Sexe, genre et évolution : dimorphisme sexuel

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Evolution du dimorphisme de poids et de taille

Autres dimorphismes

Sex differences in the pelvis did not evolve de novo in modern humans [PDF]
Fischer et al.
Nature ecology & evolution, 2021

It is commonly assumed that the strong sexual dimorphism of the human pelvis evolved for delivering the relatively large human foetuses. Here we compare pelvic sex differences across modern humans and chimpanzees using a comprehensive geometric morphometric approach. Even though the magnitude of sex differences in pelvis shape was two times larger in humans than in chimpanzees, we found that the pattern is almost identical in the two species. We conclude that this pattern of pelvic sex differences did not evolve de novo in modern humans and must have been present in the common ancestor of humans and chimpanzees, and thus also in the extinct Homo species. We further suggest that this shared pattern was already present in early mammals and propose a hypothesis of facilitated variation as an explanation: the conserved mammalian endocrine system strongly constrains the evolution of the pattern of pelvic differences but enables rapid evolutionary change of the magnitude of sexual dimorphism, which in turn facilitated the rapid increase in hominin brain size.

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Bilateral asymmetry and developmental plasticity of the humerus in modern humans [PDF]
Zelazny et al.
American journal of physical anthropology, 2021

Hunter‐gatherers from Indian Knoll have significantly higher levels of asymmetry than other groups and are more sexually dimorphic, particularly in cross‐sectional properties of the diaphysis.
[…] There is significant sexual dimorphism present in the Indian Knoll sample, but females from Indian Knoll also show markedly higher bilateral asymmetry for Z p at 40% compared to females of most other samples. It is perhaps notable then, that almost a quarter of the atlatls found at Indian Knoll were found in the graves of females (Webb, 1946)

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Impact Protection Potential of Mammalian Hair: Testing the Pugilism Hypothesis for the Evolution of Human Facial Hair [Texte]
Beseris et al. (Carrier)
Integrative organismal biology, 2021

Because facial hair is one of the most sexually dimorphic features of humans (Homo sapiens) and is often perceived as an indicator of masculinity and social dominance, human facial hair has been suggested to play a role in male contest competition. Some authors have proposed that the beard may function similar to the long hair of a lion’s mane, serving to protect vital areas like the throat and jaw from lethal attacks. This is consistent with the observation that the mandible, which is superficially covered by the beard, is one of the most commonly fractured facial bones in interpersonal violence. We hypothesized that beards protect the skin and bones of the face when human males fight by absorbing and dispersing the energy of a blunt impact.

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Sexual Dimorphism in the Functional Development of the Cochlear Amplifier in Humans [Texte]
Mishra et a
Ear and hearing, 2021

Male children showed a significant reduction in otoacoustic emission magnitudes with age, whereas female children did not show any such changes. Females showed higher stimulus-frequency otoacoustic emission magnitudes compared with males. However, the effect size of sex differences in young adults was larger compared with children. Unlike the otoacoustic emission magnitude, the noise floor did not show sexual dimorphism; however, it decreased with age.

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Female climbers are some of the best in the world irrespective of gender, a trend that is not found in any other major sport. I conclude that the exceptional ability of female rock climbers relative to male rock climbers is further evidence of the existence of sex-blind musculoskeletal adaptations, which developed over the course of human evolution – as a result of external (non-sexual) selection forces – to facilitate essential movements. These adaptations abate some of the general physical sexual dimorphism which exists in humans. This paper provides more evidence that the human body was shaped, in part, by pressure to climb well.

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Expanding the evolutionary explanations for sex differences in the human skeleton [PDF]
Holly M. Dunsworth
Evolutionnary anthropology, 2020

While the anatomy and physiology of human reproduction differ between the sexes, the effects of hormones on skeletal growth do not. Human bone growth depends on estrogen. Greater estrogen produced by ovaries causes bones in female bodies to fuse before males’ resulting in sex differences in adult height and mass. Female pelves expand more than males’ due to estrogen and relaxin produced and employed by the tissues of the pelvic region and potentially also due to greater internal space occupied by female gonads and genitals.

Is there anything wrong with the estradiol explanation of human
SSD? We don’t think so. But Dunsworth takes the argument one step
further and claims that accepting the estradiol explanation should stir skepticism about an alternative explanation based on sexual selection

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Sexual dimorphism in human arm power and force: implications for sexual selection on fighting ability
Morris et al. (Carrier)
Journal of experimental biology, 2020

our results indicate the presence of pronounced male-biased sexual dimorphism in muscle performance for protracting the arm to propel the fist forward. We also compared overhead pulling force between males and females, to test the alternative hypothesis that sexual dimorphism in the upper body of humans is a result of selection on male overhead throwing ability. We found weaker support for this hypothesis, with less pronounced sexual dimorphism in overhead arm pulling force. The results of this study add to a set of recently identified characters indicating that sexual selection on male aggressive performance has played a role in the evolution of the human musculoskeletal system and the evolution of sexual dimorphism in hominins.

Evolution built men to pack a punch [Texte]
Kathrin Knith
Journal of experimental biology, 2020
(vulgarisation de l’article de Morris et al.)

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The sex with the reduced sex chromosome dies earlier: a comparison across the tree of life [Texte]
Xirocostas et al.
Royal society biology letters, 2020

Surprisingly, we found substantial differences in lifespan dimorphism between female heterogametic species (in which the homogametic sex lives 7.1% longer) and male heterogametic species (in which the homogametic sex lives 20.9% longer). Our findings demonstrate the importance of considering chromosome morphology in addition to sexual selection and environment as potential drivers of sexual dimorphism

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The Performance Gap in Sport Can Help Determine Which Movements Were Most Essential to Human Evolution [Texte]
Colin Carroll
Frontiers in physiology, 2019

This difference suggests that general sexual dimorphism does not explain why female performance is relatively closer to male performance at some track and field events than others. We hypothesize that this trend can be explained by the presence of sex-blind musculoskeletal adaptations (SBMA’s), which accumulate over generations to reduce the size of the PG in certain movements. We conclude that the selection trend favoring in humans should be explored further to determine whether the PG in sport can indeed be used to determine movements to which the human body is adapted.

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Cochlear shape reveals that the human organ of hearing is sex-typed from birth [Texte]
Braga et al.
Nature scientific reports, 2019

We observe a sex-differentiated torsion along the 3D cochlear curve in samples of 94 adults and 22 juvenile skeletons from cross-cultural contexts. The cochlear sexual dimorphism measured in our study allows sex assessment from the human skeleton with a mean accuracy ranging from 0.91 to 0.93 throughout life. We conclude that the human cochlea is sex-typed from an early post-natal age.

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Sexual Dimorphism in Hominin Ancestors
J. Michael Plavcan
The international encyclopedia of anthropology, 2018

The fossil record suggests that the earliest known hominin—Ardipithecus ramidus—showed little size or canine dimorphism. Later australopithecines show modest to strong dimorphism, with dimorphism approaching modern human levels in Homo erectus and Homo heidelbergensis. Behavioral inferences based on dimorphism are uncertain, but strong dimorphism in Australopithecus suggests polygynous mating, with strong male competition. Changes in dimorphism and body size suggest changes in female life history and associated patterns of male competition. Finally, the reevolution of dimorphism in hominins suggests that the modern human condition is derived and not inherited from the common ancestor of Homo and Pan.

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On The Evolution of The Sex Differences in Throwing: Throwing is a Male Adaptation in Humans [PDF]
Lombardo & Deaner
The quarterly review of biology, 2018

The development of the ability to throw projectiles for distance, speed, and accuracy was a watershed event in human evolution. We hypothesize that throwing first arose in threat displays and during fighting and later was incorporated into hunting by members of the Homo lineage because nonhuman primates often throw projectiles during agonistic interactions and only rarely in attempts to subdue prey. Males, who threw more often than females in both combat and hunting, would have been under stronger selection than females to become proficient at the ability to throw, intercept, and dodge projectiles as throwing skills became critical to success in combat and hunting. Therefore, we predict that males, more than females, should display innate anatomical and behavioral traits associated with throwing. We use data from a variety of disciplines to discuss: the sex differences in throwing speed, distance, and accuracy; sex differences in the development of the throwing motion; inability of training or cultural influences to erase the sex differences in throwing; sex differences in the use of throwing in sports, combat, and hunting; and sex differences in anatomical traits associated with throwing that are partly responsible for male throwing superiority. These data contradict the view held by many commentators that socialization rather than innate sex differences in ability are primarily responsible for male throwing superiority. We suggest that throwing is a male adaptation.
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Sexual Dimorphism [PDF]
Mori et al.
Encyclopedia of Animal Cognition and Behavior, 2017

Homo sapiens shows an adult male-to-female body weight of about 1.1–1.2; on average, males are taller than females, with a ratio of 1.06 ; they are also more muscular, have larger heart and lungs, a greater number of red cells in blood, and a beard (Dixon2009). Also, males with a broad chest and shoulders, narrow hips, and a muscular torso tend to be considered attractive partners by females, so that sexual selection comes into play. Conversely, females invest more in the production and storage of fat, mainly deposited around buttocks, hips, thighs, and breast, as a result of estrogen action, when they become mature reproductively (Pond1998). In fact, fat reserves are crucial for reproduction and it makes sense their positive selection in evolutionary terms, as they are honest signals of reproductive potential. There are quite a few other signs of sexual dimorphism in Homo sapiens, relevant to skin color, craniofacial traits, and voice pitch, some of them dependent on differential hormone actions during development. All these features make the human species a remarkably dimorphic one.

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The literature on human sexual selection has historically focused on the role of female mate choice, but cumulating experimental, correlational, and cross-cultural evidence suggests that male contest competition may have been more influential in shaping men’s phenotypes. Cross-species comparison has shown similarities between humans and our closest extant phylogenetic relatives, the Great Apes, in male–male aggression, and archeological evidence also indicates a great antiquity for male–male violence. Compared to women, men possess substantially greater muscle mass, strength, cranial robusticity, physical aggression, pain tolerance, risk-taking, weapons use, and participation in coalitional aggression. Men also exhibit displays of physical prowess and acuity to the formidability of male conspecifics, as well as possessing a suite of traits, such as facial hair and low vocal pitch, that increase perceptions of dominance. These traits are consistent with having been shaped by contest competition over mates: they are sexually dimorphic, appear at sexual maturity, and predict success in male contests as well as success in mating and reproduction. While alternative explanations for some of these sexually dimorphic traits are possible, contest competition among males throughout human evolutionary history is the most parsimonious.

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Sexual competition among women : a review of theory and supporting evidence [Google books]
Arnocky & Vaillancourt
The oxford handbook of women and competition, 2017

Arnocky 2017 sexual competition among women

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The impact of subsistence changes on humeral bilateral asymmetry in Terminal Pleistocene and Holocene Europe [PDF]
Slàdek et al.
Journal of human evolution, 2016

In males, a major decline in asymmetry occurs between the Upper Paleolithic and Mesolithic. There is no further decline in asymmetry
between the Mesolithic and Neolithic in males and only limited variation during the Holocene. In females, a major decline occurs between the Mesolithic and Neolithic, with resulting average directional asymmetry close to zero. Asymmetry among females continues to be very low in the subsequent Copper and Bronze Ages, but increases again in the Iron Age. Changes in female asymmetry result in an increase of sexual dimorphism during the early agricultural periods, followed by a decrease in the Iron Age.

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Sexual selection on male vocal fundamental frequency in humans and other anthropoids [PDF]
Puts et al.
Proceedings of the Royal Society B, 2016

In many primates, including humans, the vocalizations of males and females differ dramatically, with male vocalizations and vocal anatomy often seeming to exaggerate apparent body size. These traits may be favoured by sexual selection because low-frequency male vocalizations intimidate rivals and/or attract females, but this hypothesis has not been systematically tested across primates, nor is it clear why competitors and potential mates should attend to vocalization frequencies. Here we show across anthropoids that sexual dimorphism in fundamental frequency (F₀) increased during evolutionary transitions towards polygyny, and decreased during transitions towards monogamy. Surprisingly, humans exhibit greater F₀ sexual dimorphism than any other ape. We also show that low-F₀ vocalizations predict perceptions of men’s dominance and attractiveness, and predict hormone profiles (low cortisol and high testosterone) related to immune function. These results suggest that low male F₀ signals condition to competitors and mates, and evolved in male anthropoids in response to the intensity of mating competition.

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Les chimpanzés mâles commettent plus de meurtres que les chimpanzés femelles. Les victimes sont plus souvent des mâles, mais les chimpanzés mâles tuent aussi des femelles, généralement des femelles d’autres groupes, ou des femelles immatures de leur propre groupe (infanticide).

Lethal aggression in Pan is better explained by adaptive strategies than human impacts [PDF]
Wilson et al.
Nature, 2014

Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing. Two kinds of hypothesis have been proposed. Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates^1,2,3,4,5. Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning^6,7,8,9. To discriminate between these hypotheses we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades. Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos. We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio). Variation in killing rates was unrelated to measures of human impacts. Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported.

A rapprocher de ce qu’on trouve chez les humains

Global study on homicides [PDF]
UNODC, 2013

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This indicates that males, but not females, preferentially employed movements involving throwing motions in these hunter-gatherer and early farming groups. Based on this evidence we postulate the existence of a persistent sexual division of labour in these prehistoric European populations involving one or several strenuous activities linked to unilateral limb use.

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The importance of physical strength for males [PDF]
Sell et al.
Human nature, 2012

Fighting ability, although recognized as fundamental to intrasexual competition in many nonhuman species, has received little attention as an explanatory variable in the social sciences. Multiple lines of evidence from archaeology, criminology, anthropology, physiology, and psychology suggest that fighting ability was a crucial aspect of intrasexual competition for ancestral human males, and this has contributed to the evolution of numerous physical and psychological sex differences. Because fighting ability was relevant to many domains of interaction, male psychology should have evolved such that a man’s attitudes and behavioral responses are calibrated according to his formidability. Data are reviewed showing that better fighters feel entitled to better outcomes, set lower thresholds for anger/aggression, have self-favoring political attitudes, and believe more in the utility of warfare. New data are presented showing that among Hollywood actors, those selected for their physical strength (i.e., action stars) are more likely to believe in the utility of warfare.

Men physical strenght

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Osteological clues for the presence of a sexual division of labour in the Eurasian Middle Paleolithic [Abstract]
Japp Saers
Leiden university, 2011

In this bachelor thesis I investigate whether Neanderthals had a sexual division of labour or not. I established three hypotheses: 1) Neanderthals had a sexual division of labour where males hunt and females gather plant foods and perform other activities, 2) Neanderthals had a sexual division of labour where males and females hunt but males perform the most dangerous tasks, 3) there was no sexual division of labour and males and females hunted and gathered in equal amounts. To find out if Neanderthals had a sexual division of labour, a meta-study of two osteological analyses applied to Neanderthal bones was performed. The first methods that was used was a comparison of the shape and robusticity of male and female Neanderthal limb bones compared to samples of modern human hunter-gatherers and sedentary populations. Secondly the distribution of trauma across the skeletons of male and female Neanderthals was compared. In both of the analyses the evidence pointed towards the first hypothesis. The evidence however was too limited. The small sample size of sexable Neanderthals was the largest issue. I concluded that according to the data gathered in this thesis hypothesis 1 is the most likely. However, none of the three hypotheses can be rejected confidently due to the limited evidence.

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With growing recognition that there are universal sex differences in cognition and behavior, four theories have been proposed to account for these differences: the founder effect theory, the social structuralist theory, the evolutionary theory, and the evolutionary neuroandrogenic (ENA) theory. The latter of these theories is described in considerable detail as offering an explanation for most of 65 recently identified apparent universal sex differences (AUSDs) in cognition and behavior. Regarding “ultimate causes” (why), ENA theory asserts that (a) evolutionary-genetic factors incline females to bias their mate choices toward males who are loyal and competent provisioners of resources and (b) males are merely a genetic variant on the female sex selected for responding to female mating biases. In terms of “proximate causes” (how), the theory maintains that high exposure to androgens has evolved to alter the male brain functioning in two specific ways relative to most female brains: (a) suboptimal arousal and (b) a rightward shift in neocortical functioning. These two functional patterns are described and hypothesized to incline males and females to learn differently in many respects. The most fundamental differences involve males learning ways of either complying with or circumventing female mate preferences. Numerous universal sex differences in cognition and behavior are hypothesized to result from these evolved neurohormonal factors, including most of the 65 AUSDs herein summarized in seven categorical tables.

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Sex differences in children’s toy play are robust and similar across cultures 1, 2. They include girls tending to play more with dolls and boys more with wheeled toys and pretend weaponry. This pattern is explained by socialization by elders and peers, male rejection of opposite-sex behavior and innate sex differences in activity preferences that are facilitated by specific toys [1]. Evidence for biological factors is controversial but mounting. For instance, girls who have been exposed to high fetal androgen levels are known to make relatively masculine toy choices [3]. Also, when presented with sex-stereotyped human toys, captive female monkeys play more with typically feminine toys, whereas male monkeys play more with masculine toys [1]. In human and nonhuman primates, juvenile females demonstrate a greater interest in infants, and males in rough-and-tumble play. This sex difference in activity preferences parallels adult behavior and may contribute to differences in toy play [1]. Here, we present the first evidence of sex differences in use of play objects in a wild primate, in chimpanzees (Pan troglodytes). We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males.

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Male chimpanzees, Pan troglodytes, differ from males in most other mammalian taxa because they remain in their natal communities throughout their lives, form close bonds with one another, and cooperate in a range of activities [1]. However, males also compete fiercely for status within their groups 2, 3, and high rank enhances male reproductive success 4, 5. Males rely partly on coalitions to achieve and maintain status 2, 3, 6, 7, 8, 9, and shifts in male alliances can have dramatic political effects 2, 3, 6. It is not known what benefits are obtained by low-ranking coalition partners. Here we report that the highest-ranking (alpha) male in one well-studied community of chimpanzees rewarded his allies by allowing them preferential access to mates.

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Sexual dimorphism in primate evolution [PDF]
J. Michael Plavcan
American journal of physical anthropology, 2002

In contrast to the canine teeth, dimorphism inhominid body mass was apparently quite substan-tial. McHenry (1992, 1994a,b) provides data sug-gesting that body mass dimorphism in early homin-ids ranged from comparable to a chimpanzee inAustralopithecus robustus,to quite gorilla-like inA.boisei(Plavcan and van Schaik, 1997a). Interest-ingly, the data of McHenry (1992, 1994a,b) suggestthatHomo erectus,considered as a single species,could have shown substantial body size dimorphism.However, if traditionalH. erectusis split intoH.erectus sensu strictoandH. ergaster,then the levelsof dimorphism are reduced to levels more compara-ble to modern humans. Regardless, analyses of theface and skeletal elements of early hominids all sug-gest that body mass dimorphism in these animalswas substantial

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(Cité par Priscille Touraille)

Archaeological data are frequently cited in support of the idea that big game hunting drove the evolution of early Homo, mainly through its role in offspring provisioning. This argument has been disputed on two grounds: (1) ethnographic observations on modern foragers show that although hunting may contribute a large fraction of the overall diet, it is an unreliable day-to-day food source, pursued more for status than subsistence; (2) archaeological evidence from the Plio-Pleistocene, coincident with the emergence of Homo can be read to reflect low-yield scavenging, not hunting. Our review of the archaeology yields results consistent with these critiques: (1) early humans acquired large-bodied ungulates primarily by aggressive scavenging, not hunting; (2) meat was consumed at or near the point of acquisition, not at home bases, as the hunting hypothesis requires; (3) carcasses were taken at highly variable rates and in varying degrees of completeness, making meat from big game an even less reliable food source than it is among modern foragers. Collectively, Plio-Pleistocene site location and assemblage composition are consistent with the hypothesis that large carcasses were taken not for purposes of provisioning, but in the context of competitive male displays. Even if meat were acquired more reliably than the archaeology indicates, its consumption cannot account for the significant changes in life history now seen to distinguish early humans from ancestral australopiths. The coincidence between the earliest dates for Homo ergaster and an increase in the archaeological visibility of meat eating that many find so provocative instead reflects: (1) changes in the structure of the environment that concentrated scavenging opportunities in space, making evidence of their pursuit more obvious to archaeologists; (2) H. ergaster‘s larger body size (itself a consequence of other factors), which improved its ability at interference competition.

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Fast and accurate throwing was undoubtedly important to ancestral hominids, and was subject to sexual-selection pressures that generated a male advantage in throwing accuracy that persists in modern humans. The balance of evidence, including that from a recent comparative study of throwing in humans and capuchin monkeys, suggests that high-performance throwing involves unique adaptations in the domains of spatial targeting, precision timing, and multi-joint motor control.

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(cité par Priscille Touraille)

Late in the Pleistocene the development of food processing techniques led to the reduction of both male and female dental dimensions. Dental sexual dimorphism, however, was maintained until the very end of the Pleistocene when the hunting of large game animals by crude techniques was replaced by a focus on great numbers of small game caught by more sophisticated means and by an increasing utilization of plant foods. The subsequent reduction in dimorphism represents the actions of the Probable Mutation Effect operating under conditions of relaxed selection. The conclusion offered is that the smallest degree of sexual dimorphism visible in the modern world is to be found among those populations that are separated by the greatest interval of time from precursors who depended for their survival on a Pleistocene big game hunting mode of subsistence.

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Divers visuels non issus de publications scientifiques

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Différences physiques entre hommes et femmes

Evolution du dimorphisme de poids et de taille

Autres dimorphismes

Divers visuels non issus de publications scientifiques

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