Pan pansicus), is well-known enough that when people say "chimpanzee" they often mean "as opposed to a bonobo", rather than referring to both species collectively. For clarity, I'm going to use the more specific term of "common chimp" for P. troglodytes in this post.
At any rate, being very closely related, the two species have a lot in common. (Neither, incidentally, is closer to humans than the other, in much the same way that if your aunt has two children, neither of those children is more closely related to you than the other). But there are also striking differences, particularly in the way that they interact with one another socially. A recent study, for example, shows that bonobos make eye contact with one another more frequently than common chimps do with their own kind. This is the sort of finding we would expect for an animal where social cooperation and coordination are all-important; in contrast, common chimps are more likely to focus on the mouth, and on whatever object their fellow is currently interested in himself.
The view of common chimps as macho, action-oriented, aggressive types, and of bonobos as commune-living, free love hippies is something of a simplification, but it is one that's based on a degree of underlying truth. For instance, both species get into fights among their own kind when they're angry, but bonobos do so a lot less than their cousins do.
One thing they do have in common, though, is that they both live in philopatric fission-fusion communities. "Philopatric" means that the core of the group is the males, and specifically, that males stay put and their daughters wander off in search of partners elsewhere when they're old enough to be interested in such things. This, it should be noted, is the opposite way round to how it works in the great majority of mammal species.
Normally, it's the males that get wanderlust, not wishing to stay around with grumpy old dad monopolising all the females, and travelling away in search of mates. But not so in either chimpanzee species. This is not unique, of course - spider monkeys do much the same, and there are non-primate examples, too. Oh, and, when you think about it, the female leaving her family to go and live with her partner's community is the way most human cultures work, too. (That's less true in a physical sense in modern western society, perhaps, but even so... which one is changing their surname to "join" the other family?)
As for fission-fusion, this is rather more common, at least among primates. It means that the membership of groups is fluid, with individuals coming and going between small neighbouring groups as the whim takes them, although there may be more hostility between more widely-spaced groups. This too, is a feature shared with we humans - again, noting that we're talking more about social groups of friends or co-workers than we are about, say, nations we might be at war with.
Yet re-joining a group after some time away can be a stressful situation, especially if you're low on the social totem pole. What will their reaction be? Has the social hierarchy changed since you were last here? Are there perhaps shortages of food or other resources that might make things difficult? We know that in common chimps, as in other animals with fission-fusion societies, a common reaction to a former member re-joining the group is aggression. Being the outnumbered party, it is clearly in the interests of the outsider to find some means to defuse the situation before it all gets nasty.
Since they clearly don't spend a lot of time beating one another up, bonobos in particular must be rather good at defusing these sorts of tensions. Spider monkeys do it by hugging one another, and common chimps seem to be similar. (One of the oddest such strategies, though, is that female hyenas, which also live in fission-fusion groups, defuse aggression by pretending to have an erect penis. For anatomical reasons, this is a lot easier for them to do than you might imagine).
However, given their other differences, bonobos may well have different strategies from their stronger cousins. Or, then again, perhaps they're sufficiently peaceful that it just isn't a big deal for them, and everybody's welcome, so long as there's enough food to go around. The best way of finding the answer would be to watch them in the wild, but, failing that, it's also possible to do something similar with animals in zoos.
There are a couple of caveats to that, however. For one thing, it has to be the right kind of zoo. Not only do the animals have to be an environment that's not inherently stressful (such as a bare cage), but they have to have the opportunity to have their social group break up and reform. Which means that you need at least two enclosures, that animals can move between at some times, but not others - and, ideally, do so of their own volition. As it turns out, Frankfurt Zoo has just such a facility, and, back in 2011, researchers spent three months observing the bonobos there, to see how they handled moving between different social groups.
The second caveat is the rather obvious one that, no matter how good the enclosure, it's still not natural. For the purposes of this experiment, the most likely issue there is that, since the two enclosures have to be close enough for animals to move between them at certain times, they are also going to be close enough for members of each group to be able to hear, smell, and quite possibly see members of the other one. In the wild, they'd travel a lot further, and become completely out of contact, possibly for a few weeks at a time. So, we have to concede that the natural situation is likely more extreme than the artificial one.
Having said that, how did the bonobos react? One of the first things that was apparent was that the situation seems to be considerably less stressful than it is for common chimps. Notably, the levels of the stress hormone cortisol remained unchanged as the female bonobos moved around between groups (the males, remember, tend to stay put). And perhaps with good reason, because aggression against them was rare, and, when it did occur, consisted only of putting on a threatening posture, or making a mock charge to make the target flinch. Even this is more than bonobos normally experience, but the almost complete absence of actual physical violence probably does mean that there's less to worry about.
The most common way that primates defuse tension within a group is by grooming one another. And, indeed, while the bonobos were regularly grooming one another anyway, those returning to a group were slightly more eager to engage in it. But not really by much, which may indicate that they have some other, more effective way of defusing potential trouble. And, no, it's not just hugging.
Because there's one thing bonobos are renowned for, besides being peaceful: having lots of sex. But there's an issue here, in that it's only the females who are moving between groups, and the males don't really seem to care about that. New females don't represent competition, and there's no sound reason to be aggressive towards them, so why should the males be that bothered? No, it's the other females you've got to persuade.
But, yeah... bonobos. They really don't care about that sort of thing. And so it proved in this study; females rejoining a group after some time away offered to have sex with just about every other female they met (tribadism, if you must know). Granted, it's not like they don't do this from time to time anyway (especially if they want some sort of favour in return), but it seems to be much more common after one bonobo has re-joined the group. It's basically their way of shaking hands - a way to show peaceful intent, and the foundation for a cooperative sorority that shares food and helps to look after infants.
In fact, so useful is this sort of bonding, and so eager are the bonobos to ensure that everyone's happy and working together, that those rejoining the group received more solicitations from those already within it than they offered in return. So it seems that the resident females want to see how things stand with the newcomers, and want to make sure there isn't any trouble heading their way.
It's also worth noting that just because it was solicited doesn't mean that it actually happened - females can, and do, sometimes refuse to take part. This may be a way of asserting a high-ranking female's social dominance over the one making the offer. (Perhaps surprisingly, for all their peaceful ways, bonobos may be less egalitarian than common chimps). Nonetheless, those rejected did keep coming back in the hope that their prospective partner might have changed her mind... and, more often than not, the one doing the rejecting would eventually give in.
It is, to our eyes, a rather over-the-top way of cementing social bonds with random acquaintances you haven't seen in a while. But, for bonobos, it seems to work, and, judging from the lowered cortisol levels, they seem a lot happier for it.
Hans Hillewaert, from Wikimedia Commons]