When I read Ed Yong's post about Kahlenberg and Wrangham's new article on gendered stick-carrying behaviors in chimpanzees, I wrote Stephanie and asked her what she thought. She wrote me a detailed, thoughtful response -- with citations -- after thoroughly reading the original manuscript. I liked it so much that I asked her if I could post it to my blog, and she graciously accepted.
After reading the article, I have the same questions and cautionary feelings about the data that I did when I heard Wrangham give this talk at the AAPAs in ABQ. According to the paper (Kahlenberg & Wrangham, 2010), we're talking about sex differences based on 117 observations of the behavior over 14 years (which was more than I remembered, probably in error, from the talk, but it's still not all that many). The median rates of stick-carrying for females, who carry more frequently, top out at 1.6 times per 1000 hours of observation for a set of 4 females in the 6-8 year age group. All in all, this seems to be a very rare behavior. In some cases, this behavior lasts for 4 hours, in some cases only for a minute. Of course, evolutionarily important behaviors certainly can be rare and of short duration (e.g., infanticide). But since we're obviously not talking about a behavior that characterizes every day in the lives of male and female juvenile chimps, its evolutionary importance can't be taken for granted, either, and I think we need to be careful and clear about exactly what conclusions we draw about its potential evolutionary significance.
My questions are really just that—questions about the nature of the data that will not be answered in published articles but can easily be answered over a beverage with the authors. I'd like to know how much variation there is among individuals in rates of these behaviors--do some individuals carry a lot and some not at all? Do some individuals typically carry for a long time and others typically carry for only a minute or two? Are most of the carrying bouts long, or are most short? Are most of them characterized by overt infant-care behavior, or does that characterize only a few of them? Do individuals continue to carry throughout their juvenility or is this an ephemeral phenomenon (it’s possible that one can infer this from the paper, but the way they binned individuals into age categories made it difficult for me to get a sense of the how much longitudinal data they have for each individual)? Do animals with younger siblings around do it more than ones without? Sometimes, this sort of information from the people who’ve watched the animals can nuance those dry and seemingly unimpressive numbers in such a way that you better understand how this particular small but significant difference really might be biologically important. For that reason, I'm generally curious about the details of the observations that allow for subjective, qualitative impressions about what's going on with whom and in what conditions.
Unanswered questions aside, there is the obvious issue of how best to interpret the importance of an apparent sex-difference in a very rare behavior with no obvious function. The authors’ interpretation is that this stick-carrying is effectively “practicing for motherhood” by juvenile females. That seems perfectly reasonable. Given that interpretation, though, this behavior is just another manifestation of a sex-difference in infant-interest and it can simply be rolled into a larger set of behaviors we call "infant interest." You could dissect primate infant interest behaviors if you wanted—grooming directed at infants, lip-smacking at infants, increased rates of grooming of mothers of newborns, infant carrying, etc.—but they're all a part of the same phenomenon, which is getting close to and interacting with infants in order to practice mothering behaviors so that you will have a better chance of being a successful mother yourself. But that isn't very catchy because it isn't very different from every other primate that shows sex-differential infant interest. No one disputes that there is a plesiomorphic sex-difference in infant interest for chimps and humans (and many other primates). And no one disputes that this sex-difference and the practicing it facilitates are adaptive, since female primates that have no access to infants before they themselves have their first offspring tend to be crap-tastic mothers.
So, the authors go on to suggest that the real find here is that sex-differential object play that is not socialized by adults may be a plesiomorphic characteristic of chimps and humans. Well, ok. That would be true if sex-differential object play that is not socialized by adults characterizes both species. So far, it only characterizes one group of wild chimps. Admittedly, I'm a bit confused about whether that's really the case, because the chimp people are a tight-knit little group of people, and before writing a sentence like "Given that regular stick-carrying has not been reported outside Kanyawara, a social learning component appears important," the obvious thing to do would be to ask the other chimp people if they've seen this but not reported it. In fact, I’d be amazed if that very conversation didn’t come up sometime before or after Wrangham’s talk at the AAPAs. After doing a little asking around, you could then simply state that it hasn't been seen in other communities or that it has and needs further quantification. If it hasn't been seen in other communities, it isn't species-typical and can't be used to suggest a symplesiomorphic behavioral sex-difference of chimps and humans. If it has been seen in other populations but not reported or quantified, then their assertion that it is symplesiomorphic is actually stronger than they’ve stated. If you’re a chimp person, this should be easy to resolve. Regardless, though, of whether or not this characterizes chimps as a species, sex-differential object play that is not socialized by adults has already been demonstrated in captivity in rhesus macaques (Hassett et al., 2008) and vervet monkeys (Alexander & Hines, 2002). So plesiomorphic sex-differences in object preference (to the extent that one accepts that this characterizes humans) has already been demonstrated, and for far more distant cousins than chimps.
I think the interesting point here is that we're talking about sex-differential use of the same objects in juveniles--when females play with sticks, they exhibit infant-care related behaviors, and when males play with sticks, they tend to exhibit weapon-related behaviors (although they also sometimes exhibit infant-care related behaviors). That’s something different than sex-differences in object preference. That hasn't been demonstrated experimentally with monkeys. And it’s not about the object and its characteristics, but is something about differences in individuals’ responses to the same stimuli. But not much was made of that point. I wonder if they're going to have a companion paper about weapon stick-use in males. I think it'd be a more compelling story presented all together. Of course, to suggest that this is a plesiomorphic characteristic of chimps and humans, it still has to be established for other populations of chimps. That might take a while, given how infrequent a behavior it is. But I do think that the sex-differential use of the same objects during play is pretty cool. Insofar as those uses are related to sex-differences that will later emerge in maternal care and fighting/displaying in adults, they are not altogether surprising, but it is still cool in that it is one more way in which humans aren't actually unique, which means it's one more line of evidence/inquiry that can be used to better understand human behaviors from an evolutionary point of view (and that is true regardless of whether this "shared trait" is symplesiomorphic or homoplastic in this population and humans).
Stephanie Meredith's bio
I am interested in understanding how developmental systems produce primate behavioral sex differentiation, with an eye toward elucidating the evolutionary histories of different system components (e.g., particular components of socialization, particular aspects of individual physiology, etc.), as these data are useful for testing hypotheses about the evolutionary history of human gender. My current research focuses on social and endocrine factors that shape the development of behavioral sex differences in ring-tailed lemurs, which are gregarious, female-dominant strepsirrhine primates. You can find my website here.
Alexander, G., & Hines, M. (2002). Sex differences in response to children's toys in nonhuman primates (Cercopithecus aethiops sabaeus) Evolution and Human Behavior, 23 (6), 467-479 DOI: 10.1016/S1090-5138(02)00107-1
Hassett, J., Siebert, E., & Wallen, K. (2008). Sex differences in rhesus monkey toy preferences parallel those of children Hormones and Behavior, 54 (3), 359-364 DOI: 10.1016/j.yhbeh.2008.03.008
Kahlenberg SM, & Wrangham RW (2010). Sex differences in chimpanzees' use of sticks as play objects resemble those of children. Current biology : CB, 20 (24) PMID: 21172622