The Primate Diaries

The Primate Diaries

Notes on science, politics, and history from a primate in the human zoo.

Does Nature Need to be Nurtured?


Some say that the differences between boys and girls are just aping nature, but studies of primates tell a more complex story

"Boys will by boys" by Nathaniel Gold

"Boys will by boys" by Nathaniel Gold

“Boys will be boys” is a popular refrain in schools. A bit of rough and tumble at break time? That’s natural. Likewise, “girls will be girls” is accepted without question. Some feel justified in assuming that female students will choose to compete only in small peer groups and will do so by criticizing or excluding one another. Again, this is deemed “natural.”

These preconceptions about the behavioral tendencies of the sexes tend – consciously or unconsciously – to influence how teachers react to students’ actions and thus how behavior management in schools is conducted. But would we respond differently if it could be proved that the behaviors we labelled as natural were nothing of the sort? Could it be that we are encouraging behaviors that are nurtured rather than innate?

Classroom dynamics are complicated and teachers are rightly conflicted about how to proceed when certain gender norms seem pervasive. How do we reconcile nature and nurture when conducting our lessons?

I am the father of a five-year-old boy and have had experience in elementary school classrooms, teaching students aged 4 to 12, from kindergarten through to the 6th grade. You would think that this would provide a lot of answers. But most of what I have learned about childhood behavior comes from the study of primates.

During my PhD in evolutionary anthropology, my research looked at the common features we share with monkeys and apes. Because our species relies so heavily on socialization, it can be difficult to untangle the nature-nurture issue. However, it turns out that we can answer questions about human nature by looking to our closest evolutionary relatives.

The first fact to make clear is that all social mammals play. Chasing, climbing and play fighting are ways for young animals to gain the behavioral and social skills they will need as adults. But to understand why we evolved to “monkey around,” we are best off looking at primates.

Like us, monkeys and apes have a long period of childhood dependency and rely on nuanced social cues to navigate their world as adults. By investigating how primates use social play we can better understand the same behavior in ourselves.

In February this year, the internationally renowned primatologist Jane Goodall joined a team of scientists for a study of play behavior among chimpanzees at the Gombe National Park in Tanzania. Led by Elizabeth Lonsdorf of Franklin and Marshall College, the researchers analyzed the behavior of 20 male and female chimps aged 30-36 months, a period when they begin playing out of arm’s reach of their mothers. By measuring how many individuals the young chimpanzees came into contact with, how they played and who they played with, the researchers were able to find patterns among the sexes.

It turns out that male chimpanzees have a much larger social network than females – nearly twice as large – and are much more likely to engage in rough-and-tumble play, a pattern the researchers note has also been documented in multiple monkey species. The preconceptions of teachers, it would seem, are right.

In their report, the researchers conclude: “While gender socialization in humans may play a role in magnifying the differences between young males and females, these behavioral sex differences are fundamentally rooted in our biological and evolutionary heritage.”

In other words, boys and girls play differently today because gendered behavior allowed more of our ancestors to survive and reproduce.

However, this is only half the story. Humans and chimpanzees are sister species, but we have a rarely acknowledged third sibling known as the bonobo. All three shared a common ancestor approximately 6 million years ago. Most studies looking to understand the evolution of human behavior use chimpanzees as a model to explain who we were and how we came to be the way we are today. But if we want to understand whether certain behaviors are natural for humans, we need to compare ourselves to both our close relatives.

It turns out that when we include bonobos along with chimpanzees, we get a very different picture about the potential range of natural behavior. In 2006 at the University of Pisa in Italy, Elisabetta Palagi conducted a study comparing the social play between bonobos and chimps that turned the gender question on its head. What she found was that bonobos engaged in significantly more rough-and-tumble play than chimpanzees and that young females were the roughhousing champions, displaying the behavior more than twice as often as males. So are females just as naturally wired for this form of play?

The answer may have less to do with gender itself and more to do with the mechanisms of gendered behavior. The hormone testosterone has a catalyzing effect on behavior, analogous to the way that yeast is required for bread to rise.

Experiments with monkeys have shown that injecting testosterone into the uterine environment during the late stages of pregnancy causes females to engage in rough play at nearly the same rate as males. During this period of fetal development, testosterone triggers a cascade of brain organization that permanently alters the range of behaviors that an individual will engage in. Too much testosterone will result in the masculinization of female sexual anatomy, but the right amount at this key stage is required for certain gendered behaviors to emerge in the first place. Eliminate testosterone entirely and rough play won’t arise in either sex.

It is worth noting that it is known that stress causes an increase in circulating testosterone levels for pregnant mothers in all primate species.

Last year the Harvard evolutionary biologist Victoria Wobber revealed that testosterone production in infants was the key to explaining the difference between certain chimpanzee and bonobo behaviors. She found that bonobos – both male and female – had significantly higher testosterone levels than chimpanzees throughout their childhoods. Chimp males will experience a spike in testosterone at puberty (just like human males) that more than doubles the amount of this circulating hormone. But bonobos have significantly higher testosterone levels until then, which seemingly explains why they are more inclined to roughhousing.

Wobber et al. (2013)

Testosterone levels in chimpanzees and bonobos at different developmental stages. Figure from Wobber et al. (2013)

This all suggests that so-called natural behavior for boys and girls is highly flexible, depending on their levels of testosterone.

Interestingly, however, like Palagi, Wobber also notes that female bonobos will consistently outrun, outclimb and outcompete their male counterparts. Yet both male and female bonobos have about the same testosterone levels throughout infancy and adulthood, revealing that this hormone promotes but is not the exclusive cause of female dominance in this species.

So the study of primates does not give us an unequivocal answer about the root cause of certain behaviors, but it can at least teach us that how much, or how little, a child chooses to engage in certain behaviors may be determined less by sex than the biological environment they are exposed to.

It should also remind us that myriad influences shape a child’s personality, so we shouldn’t assume that gender is a fixed variable when considering our strategies for managing behavior. Teachers need to be careful not to reinforce ideas about actions deemed natural to a certain sex. Nature is, of course, an important component of who we are as a species, but the right environment is just as crucial.

Cross-posted at Times Education Supplement


Lonsdorf, E V, Anderson, K E, Stanton, M A et al (2014) “Boys will be boys: sex differences in wild infant chimpanzee social interactions,” Animal Behaviour, 88: 79-83.

Palagi, E (2006) “Social play in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): implications for natural social systems and interindividual relationships,” American Journal of Physical Anthropology, 129: 418-426.

Wallen, K (2005) “Hormonal influences on sexually differentiated behavior in nonhuman primates,” Frontiers in Neuroendocrinology, 26/1: 7-26.

Wobber, V, Hare, B, Lipson, S, Wrangham, R, Ellison, P (2013) “Different ontogenetic patterns of testosterone production reflect male reproductive strategies in chimpanzees and bonobos,” Physiology and Behavior, 116-117: 44-53.

The views expressed are those of the author and are not necessarily those of Scientific American.

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