The 81st Annual Meeting of the American Association of Physical Anthropologists (2012)

The „gray ceiling“: why apes are not as large-brained as humans


Anthropological Institute and Museum, University of Zurich, Switzerland

Friday 8:30-8:45, Grand Ballroom II Add to calendar

To explain the evolution of human encephalization, we need to give compelling reasons why extant nonhuman apes do not have similarly large brains. Using a comparative phylogenetic approach on large datasets, we tested several hypotheses about energetic constraints on brain size evolution. We propose that early Homo met the increasing energetic demands of their brains through a stable increase in energy intake, both higher on average and with less fluctuations, and a reduction of energy expenditure through efficient bipedalism and the abandonment of climbing. We present comparative evidence that this was accomplished not through a trade-off with other expensive organs, but rather by a combined strategy of cognitive and physiological buffering against lean periods.

However, this does not explain why some other great apes could not have done the same. We argue that the limiting factor on relative brain size (the „gray ceiling“) is its tight negative relationship with life history pace, fertility, and maximum population growth rates. Any further reduction in fertility would not allow demographically viable populations in nonhuman apes. Again utilizing comparative data, we show that the human combination of large brains and high fertility is only feasible if additional infant care is provided by non-mothers. We conclude that great ape encephalization is limited by a combination of relatively inefficient locomotion and their breeding style, as ape mothers carry the bulk of the reproductive effort alone.

This study was funded by the Swiss National Science Foundation grant 3100A0 –117789, with additional support from Synthesis and the A. H. Schultz Foundation.

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