The Surprising Roots of Primate Sexuality: What Baby Monkeys’ Behavior Reveals About the Origins of Sexual Behavior
– by Irene Delval and Solimary García Hernández
Traditionally, scientific and popular assumptions about sexual behavior have struggled to decouple it from its reproductive purpose. From primates to nematodes, sexual behaviors that didn’t lead to conception were often dismissed as errors, curiosities, byproducts, or evolutionary leftovers.
Recently, however, this perspective has shifted. In Western societies, as social standards have changed and the visibility of LGBTQIAPN+ communities has grown, science has moved away from restrictive moral ideals to explore the diverse functions of sexuality. The scientific community now recognizes that across many species, including ours, sexual behavior serves a range of social functions like bonding, conflict resolution, reconciliation, or relationship maintenance. This has fueled growing interest in non-conceptive sexual behavior. Much of the research in this area has focused on Same-sex Sexual Behavior (SSB). Once framed as a Darwinian paradox, SSB is now understood as a functional and adaptive component of mammalian social systems. We now know that SSB is a prevalent trait in primates, driven by ecological and social pressures in socially-living species.
Yet, one form of non-conceptive sex remains largely unexplored: the sex-like behaviors that happen before sexual maturity, in prepuberal subjects, long before individuals develop reproductive drives. This begs a major question: how and when do these behaviors first emerge? Understanding this timeline can tell us a lot about their evolved functions.
In our new research published in Evolution and Human Behavior (March, 2026), we explored how sexual behavior emerged during the first year of life in two species of wild robust capuchin monkeys (Sapajus libidinosus and S. xanthosternos). By following 16 wild infants longitudinally in Brazil, our team provided a rare window into the development of sexual behavior under natural conditions. Our findings flip the traditional view that young primates (including humans) engage in sexual behavior merely to practice for adult reproduction, providing strong evidence for the growing consensus among researchers that, across primate species, sexual interactions help animals navigate their social worlds long before they can reproduce.
Starting from Month One
The biggest surprise was just how early these behaviors begin. Both male and female infants exhibited socio-sexual behaviors (i.e., behavior sexual in form but serve but serve social functions), such as mounting, genital inspection, and courtship gestures, within their first month of life. This remarkably early onset parallels observations in humans and shows that sexual behavior doesn’t simply lie dormant until puberty. Instead of appearing suddenly at sexual maturity, it seems to be part of the behavioral repertoire from the very beginning, at least in its early-developing form.
Even though the two capuchin species live in very different environments (one in semi-arid scrubland, the other in tropical rainforest), their developmental trajectories were similar. Among a repertoire of 25 analyzed sexual behaviors, both species and sexes shared the six most frequently performed behaviors. However, although males and females initially performed the same types of behaviors, like eyebrow raising, chest rubbing or mounting, which are usually performed during adult heterosexual courtships, sex differences emerged as the infants grew.
Early and Marked Sex Differences
Male infants showed higher rates of sexual behavior, greater behavioral diversity, and an earlier onset compared to females. The increase in the emergence of sexual behavior over time was also steeper in males, indicating a sex-specific developmental trajectory. These findings align with research in other primates, such as macaques, and suggest that sex differences in sociosexual behavior emerge early in ontogeny, likely shaped by genetic and hormonal effects right from the start.
What makes this particularly fascinating is that in adult robust capuchins, courtship is predominantly female-led. Adult females are the proactive agents, using elaborate facial expressions and even throwing stones to get the attention of often-indifferent males. If the classic “practice hypothesis” were true, which assumes infants perform early forms of these behaviors to rehearse their future adult roles, we would expect infant females to be the ones practicing the most.
However, our data revealed the exact opposite. There were large differences in the directionality of sexual behaviors. Infant males engaged in these interactions far more often, while females were usually the receivers of such behaviors. This points to a male-specific developmental pathway of arousal that clearly doesn’t just mirror adult courtship roles.
Different Partners, Different Goals
We also found a clear divide in who the infants chose to interact with. Infant males acted as sexual “extroverts,” engaging with a wide range of partners. Most notably, their most frequent partners were adult males, making same-sex interactions very common among them. Infant females, on the other hand, had much narrower preferences, mostly interacting sexually with other infants of both sexes. This divergence suggests something important: early sexual behavior likely serves completely different functions depending on the sex.
For males, these interactions might help establish social bonds, navigate dominance hierarchies with older males, or regulate arousal in complex social settings. For females, these behaviors, at this stage, seem more embedded in general peer play and social exploration. We need to further understand when sexual “awakening” occurs in female robust capuchins.
Why This Matters
Our research highlights that primate sexuality is flexible and multifaceted right from birth. Because sex differences emerge so early and don’t simply mirror adult roles, these findings open up new questions about the true origins of sexual behavior in all primates, humans included. If we want to understand the “why” of adult behavior, we first need to look closely at the “how” in the infant’s world.
Just as grooming behavior, which goes far beyond hygiene and removing parasites to act as a social glue, early sexual behavior seems relevant for building and maintaining social bonds, and canalizing arousal, rather than just preparing individuals for reproduction. Our findings also point to a broader issue: studying infant sexuality remains a highly sensitive and often avoided topic, especially in humans. This is where research on non-human primates becomes so valuable, giving us a comparative window to investigate these processes rigorously and without cultural constraints.
