While searching for something else, I discovered: Matrilineal kin relationship and social behavior of wild bonobos (Pan paniscus): Sequencing the D-loop region of mitochondrial DNA by Chie Hashimoto, Osamu Takenaka and Takeshi Furuichi. From the abstract (I haven’t read the full article although it seems to be available from Springer):

For females, there was no relationship between genetic closeness and social closeness that is represented by frequencies of proximity or grooming. After immigration into a new group, females form social associations with senior females without regard to kin relationship.

Thought you might be interested.

This event occurred very recently, long after the demise of the diverse forest apes of the miocene

(see:

http://scienceblogs.com/gregladen/2009/02/how_diverse_were_early_hominoi.php)

It occurred in a rain forest, where Australopiths were perhaps not present.

So I’m betting on ape-free habitat, but I would not bet too much.

First, have you ever noticed the facial similarities between bonobos and gorillas, compared to chimps?

Second, as I recall the Wrangham explanation included no only larger groups, but the ability for the entire troop to be within sight (and immediate response) of one another while foraging. This means that when a young male has been subdued by a coalition (of, e.g. females), he doesn’t have the option of waiting a few days and catching each of the coalition members alone for vengeance.

This makes the contribution of Gorillas and their diet key. Now, considering the facial similarities of gorillas and bonobos, if they were co-habiting a territory, the gorillas would probably react violently to the consumption of their common food. By evolving facial features different from gorillas, and learning to avoid their common food, chimps became able to cohabit with gorillas.

IMO this leads to the conclusion that, originally, the common ancestor was more like bonobos, occupying a range including that of both modern species. Gorillas moved in from the east (probably due to climate change), but were never able to cross the Congo. Where the gorillas showed up, the ancestor was forced to adapt in facial feature as well as feeding habits, with their concomitant social changes.

BTW, given that bonobos practice female migration as do chimps, the “matralines” aren’t nearly as clear-cut as those of, for instance, many baboons and related OWL’s.

We know that when you take bonobos out of the wild and raise them in captivity in totally unnatural conditions, they do not easily recreate their original social system. They become, rather, utterly f-ed up. Many many things go wrong.

In the larger scale, there is not that much different between chimps and bonobos. The differences happen to be of interest to us because of all the interesting sex involved, and the relative role of females.

There has not been demonstrated a significant fitness related difference between these two strategies (chimp vs. bonobo).

For all of these reasons, I tend to agree that this may not be a difference with an adaptive explanation. Efforts to explain the difference adaptively have not worked out.

However, I quickly add that there are numerous reasons to ignore any argument that not finding an explanation to date means anything yet. It is still the case that there is not enough known.