The expensive brain hypothesis predicts an interspecific link between relative brain size and life-history pace. Indeed, animals with relatively large brains have reduced rates of growth and reproduction. However, they also have increased total lifespan. Here we show that the reduction in production with increasing brain size is not fully compensated by the increase in lifespan. Consequently, the maximum rate of population increase (r_max) is negatively correlated with brain mass. This result is not due to a confounding effect of body size, indicating that the well-known correlation between r_max and body size is driven by brain size, at least among homeothermic vertebrates. Thus, each lineage faces a ‘grey ceiling’, i.e. a maximum viable brain size, beyond which r_max is so low that the risk of local or species extinction is very high. We found that the steep decline in r_max with brain size is absent in taxa with allomaternal offspring provisioning, such as cooperatively breeding mammals and most altricial birds. These taxa thus do not face a lineage-specific grey ceiling, which explains the far greater number of independent origins of large brain size in birds than mammals. We also predict that (absolute and relative) brain size is an important predictor of macroevolutionary extinction patterns.

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This Issue

23 February 2009

Volume 5Issue 1

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DOI:https://doi.org/10.1098/rsbl.2008.0469
PubMed:18842563
Published by:Royal Society
Online ISSN:1744-957X

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Manuscript received19/08/2008
Manuscript accepted22/09/2008
Published online08/10/2008
Published in print23/02/2009

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