Brain size, life history, and metabolism at the marsupial/placental dichotomy
Vera Weisbecker a,b,c,1 and Anjali Goswami c
- Author Affiliations
aDepartment of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, United Kingdom;
bInstitut für Spezielle Zoologie und Evolutionsbiologie, Friedrich-Schiller Universität Jena, 07743 Jena, Germany; and
cDepartment of Genetics, Evolution and Environment and Department of Earth Sciences, University College London, London NW1 2HE, United Kingdom
Edited by Robert Martin, The Field Museum, Chicago, IL, and accepted by the Editorial Board August 5, 2010 (received for review June 10, 2009)
Abstract
The evolution of mammalian brain size is directly linked with the evolution of the brain's unique structure and performance. Both maternal life history investment traits and basal metabolic rate (BMR) correlate with relative brain size, but current hypotheses regarding the details of these relationships are based largely on placental mammals. Using encephalization quotients, partial correlation analyses, and bivariate regressions relating brain size to maternal investment times and BMR, we provide a direct quantitative comparison of brain size evolution in marsupials and placentals, whose reproduction and metabolism differ extensively. Our results show that the misconception that marsupials are systematically smaller-brained than placentals is driven by the inclusion of one large-brained placental clade, Primates. Marsupial and placental brain size partial correlations differ in that marsupials lack a partial correlation of BMR with brain size. This contradicts hypotheses stating that the maintenance of relatively larger brains requires higher BMRs. We suggest that a positive BMR–brain size correlation is a placental trait related to the intimate physiological contact between mother and offspring during gestation. Marsupials instead achieve brain sizes comparable to placentals through extended lactation. Comparison with avian brain evolution suggests that placental brain size should be constrained due to placentals’ relative precociality, as has been hypothesized for precocial bird hatchlings. We propose that placentals circumvent this constraint because of their focus on gestation, as opposed to the marsupial emphasis on lactation. Marsupials represent a less constrained condition, demonstrating that hypotheses regarding placental brain size evolution cannot be generalized to all mammals.
encephalization maternal energy hypothesis altricial basal metabolic rate
Footnotes
1To whom correspondence should be addressed. E-mail: vw248@cam.ac.uk.
Author contributions: V.W. and A.G. designed research; V.W. and A.G. performed research; V.W. and A.G. analyzed data; and V.W. and A.G. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. R.M. is a guest editor invited by the Editorial Board.
This article contains supporting information online at
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