Brain size and diet in orangutans

This is a paper in press in the Journal of Human Evolution. They found that individuals in one well fed group of orangs had larger brain sizes than in another not so well fed group. They say that this lends some support to the expensive tissue hypothesis. The sample sizes are small and, if I'm not mistaken, the differences are only significant for females. From the abstract (the link to H. floresiensis is interesting):

Our results, therefore, provide conditional support for the hypothesis that decreased brain size is related to prolonged episodes of food scarcity, and suggest a correlation between brain size, diet quality, and life history at the lowest macroevolutionary level. The association of a relatively small brain and poor diet quality in Pongo further suggests that ecological factors may plausibly account for such a reduction in brain size as observed in the recently recovered Homo floresiensis from Indonesia.

here's a press release from EurekAlert

Variation in brain size and ecology in Pongo

Andrea B. Taylor, and Carel P. van Schaik

Journal of Human Evolution Arcticle in Press

Abstract: Numerous hypotheses have been advanced to explain relative increases in brain size in primates and other mammals. However, notably less attention has been directed towards addressing the biological limits to increasing brain size. Here we explore variation in brain size in orangutans. We evaluated both raw and size-adjusted cranial capacity (CC) in adult Pongo pygmaeus pygmaeus (n = 147), P. p. wurmbii (n = 24), P. p. morio (n = 14), and P. abelii (n = 36). Results demonstrate significant variation in CC among orangutan taxa. Population differences in raw CC are significant for females (p = 0.014) but not males. Post-hoc pairwise comparisons among females further reveal that raw CC is significantly smaller in P. p. morio compared to both P. abelii and P. p. pygmaeus. When evaluated for proportionality, geometric equivalence in CC is not maintained in orangutans, as P. p. morio has a significantly smaller CC when compared to one or more other orangutan groups. Even after statistically partitioning size and size-correlated shape, P. p. morio has a significantly smaller CC compared to most other orangutan groups. These observed differences in relative brain size are consistent with known variation in resource quality and life history amongst orangutan populations. Specifically, P. p. morio is characterized by the least productive habitat, the lowest energy intake during extended lean periods, and the shortest interbirth intervals. Our results, therefore, provide conditional support for the hypothesis that decreased brain size is related to prolonged episodes of food scarcity, and suggest a correlation between brain size, diet quality, and life history at the lowest macroevolutionary level. The association of a relatively small brain and poor diet quality in Pongo further suggests that ecological factors may plausibly account for such a reduction in brain size as observed in the recently recovered Homo floresiensis from Indonesia.