My student, Marta, exploded the other day.

She was sitting there in class two weeks ago and exploded. She does not know that I know this, but I noticed it happen. Since she was sitting, as usual, in the front row, and it was all in her face, the other students did not see it but I definitely did.

By “exploding” in this case I mean that her brain suddenly filled with unanswered questions, which she then started sending me in frantic emails. Many of these questions are about things we will eventually get to in class, but some are on issues that we won’t touch on at all. I decided, and I received her kind permission to do this, to answer her questions by blogging them. This way I get to kill two birds with one stone, which is usually a good thing (unless of course you are the second bird).

In some cases I’ve re-written the question a little, but in all cases, they are good questions. I cannot guarantee that all of my answers will be good. But I do appreciate Marta’s inspiration, and find it inspiring myself. My only concern is that Marta gets interested enough in this material to become a biological anthropologist and thus wastes an otherwise potentially productive life. I’m hoping she becomes a doctor or a world leader instead, but we’ll see…

OK, on to the first question (I’ll deal with others in later posts):

Why did bonobos (Pan paniscus) evolve from common chimps (Pan troglodytes)?

The flippant answer is that we never really know or care why one species gives rise to another, we just see that it happens and document it. But underlying this question is the presumption that bonobos have interesting features that we do not expect to see in apes, and this is an opportunity to think interesting thoughts about evolution.

Bonobos are different from chimps in a few ways that we can presume are derived … meaning that they are novelties found in bonobos that arose as part of their differentiation from the common chimps, or subsequently. Since none of these features have a fossil record at this time it will be hard to say which known differences, if any, account for the speciation event itself.

The differences (that we’ll talk about now … this is not an exhaustive list):

• Bonobos have a female matriline in which dominance ranking of the females determines the dominance ranking of the female’s male offspring.
• The “chief bonobo” in a group is typically the highest ranking female. With chimps, most males are dominant over most females.
• Bonobos seem to have a lot of sex, and at least as interesting, they seem to have all kinds of sex, whereby all combinations of age, gender, and which body part is involved seem to occur except one: Males do not have sex with their mothers.
• Overall, there seems to be a higher level of agonistic (nasty) behavior among common chimps, especially involving males, while bonobos prefer sexual encounters over agonistic ones. One way of thinking of this is expressed on the Columbus Zoo web site: Chimps resolve sex issues with power; bonobos resolve power issues with sex.
• Bonobos probably live in larger groups. It is thought that having a large number of females in a group allows the females to dominate.

So, how, why, when, where, etc?

A key fact is that common chimpanzees have lived forever in a more or less continuous distribution (but there are important details of that I’m leaving out) across the breadth of the African Rain Forest north of the Congo/Zaire river, while bonobos live only south of the river. The genetic diversity and geographical range of the common chimps, and the fact that gorillas also live only north of the river, leads us to assume (this could be wrong but probably not) that at some point in time (about a million and a half years ago, maybe) a group of common chimpanzees somehow ended up on the other side of this river … a formidable geographic boundary … and subsequently gave rise to bonobos.

Richard Wrangham’s original hypothesis on this was that in the absence of competition for terrestrial herbaceous vegetation (THV), a major fall back food for chimps and THE major food for gorillas, bonobos were able to live in larger groups, leading to the large-number-of-females effect (see above) and thus ultimately to a totally different social system.

Subsequent studies by Wrangham and others have failed to show the expected difference in access to THV, thus probably falsifying this hypothesis. I’ve often thought that a very thorough study of all of the elements of the forest, not just THV, would show a number of candidate differences between the two regions (including other THV eaters, human hunter-gatherers, etc.), and I don’t think this kind of explanation should be ruled out yet.

However, there is another distinct possibility that should be considered, not necessarily as an explanation per se, but as a context for any explanation: I call this the “Fish or Cut Bait” phenomenon.

Imagine that there are two ways to do something, each of about the same effectiveness and of about the same cost. Or at least, the cost-benefit ratio of each approach is so close that you can’t say that they are different. In an evolutionary sense, either strategy could be an “evolutionary stable strategy” … one is not clearly going to invade and take over the other, in game theory terms … but only one will work at at time. They can’t both happen together in a given system. So it has to be one or the other. Fish or cut bait.

In this case, we have agonistic interactions to mediate social issues vs. erotic encounters to mediate social issues. For the former, how this works is obvious. Individuals perform nasty acts against each other, coalitions are formed, politics happen. This should be easy for any human to understand because this is the core of our own social system. Unfortunately. The latter may be harder to understand, but this also is part of our own social system as well (to a lesser extent) so we can relate to it.

In the case of agonistic (nasty) interactions, there is a psychological/emotional state that individuals strive for. Individuals are selected to do what it takes to not be in pain, not threatened, not stressed. You may do this by forming a tight coalition and the members of the coalition get left alone, occasionally groomed, not challenged very often. Or you might do this by avoiding the dominant individuals or coalitions. That’s all pretty obvious. Please don’t bite me, I wasn’t really looking at your girlfriend. Sure, you can have my banana.

But there may also be a state of being that one strives for that is accomplished by erotic interactions. It’s a temporary state of comfort or satisfaction that occurs through erotic interaction (including but not limited to “sex” … let’s not go into that definition too much. Here I mean sex to have babies, a behavior that is selected for regardless of the social dynamic, as opposed to all that other stuff bonobos do, which does not produce babies and is thus not selected for directly in relation to reproductive success).

Perhaps agonistic social politics and erotic bonding can each work about the same in the group dynamic of a medium sized frugivorous social ape. It is highly unlikely that either could work without interfering with each other. You can have a culture of general nastiness or a culture of erotic satisfaction. (I’m sure there are some who will say you can have both, but I’m pretty sure that is illegal in Minnesota.)

So now you have these common chimpanzees wandering around in a forest in which they or any other ape never before existed, because they somehow got across the Zaire river. I’m actually imagining that these come from a subgroup of chimps that lived in somewhat less forested environments, because they probably were the southeastern extension of the chimpanzee range, down south of present day Gombe, where they could stay in a forest and make it to the region south of the river, in times of maximal forest expansion during an interglacial.

As they move into the central forest south of the Zaire River, they find themselves in an environment that may have been richer than where they previously lived because of this shift from savanna-forest mixed habitat to true forest. (I quickly add that since chimps and fruiting trees probably have some important seed dispersal relationships, one might have to expect a difference in the distribution of key food sources, but let’s ignore that for now).

The point is that these are apes moving into a forest with virtually no conspecific or even congeneric neighbors against which to compete. In modern chimps, males spend considerable time and effort patrolling borders, and occasionally killing individual chimps from neighboring groups. According to Wrangham’s model of this behavior, the long term goal is to diminish the numbers of chimps in your neighboring group, so that if and when you need to expand your territory because of food stress, you can go over to the neighbor’s place, run them off and/or do them in, and at least temporarily enjoy a much expanded territory.

It is almost certainly true that day to day male agonistic interaction and the related coalition building in common chimps is essential for this strategy to work.

But these wandering chimps south of the big river, which would ultimately give rise to the bonobos, are expanding across an ape-free landscape, so this pattern is rarely selected for. Over several generations, in the absence of selection for the more traditional chimp-like social behavior, both the agonistic model and the erotic model of society are roughly equal in likelihood. We don’t need to postulate a specific reason why the erotic model emerged instead of the agonistic model. There would have been, according to this hypothesis, a roughly 50-50 chance of one or the other.

It is possible, though, that in this chimp-free landscape (at the outer edge of their range of expansion, at least) females could get into larger groups, and thus be more in charge. Since females will always lose in the agonistic model, but win in the erotic model, the erotic model was more likely to happen.

Every now and then I wonder about this question, not one that Marta asked: What would happen if a group of chimps and a group of bonobos, in the wild, found themselves as neighbors?