Thursday: Comparing Primate Sexuality
What makes us human, and what makes a chimp a chimp? In this discussion, visitors to our web site had the chance to ask two researchers who have studied our simian relatives about the similarities and differences between the species.
Thursday's Question:Comparing Primate Sexuality
“Okay. Quite a sensitive one this. Unlike chimps, bonobos utilize sex as a social networking tool. Their activity (in human terms) could be described as heterosexual, homosexual, incestuous, pedophilic, etc. Given the genetic overlap alluded to by the 98.5% question in the headers for this site, do you think the genes that give rise to this behavior in bonobos might somehow be present in humans and so give rise to what we commonly consider to be deviant behavior?”
—David in Cambridge, England, is a freelance science journalist.
Daniel Povinelli
The history of studying nonhuman primates is littered with examples of trying to find the origins of every human trait in the ape. Poor, ape!
For a while, bonobos were shouldered with this burden. Given certain aspects of their social behavior, it became fashionable to talk about bonobos as being more human-like than other apes. Of course, any such judgment depends on what behaviors you wish to emphasize. In our hyper-sexualized culture, bonobo sexuality is certainly titillating. But most evolutionary biologists had a hard time with placing such a burden on the bonobo. After all, they arose from a common ancestor with the common chimpanzees roughly 2 million years ago, whereas the human lineage split from the African apes 6 million years ago. What this means is that bonobos are genetically more similar to chimpanzees than they are to humans. Oops.
But what about the deeper assumptions behind this question: Is human behavior in all its manifestations in some way influenced by our genes, some of which we share in common with the great apes and other species? Well, in some sense, the answer must be yes. But “genes” don’t by themselves code for anything. Genes are expressed as part of an inseparable relationship to the environment in which they operate — both the external environment and the cellular environment in which they are suspended. Trying to find genetic relationships between superficially similar behaviors across species is a very difficult task.
And while we’re discussing the genetic similarities of humans and apes, here’s a well-kept secret: we don’t share 98.8% of our genes in common with chimpanzees. What we share, for sure, is about that much similarity in our overall nucleotide sequence for the regions of DNA we share in common. Why is this distinction important? First, a single gene may be 400 nucleotides in length, but a change in just one of those nucleotides (far less than 1%!) can code for a different gene. Second, there are upwards of 700 genes in humans that are not found in chimpanzees. Third, there are nearly 5 million “indels” in each species (regions of the DNA that have been inserted or deleted). And fourth, there are major differences in the rate and timing at which genes we do share in common are turned on and off during growth and development.
This later point is especially important: Major changes in evolution are driven by the turning on and off of genes. Here’s a well-known example among evolutionary biologists: Modern birds are descendants of reptiles, and, as we all know, they have no teeth. However, the “genetic potential” for teeth still exists in moderns birds, as experiments in the laboratory clearly show: It is quite easy to get a chicken embryo to develop teeth by exposing the cells in its developing jaw tissue to specific certain environmental triggers.
Finally consider this: To the extent that we can make such comparisons, we share about 50% of our genetic sequence in common with the garden pea. Now I don’t know about you, but there is no sense in which I’m puzzled as to why I’m not half-pea.
What all of this means is that despite crude similarities in our nucleotide sequence, all of the “genetic” information necessary to make chimpanzees comfortably chimpanzee, and humans comfortably human, are in place. How could it be otherwise?
Before we go about trying to draw simplistic “genetic” connections between superficially similar behaviors in humans and other great apes, we need to clean up our understanding of what the real genetic (read: epigenetic), cognitive, and cultural differences are among these species. This website is an excellent place to start.
William Fields
I do not believe bonobos, or any of the great apes are exclusively biological preparations. As my student Daniel Musgrave has coined the "c-value paradox" (in honor of Chomsky) — there seems to be a “poverty of genes” in great apes. In humans, I believe the c-value is 3.50 and it varies within Pan, but it remains significantly lower compared to the genetic information contained in an epiphytic fern or a frog. There does not seem to be enough gene-stuff for behavior or sexual behavior to be the exclusive expression of the genes in humans or apes. Terry Deacon says it well: “Genes are under the control of epigenetics. Epigenetics are under the control of social behavior. Social behavior is under the control of culture.” Variations in sexual behavior are cultural, just as the notion of deviance is cultural.
With respect to the sexuality of bonobos and their sexual practices, in my view, this is strictly a cultural phenomenon. Bonobos are individuals and vary in their many tendencies to sexually behave based upon their origins and rearing history. The many expressions of human sexuality, as described in the question, are human ideational categories. These categories are exclusively human constructions that do not overlay easily upon how bonobos use sex or for what purposes they use their genitalia to negotiate social ambiguity. However, I find that the bonobos I know, tend to engage heterosexual sex and avoid incest and pedophilia.
I have observed the incest taboo strongly present in some bonobos and I have also observed avoidance of male-to-male genital contact. I have also observed males engage in ritualistic forms of erect penis fencing where there is no contact, sometimes where the individuals are separated by as much as 15 meters. Sometimes, what appears to be sex is not. The goal of pressing up against another’s groin, when observed in a highly synthetic stylized abbreviated manner reveals itself for something other than sex as humans define it. There is variability among bonobos as there is in all groups. I would not be surprised to meet a bonobo who sexually preferred others of the same sex, but so far, I have not.
I am not sure that bonobos use sex as a networking tool. And I disagree that bonobos, with respect to the notion of species-specific behaviors, are more sexual than other great apes. I do not discredit the observations of the Japanese scientists in Wamba or the observation by De Waal; however, it is my experience that bonobos often use their genitalia in social circumstances as we use our hands, as in hand shaking or patting for reassurance. It can have nothing to do with sex. It may be the case that certain captive bonobos engage in more sex than free-ranging ones, but this might be adaptive, for what else is there to do in certain types of captivity? As for ethological observations of bonobo sexuality, there is a critical distance between the observer and observed in the arcuate of the Congo — where visibility drops off at 10 meters and the bonobos travel mainly in trees, high in the canopy. Human observation of bonobos in free-ranging habitats is difficult. And the arcuate is the only place they naturally occur.
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