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Showing posts with label correlation. Show all posts
Showing posts with label correlation. Show all posts

Friday, August 27, 2010

Genes and culture in animals

Recent UM Ph.D. Kevin Langergraber and others (including UM primatologist John Mitani; 2010) recently reported on a high correlation between genetic relatedness and 'cultural' behavioral repertoire in wild chimpanzees.

Chimpanzees, and other animals, have been observed to display behaviors that appear 'cultural,' since the behaviors are variously a) learned from other individuals, b) specific to certain chimp populations, and/or c) not recognizably adaptive. Such behavioral variants include, for example, how hands are clasped during grooming (photo from Whiten, 2005), or whether/how insects are acquired.

Researchers have debated whether these behavioral variants actually represent culture, in the sense that humans have culture. This itself is tough because anthropologists have had a helluva time defining what 'culture' is simply for humans, let alone animals. I'm a bit anthropocentric myself, and I'm wont to view culture as something uniquely human, the adaptation (or set of adaptations) that has essentially shaped our evolution for over a million (2 million?) years.

Anyway, back to Pan, Langergraber and colleagues set out to test whether genetic variation may help explain some of the behavioral variation between different chimp populations. Lo and behold! there was a significant correlation between groups' genetic dissimilarity and behavioral dissimilarity. This isn't at all to say the authors have found the genetic basis for cultural behaviors, but rather that some genetic variation may underlie some behavioral variation we see in chimpanzees. Indeed, the authors note that the mtDNA used in the study doesn't 'code for' any of the putatively cultural behaviors; it's a proxy for genetic relatedness. However, there was no clear pattern of which types of behaviors (e.g. grooming- vs. feeding-related) correlate with genetic relatedness.

The results are a bit tough to interpret. The authors state that the finding of a correlation does not mean that many chimp behaviors analyzed are not cultural. But it doesn't necessarily mean that the behaviors are cultural, either. This gets really tricky for a number of reasons.

First, identifying "the" or "a" genetic basis for phenotypes is difficult, and it's especially difficult for complex phenotypes like behaviors (in general, if you ever hear about a "gene for" some behavior, immediately disbelieve it). The analysis uses an allegedly neutral DNA marker, that admittedly does not 'code for' any of the behaviors in question. All the DNA can do here is attempt to indicate relatedness among groups. To say that "genetic differences cannot be excluded as playing a major role" in patterning behavioral variation (p. 7), basically means that some unexamined genetic region may be patterned among populations the same way as the mtDNA marker, and might be responsible for specific, fine-tuned, non-adaptive aspects of their behavior. The authors discount the possibility of the link being due to kin teaching behaviors to kin, but I would suppose a higher resolution (like looking at relatedness and behavior between individuals rather than groups) would put that matter to rest.

Next, how much of a correlation is biologically (and here culturally?) meaningful? In various permutations of their analysis, the correlations between the behavioral and genetic dissimilarity matrices ranged from r = 0.37 - 0.52, most of which were significant. "Significant" here means that the correlation coefficients, r, are different enough from zero - there isn't no relationship between the variables (I mean to say the double negative). Put another way, we can square the r coefficients to get the 'amount of variance explained': 13.7 - 27.0% of the behavioral dissimilarity can be 'explained' by genetic dissimilarity. What if the correlation coefficients had been higher - would this be better evidence for some genetic basis for chimp behavioral variants? I love correlation as much as the next guy, but aside from significance level, variation in linearity is not always completely understandable.

So, regardless of the results of the analysis, do apes (or other non-human animals) have culture? An interesting conundrum is that when people describe the subtle variants of behavior as cultural, they're assuming the variation itself is non-adaptive, while the grand behavior itself purportedly is. Can things that are readily adaptive (ecological explanation) not also be cultural? Moreover, how widespread in a population must a behavioral variant be to be cultural? How many variants on a theme are permissible within a population? Questions like these are why I tend to shy away from the topic of culture, in humans and animals.

References
Langergraber K, Boesch C, Inoue E, Inoue-Murayama M, Mitani JC, Nishida T, et al. Genetic and 'cultural' similarity in wild chimpanzees. Proc R Soc B in press. Proc. R. Soc. B doi:10.1098/rspb.2010.1112 (2010)

Whiten A. 2005. The second inheritance system of chimpanzees and humans. Nature 437: 52-55.