What can the ape world teach us about gender and biology?

As I said, brave. Whether he’s convincing is another matter.

Most of “Different” is devoted to our closest evolutionary cousins, the bonobos and chimpanzees, but it includes instructive and entertaining examples from other primates too, such as baboons, macaques, spider monkeys and the fatherly cotton-topped tamarin. Based on his studies, de Waal’s central premise is that males and females behave differently and that those behavioral differences have some basis in biology. He is not the first to propose this, but as a biologist who studies animal behavior, I was curious how de Waal presented his observations and what new evidence he could bring to a discussion that has become deeply politicized.

To understand de Waal’s premise, we first must wrestle with some dichotomies that are notoriously muddy. One is the distinction between biology and culture. To neutralize this, de Waal turns to Hans Kummer’s analogy of drum and drummer. If you hear the sound of drumming, is it coming from the drum, or the drummer? The answer is obviously both; neither alone makes a sound. Likewise, human behavior is a result of the interaction between genes and the environment, between biology and culture, the drum and the drummer. But if we take the analogy a step further and ask, if we hear drumming from two places and they differ in their sound, is it because of a difference in the drum, or the drummer?

De Waal takes issue with those who argue that culture — let’s call that the drummer — is solely responsible for the differences we hear in the “sound” of gender. As a primatologist, he argues for a role of the drum. He points to differences in the behavior of male and female primates as evidence that biology explains more of human gender differences than many would like to believe. That is not necessarily a baseless argument, but de Waal never provides a concrete definition of “biology” for us to consider. It’s safe to assume it has something to do with genetics, but his clear definition of gender, as “the culturally circumscribed role and position of each sex in society,” seems to reject a role for genetics even as he asserts its biological basis.

That left me confused. What exactly is the drum in this scenario, and how might it affect the expression of gender? An explicit definition of biology, by way of hypothetical genetic mechanisms, would have gone a long way in supporting the central premise.

Another dichotomy that gets muddy is the distinction between males and females. De Waal provides a medical definition of sex, based on chromosomes — “XX for female, XY for male” — and genital anatomy. He doesn’t name the genitalia in his definition, but they come up regularly in reference to bonobo behavior, and we get the picture. But sex is more complicated than that.

At the cellular level, sex is binary: there are sperm and there are eggs. But understanding how that maps onto our chromosomes and our bodies is a work in progress. Gender is even more confusing — a mosaic of body parts, beliefs and behaviors that don’t separate neatly into two happy summer camps across the lake. De Waal agrees, writing that “gender resists division into two neat categories and is best viewed as a spectrum that runs smoothly from feminine to masculine and all sorts of mixtures in between.” We don’t yet know how or why people identify as “male” or “female,” why some people don’t identify as either and why people vary in the degree to which they identify as anything at all. So the central premise, that behavioral differences between males and females have a basis in biology, is undermined by muddy definitions of males, females — and biology.

Still, de Waal’s book is a valuable addition to the public discussion of sex and gender. Analyzing the behavior of close evolutionary relatives is a scientifically sound way to understand the origins of our own behavior. Evolutionary biologists regularly chart traits of interest onto evolutionary trees to test hypotheses about evolutionary origins, and the book gives us plenty to ponder from that perspective. One hypothesis de Waal discusses is the possibility of a genetic predisposition to learn the behaviors of your own sex — for example when girls imitate the nurturing behaviors of their mothers. None of the evidence he cites is conclusive but the book adds an important evolutionary dimension to one of the most complex issues of our time.

Even if “Different” doesn’t provide satisfying answers to our questions about gender and sex, it is still a valuable collection of amusing, heartwarming and sometimes heartbreaking anecdotes about our animal cousins. There’s the chimpanzee Kuif, who kept losing her babies from insufficient lactation, became deeply depressed and withdrew from society until de Waal taught her to bottle feed an orphaned baby, and she became a successful and devoted mother many times over. He also demonstrates how a “secondary sisterhood” of unrelated females dominate bonobo society; his detailed descriptions of female leadership in bonobos and chimpanzees are refreshing reminders of the difference between physical and political power. And whatever your views on the Kinsey scale of sexual orientation, ranging from 0 for exclusively heterosexual to 6 for exclusively homosexual, it’s fascinating to learn that every bonobo would score a perfect 3.

The beauty of a book by Frans de Waal is that once you read it, you’ll never look at your own species the same way again. “Different” may not tell us how much of our behavior is genetic, hormonal, culturally driven, or all of the above. That’s an impossibly tall order given our current state of knowledge. But it will give you a lot to think about and, if you’re brave enough, plenty of fodder for debate.

Tamra Mendelson is a professor of biology focusing on animal behavior at the University of Maryland Baltimore County.

Gender, through the Eyes of a Primatologist

Norton. 408 pp. $30