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Why are there so few smart mammals (but so many smart birds)?


Isler, K; Van Schaik, C P (2009). Why are there so few smart mammals (but so many smart birds)? Biology Letters, 5(1):125-129.

Abstract

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 (rmax) 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 rmax 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 rmax is so low that the risk of local or species extinction is very high. We found that
the steep decline in rmax 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.

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 (rmax) 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 rmax 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 rmax is so low that the risk of local or species extinction is very high. We found that
the steep decline in rmax 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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50 citations in Web of Science®
59 citations in Scopus®
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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Anthropology
Dewey Decimal Classification:300 Social sciences, sociology & anthropology
Language:English
Date:2009
Deposited On:19 Jan 2009 10:11
Last Modified:05 Apr 2016 12:35
Publisher:The Royal Society
ISSN:1744-9561
Publisher DOI:10.1098/rsbl.2008.0469
PubMed ID:18842563
Permanent URL: http://doi.org/10.5167/uzh-6023

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