Here's another interesting paper waiting in the publication pipeline at the Journal of Human Evolution on brain size and life history in primates. Should be fun to chew on following on the heels of Heidi Rivera's post about cellular scaling rules which has generated some interesting discussion. Look below the fold for the abstract of the paper, and my own first impressions (and email me if you want the pdf):
Life history costs and benefits of encephalization: a comparative test using data from long-term studies of primates in the wild
Nancy L. Barrickman, Meredith L. Bastian, Karin Isler, and Carel P. van Schaik
The correlation between brain size and life history has been investigated in many previous studies, and several viable explanations have been proposed. However, the results of these studies are often at odds, causing uncertainties about whether these two character complexes underwent correlated evolution. These disparities could arise from the mixture of wild and captive values in the datasets, potentially obscuring real relationships, and from differences in the methods of controlling for phylogenetic non independence of species values. This paper seeks to resolve these difficulties by (1) proposing an overarching hypothesis that encompasses many of the previously proposed hypotheses, and (2) testing the predictions of this hypothesis using rigorously compiled data and utilizing multiple methods of analysis. We hypothesize that the adaptive benefit of increased encephalization is an increase in reproductive lifespan or efficiency, which must be sufficient to outweigh the costs due to growing and maturing the larger brain. These costs and benefits are directly reflected in the length of life history stages. We tested this hypothesis on a wide range of primate species. Our results demonstrate that encephalization is significantly correlated with prolongation of all stages of developmental life history except the lactational period, and is significantly correlated with an extension of the reproductive lifespan. These results support the contention that the link between brain size and life history is caused by a balance between the costs of growing a brain and the survival benefits the brain provides. Thus, our results suggest that the evolution of prolonged life history during human evolution is caused by increased encephalization.
Here's the key figure demonstrating the main correlations between brain growth (x-axis) and (a) lactational period, (b) juvenile period, and (c) Age @ First Reproduction, all after controlling for the effects of body growth:
This leads the authors to argue that the high cost of growing all that brain tissue is offset by prolonging juvenile and reproductive phases. The juvenile phase is when the brain apparently grows faster than the body in most primates (which is why there is no correlation in panel (a) of the figure). In turn, species with larger brains have better survival over longer juvenile and reproductive phases in their life history. In other words, species that have bigger brains pay the cost of growing more of the most expensive tissue in the body, but benefit by being smarter at foraging, avoiding predators, and generally living and fornicating over longer periods than their smaller-brained counterparts. Note that having a bigger brain is not correlated with an increase in reproductive output at any given time, just the prolongation of the reproductive phase oveall - which, I suspect, may leave you with greater lifetime reproductive success (=fitness) in any case.
The argument of the paper is strengthened by the fact that they draw mostly upon datasets from wild populations that have been studied for long enough to document a boat-load of life history parameters (including gestation length; interbirth interval between surviving offspring; lactational period; juvenile/adolescent period; age at first reproduction for females; life expectancy at AFR, or the inverse of mortality rate; adult lifespan) - so they are analyzing real-life situations faced by these primates in the wild, and not more benign or stressful conditions in captivity. They also used a phylogenetic comparative method in analyzing correlations such as shown in the above figure - i.e., they removed or controlled for effects of phylogeny.
So, do you buy the causal argument (at the end of the abstract) that increased brain size is what caused the life-history phases to lengthen? Or is it the other way around? What does the correlation tell us about causation in this case? Myself - I'd have to say I like the correlations, but have to digest the paper some more before I completely buy the causal argument.
So what else are we doing as the primate with the most brains, and longest active sex lives? Well, cutting down on our reproduction (or trying to anyway), for one thing. And also, apparently trying to decouple brain growth from body growth entirely - in other domesticated animals - even thinking about producing animals with more meat and much less brain! Why? So we can have more meat to eat, of course, without having to worry about animal rights issues, presumably! Now that's an interesting direction to go in, opposite from the self-sacrificing talking cow imagined by Douglas Adams in "The Restaurant at the End of the Universe"!
[Hat-tip to Afarensis]
BARRICKMAN, N., BASTIAN, M., ISLER, K., VANSCHAIK, C. (2007). Life history costs and benefits of encephalization: a comparative test using data from long-term studies of primates in the wild. Journal of Human Evolution DOI: 10.1016/j.jhevol.2007.08.012