Chimpanzee grooming influenced more by mothers than conformity

Chimps exhibiting "palm to palm clasping" a type of socially learned grooming. Photo by Suzi Eszterhas. 

This week's paper comes from Richard Wrangham's lab at Harvard. They examined the grooming behavior of 18 wild chimpanzees in one group in Kibale National Park, Uganda. Wrangham and colleagues were interested in a very specific variant of grooming behavior. Chimps often exhibit "high-arm grooming", sometimes with clasped palms (photo above) and sometimes with just their wrists or forearms touching. There is observational evidence that high arm grooming style is learned socially. The researchers investigated why there is variation in palm clasping between individual chimps within a population. Some chimps rarely use palm clasping (6% of the time) and some frequently use it (68% of the time). Additionally, they show that there is no correlation between the number of years that an individual has been in a group and the difference between the percent of time an individual spends palm-clasping and the median percent of palm clasping in the population. They propose that if individuals were conforming, one might expect that with an increasing number of years in the population individuals would converge on the average percent of grooming bouts with palm-clasping. I am not sure I buy this, a chimp either palm-clasps or it doesn't in a grooming bout. I can imagine that a population would converge on all (or predominantly) palm-clasping, or not (or rarely) palm-clasping, but it doesn't make sense to me that they would conform on palm-clasping 35.1% of the time, as the authors propose. Unless, some particular grooming condition occurred 35.1% of the time and palm-clasping was always used in that case, but there appears to be no evidence for a particular grooming condition in which palm-clasping occurs. 

The authors show that the percentage of grooming bouts in which a chimp palm-clasps is clustered by matriline. Some matrilines of chimps (mothers and their children) palm-clasped often, and some almost never did. They therefore propose that whether or not a chimp palm-clasps is determined by what their mother did rather than the population median. In conclusion, the authors state that "when incentives are low, chimpanzees tend to maintain their first-learned strategy rather than conform to the group". 

I have some beef with this paper. First of all, it is one population containing 18 chimps (and only 14 for which they know the matriline). I know that research with chimps is incredibly difficult, but it seems dubious to make a general claim about population conformities (or lack thereof) when you only study one population. Secondly, as mentioned above, it is not clear to me that the expectations for conformity make sense. I would be interested to see if a chimp raised by a mother who did not palm-clasp and introduced to a population where the majority of individuals palm-clasped conformed to the population norm, or not. There just doesn't seem to be a population norm to conform to in this case. But regardless, it is interesting to consider the conditions in which social learning from a parent may outweigh social learning from other members in the community.