycle duration	5.20	1.25	4.36	0.66	4.18	0.81	110.67	<0.0001	0.84	ns	1.22	ns
	Stride frequency	0.21	0.06	0.24	0.04	0.25	0.04	106.24	<0.0001	0.77	ns	1.42	ns
	Stride length	3.77	0.55	3.53	0.44	3.82	0.47	28.53	<0.0001	0.87	ns	1.04	ns
	Speed	0.79	0.24	0.85	0.21	1.00	0.24	N/A	N/A	0.15	ns	N/A	N/A
	Dimensionless values (shank + thigh length = scaling factor)
	Duty factor	0.71	0.03	0.67	0.04	0.65	0.03	32.24	<0.0001	8.29	0.002	5.92	0.007
	Cycle duration	3.70	0.90	3.11	0.46	2.94	0.54	109.89	<0.0001	0.06	ns	1.43	ns
	Stride frequency	0.30	0.08	0.33	0.05	0.36	0.05	110.17	<0.0001	0.06	ns	1.41	ns
	Stride length	1.90	0.27	1.80	0.24	1.90	0.24	30.16	<0.0001	0.04	ns	1.22	ns
	Speed	0.56	0.17	0.61	0.16	0.71	0.17	N/A	N/A	0.11	ns	N/A	N/A

TABLE 4 .

 4 Statistical results of the mixed effects models applied on the mean and amplitude of the joint and segment anglesSignificant results are indicated in bold and results that are close from the significant threshold are indicated in italic. "ns" indicates a result that is not significant.

		Speed effect	Age effect	Interaction effect
		F value	P value F value	P value F value	P value
	Mean
	Ankle	11.22	0.002	1.57	ns	2.62	0.089
	Knee	2.99	ns	0.03	ns	1.30	ns
	Hip	1.27	ns	1.60	ns	0.25	ns
	Shank	1.16	ns	6.17	0.017	0.91	ns
	Thigh	4.89	0.034	1.43	ns	2.67	0.083
	Trunk	0.03	ns	2.78	0.087	1.71	ns
	Shoulder	0.01	ns	0.20	ns	0.02	ns
	Amplitude
	Ankle	0.62	ns	0.77	ns	1.40	ns
	Knee	1.78	ns	6.87	0.007	3.69	0.037
	Hip	0.03	ns	3.58	0.0499	0.24	ns
	Shank	7.83	0.009	0.61	ns	2.14	ns
	Thigh	2.29	ns	4.14	0.03	0.15	ns
	Trunk	0.64	ns	1.95	ns	0.82	ns
	Shoulder	0.84	ns	4.98	0.019	1.41	ns

Acknowledgments

We are very grateful to Guy Dubreuil, the director of the Primatology Station of the CNRS at the time the data were collected, who provided access and facilities to the animals. We are very grateful to Christophe Arnoult, the current director of the Primatology Station of the CNRS, for his support, as well as to Romain Lacoste. We thank Guillaume Daver, Kristiaan D'Août and Zohreh Anvari for their help in recording the video sequences of walking in baboons. We thank Josie Meaney-Ward who revised and improved the English of the manuscript. We are also grateful to the referees for their constructive and detailed comments on the first version of the manuscript. The Technical Platform has been initially funded by the CNRS-INEE. This project is also currently funded by ANR-18-CE27-0010-01 and CNRS-INEE International Research Network IRN-GDRI0870.

Authors declare no competing interests.

Data availability statement

All the data of the present study are available from the corresponding author upon request.


Céline Bessière

Sibylle Gollac

5 Why do women accumulate less wealth than men?

In contemporary capitalism, inequality of wealth is increasing. Some upper-class families appropriate economic capital and pass it on to their children, while others are permanently deprived of it. At the same time, while women have largely entered the labor market and the gender pay gap is on the political agenda in all Western countries, the gender wealth gap is not closing, quite the opposite. In this chapter we explore family wealth arrangements in France. Estate planning and marital breakdowns are two moments when families and legal professionals strive to preserve real estate and businesses, or to minimize taxes. For this purpose, they produce inventories, estimations and distributions of assets which end up disadvantaging women, even though shares may appear to be formally equal. Reversed accounting is a common logic of practice, in which the result comes first and computation only after, as a means to legitimize the sharing that has been (forcefully) agreed on. Thus, it is not only the wealth of the upper class that is underestimated, but more particularly men's wealth. Those mechanisms contribute to impoverish women. We conclude that class society reproduces itself thanks to the male appropriation of capital.

Consider two women. The first one is named Ingrid. Her family name, Levavasseur, is fairly common in Normandy. It means the vassal of a lord who is himself a vassal. Ingrid Levavasseur is born in 1987 in a rural area, close to the lower Seine Valley. With her three brothers and sisters, she is raised by their mother, a former cleaning woman who becomes a special needs caregiver. Their violent and alcoholic father, frequently taken charge of by the Salvation Army, is mostly out of the picture. At 16, Ingrid leaves her mother's home without graduating high school. She strings together a series of minor service jobs, waitress, cashier, and telephone operator, and then gets married. Two kids are born. A year after the birth of her second child, Ingrid and her husband divorce. She is just 24 years old. While working as a firefighter at night, she studies to become a nursing assistant. She gives up dreaming of becoming a nurse because she cannot afford the education and training. In 2018, she makes 1,250 euros a month, supplemented with a 95-euro housing credit and 200 euros in alimony for her two children aged 8 and 13 years, for whom she has sole custody. She lives in a small rented house in the town of

in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE Pont-de-l'Arche, and she puts her children in day care while she works in Rouen, 20 kilometers away. Family vacation consists of three days a year at a campground near Mont-St-Michel. She has trouble buying her kids new sneakers, and keeping the fridge stocked all month. Ingrid long ago stopped spending money on herself: no haircuts, no sports, never dining out. At any rate, when would she find the time? Every other weekend when the kids are with their father?

In the autumn of 2018, Ingrid Levavasseur becomes a national figure as a part of the French Gilets jaunes movement 1 . With her straight red hair and Botticellian face, she is an immediately recognizable figure in news reports; a face for decades-old statistics that describes the poverty of women running single parent households. In 2019 she announces she is creating a support center providing lodging, child-care, and activities for women raising their children alone.

The Gilets jaunes movement places many anonymous members of the working class in a spotlight of media attention they have not known before. The presence of numerous women is remarkable, whether camping out at highway toll booths, or leading demonstrations. 2 A number of them raise their children alone, and find it hard to make ends meet to the end of the month.

For news cameras and microphones, they speak of overdue alimony payments and long administrative waits within social services systems to receive limited public assistance. They tell how they are constantly juggling bills, putting their children's needs before their own. Women in couples voice their concern of paying the bills. Some women speak about unemployment, parttime jobs, and having to take on as many work hours as possible. Others have given up on salary work to become self-employed; this does not provide a better income. Finally, there are retired women, often widows, who receive a meager pension. Among the poor, money problems are women's problems [START_REF] Desmond | Evicted. Poverty and Profit in the American City[END_REF].

in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE The second woman is named MacKenzie. She is born in San Francisco in 1970 to a well-off family with a financial planner father and a full-time wife and mother. She gets a degree in English from Princeton, where she attends Toni Morrison's seminars, in the hope of becoming a novelist. In the early 1990s she works at the investment firm of D.E. Shaw&Co in New York, a job that pays the bills and leaves her time to write. She meets her future husband there, Jeff Bezos; a computer scientist by trade and fellow Princeton graduate, who has become a senior vice president at the hedge fund by the time they meet. Jeff is the one who hired MacKenzie; he occupies the office next to hers. In 1993, they get married. She is 23 years old. He is 30. The next year, they move to the west coast, to a little rented house in the suburbs of Seattle. During their drive across the United States, with MacKenzie at the wheel and Jeff in the passenger seat, they develop a business plan for a new company that would sell books over the internet. The business that is created the next year later takes the name of Amazon.

In the beginning, MacKenzie is fully involved with the business. She does the bookkeeping, participates in hirings and strategy sessions, and dives into the basic work of sending packages out via UPS: "I was there when he wrote the business plan, and I worked with him and many others in the converted garage, the basement warehouse closet, the barbecue-scented offices, the Christmas-rush distribution centers, and the door-desk filled conference rooms in the early years of Amazon's history", she declares in a later interview when the business has become the world's largest online retailer.

In 1999, the couple's first child is born; to be followed by three others. MacKenzie and Jeff move into a 10-million-dollar house. MacKenzie starts working less for the company. She also sets aside her ambitions of becoming a novelist to take care of the four children. (Later, she explains that she could have hired nannies, but that she preferred to look after the children in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE herself, even home schooling them during certain periods). In 2005, her first novel is published, on which she has worked for over ten years. A second novel follows in 2013. It is critically wellreceived, but sales remain modest, around a couple of thousand copies. Book stores refuse to sell the novel because her husband's business has destroyed their own 3 .

On January 9, 2019, after 25 years of marriage, MacKenzie and Jeff Bezos announce their divorce in a joint tweet: "We want to make people aware of a development in our lives […] we have decided to divorce and continue our shared lives as friends […] We've had such a great life together as a married couple, and we also see wonderful futures ahead, as parents, friends, partners in ventures and projects, and as individuals pursuing ventures and adventures". This message stages an amicable divorce settlement that is destined less for their friends than for financial markets, investors, and shareholders. The future of the world's largest private fortune is in play: an estate worth over 130 billion dollars that includes a large portion of Amazon's capital including 16% of its shares. In Washington State, where the couple lives and works, divorce laws stipulate that all assets acquired during the marriage must be divided into two equal parts.

Hundreds of newspaper articles around the world expressed concern about the future of the Bezos fortune, a large portion of which consists of companies: Amazon, but also the aerospace company Blue Origin, or the daily newspaper The Washington Post. 8% of Amazon risks falling into the hands of a woman, possibly leading to Jeff Bezos losing control of the company; a possibility that makes financial markets anxious 4 . Three months later, the details of the divorce are made public by the couple, again on Twitter: "Grateful to have finished the process of dissolving my marriage with Jeff, with support from each other and everyone who reached out to us in kindness, and looking forward to next phase as co-parents and friends. Happy to be giving me all my interest in the Washington Post and Blue in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE Origin and 75% of our Amazon stock plus voting control of my shares to support his continued contributions with the teams of these incredible companies", MacKenzie writes. Jeff Bezos thus remains the primary shareholder of Amazon stock and retains control. He is still today the richest man on Earth. Among the ultra-rich caring for capital is a man's prerogative (Herlin-Giret, 2019: p. 69).

Wealth accumulation, class, and gender

An ocean and billions of dollars separate the lives of Ingrid Levavasseur and MacKenzie Scott.

The wealth of the former would likely include her car and some modest savings, probably no more than a couple thousand euros. MacKenzie Scott exited her marriage with 35 billion dollars.

As Thomas Piketty's Capital in the Twenty-First Century has made clear to a larger public, wealth inequality is a central characteristic of contemporary capitalism [START_REF] Piketty | Capital in the Twenty-First Century[END_REF]. Even more pronounced than income inequality, wealth inequality better describes the ever-widening chasm separating the worlds of MacKenzie Scott and Ingrid Levavasseur. According to the 2018 World Inequality Report, among the inhabitants of Europe, the United States, and China, the top 1% control a third of the world's wealth; and the top 10% dispose of 70% of the world's wealth; while the poorest half of this population only possesses 2% of it.

Accumulation of economic capital shapes the contemporary social class structures more than ever before. Marx defined the relationship between the social classes on the basis of the ownership of productive capital as opposed to the ownership of labor power alone. In the 20th century, in Western countries, the relations of exploitation were transformed by the generalization of the wage system, which at first was despised, then has been progressively associated with social protections [START_REF] Castel | From Manual Workers to Wage Laborers: Transformation of the Social Question[END_REF][START_REF] Castel | From Manual Workers to Wage Laborers: Transformation of the Social Question[END_REF]). In the wage system, social hierarchy in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE largely derives from educational qualifications. It was thus between work and school that sociologists examined the construction of class relations. However, at the beginning of the 21st century, differences in living conditions and social status are increasingly linked to the family transmission of economic capital. There are two different ways of accumulating wealth: by putting money aside or by inheriting it. While during the 1950s and 1960s inheritances represented less than half of the private wealth held by individuals in France, this proportion has only increased, and represented 60% of wealth in 2010 [START_REF] Alvaredo | On the Share of Inheritance in Aggregate Wealth: Europe and the USA, 1900-2010[END_REF]. We are certainly a long way from the 1910s when inherited wealth represented 80% of total private wealth, but if current economic and demographic trends continue, the inherited portion of wealth will keep on increasing during the 21st century. Today, family economic capital is more and more crucial to obtain housing, especially in a context where property is both widespread and socially distinctive. Furthermore, while a society based on salary work slowly fades away, family economic support can prove indispensable for starting a business, maintaining its economic activities, gaining access to credit, or for obtaining added revenue from the family assets. The accumulation of academic capital also depends more and more on the mobilization of family savings [START_REF] Zaloom | Indebted. How Families Make College Work at Any Cost[END_REF]. The material conditions of life influence children's success at school from a very young age (Lahire, 2019).

In other words, Ingrid Levavasseur's precarious economic situation will likely affect her children's academic future and reduce their chances for social success. Even if her daughter and son excel in school, and find some employment with a good salary, it will still take them quite some time to start accumulating their own assets. At the same time, MacKenzie Scott's children will likely have ready access to the best schools and universities. Her three sons and her daughter in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE will probably never need to borrow money to buy a home, to start a business, or to partner in a sound investment and this, even if they have trouble proving their worth in college.

In our book The Gender of Capital we approach the institution of family from a materialistic point of view breaking with the dominant theory of a modern relationship-based family, free of financial stakes. In our perspective, the family should be designed as a unit that produces, circulates, controls and evaluates assets [START_REF] Bessière | Le Genre du Capital[END_REF]. We reconnect with feminist theory born in the wake of the women's liberation movement and inspired by Marxism: materialist feminism. Christine Delphy and Diana Leonard have shown that family wealth in the 1960s was accumulated and transferred to the next generation based on the exploitation of married women's unpaid labor [START_REF] Delphy | Familiar Exploitation. A New Analysis of Marriage in Contemporary Western Societies[END_REF]. Is this still the case today, in contemporary France, a country that celebrates gender equality?

Obviously, Ingrid Levavasseur and MacKenzie Bezos are worlds apart. Nonetheless, there are common points worth noticing between the lives of these two women. When they were part of couples, each found their proper place in a household economy by taking care of the children.

Both women made professional sacrifices, putting off or giving up on projects that had been near to their hearts. Their professional lives were parceled out into a succession of smaller tasks, rather than integrated into a linear career. Both women faced also a challenging divorce surrounded by legal professionals who promoted specific kinds of legal advice. At the very least Ingrid would have had a lawyer, MacKenzie, several. For these women divorce resulted in their relative impoverishment with respect to their former situation. The 100 euros per child per month that Ingrid receives as alimony does not even cover half of the costs for their support and education. Who could possibly house, feed, clothe, care for, and cover all of the other costs of raising a child today in France for 100 euros per month? As for MacKenzie Scott, half-owner of a colossal conjugal fortune at the time of her divorce, she had to content herself with a much smaller portion of that fortune, since the majority went instead to her ex-husband.

On both the highest and lowest rungs of the social ladder, these two women's situations raise fundamental questions. Why is it that in the working class, women are at the forefront of dealing with money problems, while higher up the social ladder, economic power is monopolized by men? Historically, legal discrimination has hindered women from accumulating wealth almost everywhere in the world. In the 19th and 20th centuries, Western societies seemed to have achieved legal gender equality concerning worker's rights, family rights, and property rights. Yet despite these formally equal rights, men still continue to accumulate far more wealth than women.

Women's work, men's salaries

For those who think that this economic inequality is explained by the fact that women earn less than men because they work less than men, it is important to remember that women have always worked as much as men, if not more [START_REF] Kessler-Harris | Women Have Always Worked: A Historical Review[END_REF].

One obvious characteristic of women's work for more than two centuries in a number of economic sectors (starting with agriculture, but also including crafts, commerce, and industry) is its invisibility, in the absence of judicial or financial recognition. Housework, primarily accomplished in family settings by women, is the archetype of unpaid work that never quite gets recognized as such [START_REF] Dalla Costa | The Power of Women and the Subversion of the Community[END_REF][START_REF] Federici | Revolution at Point Zero: Housework, Reproduction, and Feminist Struggle[END_REF]. Domestic production is not counted in the large statistical aggregates that measure production from the perspective of national accounting. National income only includes activities that produce goods and services for commercial exchange, or for those furnished as part of public administration [START_REF] Waring | If Women Counted: a New Feminist Economics[END_REF] preschool Assistant Teacher who takes care of a child contributes to the national income, while a mother who does the same work does not. If household production were to be taken into account, the gross national product (GNP) in 2010 would have been 33% larger in France, 63% larger in the UK, and 43% larger in Germany; and in 2014 in the United States 23% larger [START_REF] Bridgman | Accounting for Household Production in the National Accounts: An Update, 1965-2014[END_REF][START_REF] Poissonnier | Household Satellite Account for France. Methodological Issues on the Assessment of Domestic Production[END_REF]. This unpaid and invisible household production is financed largely by women. In France, in 2010, among couples with infants, women worked on average 54 hours per week: 34 hours of unpaid housework and 20 hours of professional activity. Within the same households, men worked only 51 hours, 3 hours less per week. Men devoted on average 18 hours per week to unpaid housework, and 33 hours to professional activities (Source: the French Time Use Survey). In the end, women worked a bit more, but were paid much less.

These figures established by the French national institute for statistical and economic studies (INSEE) from men's and women's work data, do not account for the fragmentation of women's work, both domestic and professional, which is permanently interrupted because women must always make themselves available to others [START_REF] Oakley | The Sociology of Housework[END_REF][START_REF] Oakley | The Sociology of Housework[END_REF]). Women always carry with them a domestic mental load, even during paid work 5 . Women are the first people contacted by schools and day care centers when children are sick. Women often multitask (doing housework while watching the kids) and must interrupt what they are doing at any moment when the need arises. To the contrary, men's work, whether professional or household (handiwork, repairs, gardening, or maybe cooking) is more clearly delineated in time and space.

Salary inequality thus summarizes a wide range of other inequalities that accumulate in families and in the job market, both at the top and at the bottom of the work hierarchy. Women are concentrated in less well-paid sectors: educational, care giving, and personal assistance professions notably (Ingrid Levavasseur's employment as a care assistant is typical). Because of their family duties, women are often employed in part-time jobs and their careers run on a slower track. Furthermore, glass ceilings prevent them from reaching the best-paying positions (Gustafsson and Meulders, 2000). These factors help to explain why women, in France like elsewhere in the world, earn on average about one quarter less than men do. But even ceteris paribus (for the same age, seniority, job sector, position, years of employment, etc.) the job market still discriminates against women, providing them with a salary 10.5% lower than their male counterparts [START_REF] Silvera | Un Quart en Moins[END_REF]. These persistent inequalities are intertwined with other inequalities that play out in private family life. In France, according to the French national institute for statistical and economic studies, the income of women living in a couple is, on average, 42% less than their partner. In 2011 she earned 16,700 euros while he earned 29,000 euros. This gap in incomes is only 9% between women and men who live alone. Different-sex marital relations endorse existing economic inequalities and then firmly fix them in place.

Today, Western societies would appear to have addressed questions of unequal salaries between men and women with laws focused on professional equality. Alas, even if women were paid with equal salaries for equal work, this still would not resolve everything. There exists an economic inequality between women and men that does not show up on most political and statistical radars, that, nonetheless, structures and summarizes the socioeconomic destinies of individuals, and that is transferred from one generation to the next.

From unequal pay to unequal wealth

To measure that inequality, one must become interested not only in income but also in wealth. At an individual level, what is meant by the terms assets, wealth, and capital (terms that are easily in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE interchanged in contemporary economic literature) is the total value that a person possesses at a given moment. In practice this can be land, real estate, financial assets, or businesses. Wealth consists of economic assets whose acquisition permits the conservation (or accumulation) of their value, and whose final fruition (through sale) can guarantee future cash flows 6 . Individual wealth inequality stems in part from income inequality, but it also depends on the way wealth is transmitted within families.

The investigation of wealth inequality between men and women recently has received more attention. The few statistical analyses currently available show that, throughout the world today, men possess more assets than women [START_REF] Sierminska | Wealth and Gender in Europe[END_REF][START_REF] Chang | Shortchanged: Why Women Have Less Wealth and What Can Be Done About It[END_REF][START_REF] Deere | The Gender Asset Gap: What Do We Know and Why Does It Matter?[END_REF]. This should not be surprising in itself, given the income inequality between the sexes. Yet, in France, according to recent statistical data from the French Household Wealth Survey, the gender wealth gap is widening steadily: from 9% in 1998 to 16% in 2015 [START_REF] Frémeaux | Inequalities and the Individualization of Wealth[END_REF]. The same study also shows that men retain much more capital than women, whatever the form: housing, land, financial, or professional capital. In 2015, the average wealth gap between women and men was estimated at 24,000 euros, covering a wide variety of situations: from modest differences between working-class men and women, as neither partner accumulates much wealth, to immense gender gaps among the wealthiest classes.

Investigating the making of gender wealth inequality in the family

The gender wealth gap does not emerge from Wall Street, but in the daily struggles of family life. This inequality is produced by the unspoken practices of men and women when they act as spouses and partners, fathers and mothers, daughters and sons, brothers and sisters. The inequality takes on different forms based on class: based on whether wealth consists of debts or in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE goods, on whether it includes several thousand euros in a savings account passbook, a suburban bungalow, a Parisian apartment, a family country home, a timeshare, stocks in a company, or works of art. To make the inequality visible, one must look at the family in a different way. One should consider the family as a fully integrated economic institution that produces wealth, but also organizes wealth's circulation, control, and evaluation under what we call family economic arrangements.

As sociologists, we have been studying for more than 20 years these ordinary economic arrangements of French families from the most modest to the wealthiest backgrounds. Barely visible, these arrangements can take many forms: small hand-outs, free lodging, security deposits, interest-free loans, contributions, inheritances, references, college financing, home health care for an aging parent, moving in to help out in an emergency, watching children, paying alimony, and so forth. Family economic arrangements are considered private, and the public discussion of their economic aspects is often frowned on. To study this subject, multiple methodologies and sources of input are necessary.

First, we carried out family monographs based on repeated and intersectional observations and interviews with groups of kin. These relatives invited us to participate in their daily lives, and in their more exceptional family moments: marriage ceremonies, funerals, celebrations. We stayed in their homes. Some of them entrusted us with their most intimate archives: notarized certificates, civil registrations, correspondence, and photographs. By using this method from 1997 to 2005, Céline Bessière studied how family businesses were transferred in the Cognacproducing area. Similarly, Sibylle Gollac investigated real estate property strategies in families from different social backgrounds, several of whom she followed for more than 15 years.

in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE In completing these family monographs, we described family economic transfers in some detail.

We noted that some brothers and sisters recalled quite differently the various stages of estate planning: not counting the same assets, or accounting for them differently, thus proposing extremely divergent conceptions of what a fair inheritance might mean. But family wealth arrangements are about more than money and property. As Viviana Zelizer has noted, these are intimate transactions, that is, a mixing of economic activity with intimate social relations involving emotions, moral obligations, values, principles of justice, and issues about reputations that are all inscribed in a long-term narrative of interpersonal relations [START_REF] Zelizer | The Purchase of Intimacy[END_REF].On the whole, men and women do not occupy the same place in this process: neither acts in the same way or has the same aspirations; and their loved ones do not expect the same things from both of them.

Because family monographs cannot be used to research a large number of people, and because they make it difficult to compare different social classes, we combined them with statistics, particularly data arising from the French Household Wealth Survey. To dig deeper into an analysis of the gender wealth gap, we also implemented field studies to describe two extraordinary moments which clarify and formalize family wealth arrangements: marital separation and estate planning.

The gender of capital

Certain social classes monopolize wealth and work to preserve it among themselves from one generation to the next, while other social classes are persistently deprived of it. At the same time, women accumulate less wealth than men. Class inequality and gender inequality are intertwined.

Studies led in other national contexts than France, particularly in the United States, have also documented a racial dimension to wealth inequality. Age and generation also constitute factors of inequality. Our work thus has an intersectional perspective that articulates without prioritizing several relations of power and domination. In exploring family wealth arrangements, we are studying the concrete places where these different dynamics of inequality inseparably play out.

We show that not only do the wealthy have more assets to pass on from one generation to the next, but also, they pass on these assets in a more efficient way, notably thanks to underestimation and tax optimization techniques. These techniques are all the more effectively implemented by legal professionals because of a class affinity between them and their clients.

This affinity is based both on the size and nature of the economic capital held, but also on the possession of a certain cultural capital. We show that these processes which ensure that wealth remains in the hands of certain families are also gendered. As families and legal professionals strive to preserve real estate and businesses, or to minimize taxes, they produce inventories, estimations and distributions of assets which end up disadvantaging women, even though shares may appear to be formally equal.

Reversed accounting is a common logic of practice, in which the result comes first and in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE computation only after, as a means to legitimize the sharing that has been (forcefully) agreed on [START_REF] Bessière | Reversed Accounting: Legal Professionals, Families and the Gender Wealth Gap in France[END_REF]. Thus, it is not only the wealth of the upper class that is underestimated but more particularly men's wealth.

We conclude that class society reproduces itself thanks to the male appropriation of capital. It is not only that gender inequality is found in all social backgrounds: class relations and male domination are inseparable. The reproduction of the gender order is played out in the processes of conservation and transmission of wealth within the different classes. Conversely, the reproduction of the class order is based on processes of male enrichment and female impoverishment. At a time when family wealth increasingly determines the social status of individuals, gender inequality will not be defeated without tackling class inequality, and class society will not be abolished without equalizing the gender order. in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE 5 . The mental load describes a psychological weight that crops up during both domestic and professional tasks. One must not simply complete the task, but organize when and how to complete it. For instance, while still at work thinking, a mother thinks of what to make for dinner, what groceries will be needed, and when to find the time to cook between picking the kids up at school, taking them to extracurricular activities, and helping them with their homework. 6 . We are working here off of Thomas Piketty's definition in Capital in the Twenty-First Century. Contrary to a classic marxist definition, Piketty does not limit the notion of capital to those elements of wealth used directly in the process of production or for which the owner expects a return. He includes in his definition of capital land and natural resources on which it is possible to exercise property rights, wealth as a value reserve such as gold, or rights to possession and use such as an apartment. His definition of capital is thus a synonym for contemporary economics definitions for wealth and assets.

7 . A description of the research team and its activity can be found here: http://justines.cnrs.fr

  Splitting up. Inheriting: two moments that are strictly legally codified. These are matter of family, fiscal, and civil law. Based on social class, relatives may have to meet with legal professionals who accompany them in a more or less diligent fashion throughout this confrontation with law. Our research thus brought us into other locations: the offices of family practice lawyers and notaries, and civil family courts. Though we separately conducted studies that led to the family monographs, we worked together investigating notaries. As for materials in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies ofProfit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE related to family courts and lawyers, these were collected as a part of a larger collective study begun in 2008 7 .

Notes1

  figure nationale des Gilets jaunes originaire de Pont-de-l'Arche?, Paris-Normandie, 12 January 2019; Ballet V., Ingrid Levavasseur, rond-point en suspension, Libération, 1 April 2019.

  in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies ofProfit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE

  . A in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies of Profit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE

  in Marlène Benquet et Théor Bourgeron (dir.), Accumulating Capital Today. Contemporary Strategies ofProfit and Dispossessive Policies, Londres, Routledge, 2021, pp. 224-234 -VERSION PROVISOIRE