Sex differences in the behavior of nonhuman primates

An evolutionarily conserved sexual signature in the primate brain

The question of a potential biological sexual signature in the human brain is a heavily disputed subject. In order to provide further insight into this issue, we used an evolutionary approach to identify genes with sex differences in brain expression level among primates. We reasoned that expression patterns important to uphold key male and female characteristics may be conserved during evolution. We selected cortex for our studies because this specific brain region is responsible for many higher behavioral functions. We compared gene expression profiles in the occipital cortex of male and female humans (Homo sapiens, a great ape) and cynomolgus macaques (Macaca fascicularis, an old world monkey), two catarrhine species that show abundant morphological sexual dimorphism, as well as in common marmosets (Callithrix Jacchus, a new world monkey) which are relatively sexually monomorphic. We identified hundreds of genes with sex-biased expression patterns in humans and macaques, while fewer than ten were differentially expressed between the sexes in marmosets. In primates, a general rule is that many of the morphological and behavioral sexual dimorphisms seen in polygamous species, such as macaques, are typically less pronounced in monogamous species such as the marmosets. Our observations suggest that this correlation may also be reflected in the extent of sex-biased gene expression in the brain. We identified 85 genes with common sex-biased expression, in both human and macaque and 2 genes, X inactivation-specific transcript (XIST) and Heat shock factor binding protein 1 (HSBP1), that were consistently sex-biased in the female direction in human, macaque, and marmoset. These observations imply a conserved signature of sexual gene expression dimorphism in cortex of primates. Further, we found that the coding region of female-biased genes is more evolutionarily constrained compared to the coding region of both male-biased and non sex-biased brain expressed genes. We found genes with conserved sexual gene expression dimorphism in the occipital cortex of humans, cynomolgus macaques, and common marmosets. Genes within sexual expression profiles may underlie important functional differences between the sexes, with possible importance during primate evolution.

The contribution of genetics versus environment to behavioral differences between the sexes is a fundamental question in neuroscience. We hypothesized that some differences between the sexes might be partially explained by sexually dependent gene expression differences in the brain. We further speculated that if differences in gene expression between males and females are functionally important, they may be conserved in the evolution of primates. To test these hypotheses, we measured gene expression in the brains of male and female primates from three species: humans (Homo sapiens), macaques (Macaca fascicularis), and marmosets (Callithrix jacchus). Our results point to a conserved signature of sexual gene expression dimorphism in the brains of primates. Interestingly, we found that genes with conserved sexual gene expression dimorphism in the brain also evolve under more evolutionary constraint, compared with other genes, suggesting that they may have important roles during evolution of sex in primates. Moreover, we found higher evolutionary constrains in the coding regions of female-biased genes as compared to both male-biased and non sex-biased brain expressed genes. The study of sex dimorphic genes may in the future shed light on the basis of psychiatric diseases with differences in prevalence between the sexes.

Socialization processes in primates: use of multivariate analyses II-Influence of sex on social development of captive rhesus monkeys

In a previous study, we demonstrated the importance of social interactions in the development of non-human primate infants. These results confirmed the social network concept. According to this concept, it is assumed that socialization processes would differ in various social environments. However, much variability remained to be explained. In the present study we investigated especially the influence of infants' gender on socialization processes. In relation to the previous results, the influence was tested within groups having the same social structure. At the Wisconsin Regional Primate Research Center, four mother-peer groups of rhesus macaques (Macaca mulatta) were studied. Therefore no adult male model was available to infants. Twenty infants, including eight males, eight females and four prenatally DES-treated females, were the subjects of this study. We considered six comprehensive developmental parameters to account for the two main socialization processses, the acquisition of the social behavioral repertoire and the establishment of a network of social relationships. We analyzed the variability of these parameters using a new multivariate technique previously described in a companion paper. This new technique is derived from the Principal Components Analysis and Multivariate Analyses of Variance. The model used in this analysis included two intrinsic features of infants, sex and age and two variables related to the social organization of the groups, mother's rank and social group. This 4-variable-model significantly accounted for most of the variability of the developmental parameters. The results showed that the social group and the infant's sex were the most influential variables when infants initiated interactions: male and DES female infants displayed a greater behavioral diversity than non-treated female infants. When infants were recipients in interactions, social organization variables (social group and mother's rank, respectively) had the greatest influence on the socialization processes while infants' gender was less influential. In this case the effect of sex was confounded with the social group effect; male infants then differed from both kind of female infants in being looked at more frequently, in being contacted by a larger variety of partners and in receiving a greater variety of social behaviors. This new example of applying multivariate methods to the study of individual social development confirms the potential of this method to provide new insights into behavioral development.