Wednesday, November 30, 2011

Energetics and the evolution of human brain size

Navarette et al., 2011: Figure 2

Navarrette and colleagues refute the expensive tissue hypothesis in the current issue of Nature (subscription required). They report the results of a dissection based study of 100 mammalian species.  They did not find a negative correlation between brain size and gut mass, as would be predicted by the Expensive Tissue Hypothesis.  Instead, they DID find a negative correlation between brain size and the mass of adipose (fat) deposits. 

The authors interpret these results as supporting an alternative view of the energetics of encephalization.  They argue that increasing body fat is one strategy to protect against starvation. A different strategy involves developing a larger brain, which comes along with intelligence and the behavioral flexibility needed to secure a steady food supply.  However, fat tissue is metabolically cheap, so what explains the negative relationship between brain size and fat?  They argue that fat is expensive energetically because you have to literally carry it around!  Thus, there is an energetic cost in transporting your fat reserves wherever you go. 

The final major point is in explaining humans, which are both particularly fatty AND particularly brainy.  The authors argue that hominins are able to have the best of both worlds thanks to their unique (and energetically effecient) form of locomotion....striding bipedalism.  Bipedalism, in the end, allows us to efficiently transport our fatty selves from one patch of food to the next, and to think deep thoughts while doing so.