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RESEARCH ARTICLE
Contents Vol 56(5)

Interspecific variation in relative brain size is not correlated with intensity of sexual selection in waterfowl (Anseriformes)

P.-J. Guay A C and A. N. Iwaniuk B
+ Author Affiliations
- Author Affiliations

A Department of Zoology, The University of Melbourne, Vic. 3010, Australia.

B Division of Birds, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA.

C Corresponding author. Email: p.guay@zoology.unimelb.edu.au

Australian Journal of Zoology 56(5) 311-321 https://doi.org/10.1071/ZO08082
Submitted: 16 October 2008  Accepted: 17 November 2008   Published: 20 February 2009

Abstract

The role of sexual selection in shaping the brain is poorly understood. Although numerous studies have investigated the role of natural selection, relatively few have focussed on the role of sexual selection. Two important factors influencing the intensity of sexual selection are sperm competition and pair bonding and three different hypotheses have been proposed to explain how they could influence relative brain size. (1) The ‘extra-pair mating’ hypothesis predicts that sexual dimorphism in brain size will increase with sperm competition intensity. (2) The ‘Machiavellian intelligence’ hypothesis predicts that brain size will be larger in species with intense sperm competition. (3) The ‘relationship intelligence’ hypothesis predicts that species forming long-term pair bonds will have larger brains. We investigated sexual dimorphism in brain size and tested these three hypotheses in waterfowl by studying correlations between relative brain volume and three measures of sperm competition (testicular mass, phallus length and mating strategy) and pair-bond duration using the modern phylogenetic comparative approach. We found no evidence of sexual dimorphism in brain size in waterfowl after controlling for body mass and found no support for any of the three hypotheses. This suggests that brain size may not be sexually selected in waterfowl, despite evidence of sexual selection pressures on other morphological characters.


Acknowledgements

This research was supported by funding from the Department of Zoology and the Faculty of Science of the University of Melbourne, the David Hay Scholarship, le Fonds québécois de la recherche sur la nature et les technologies, the Academy of Natural Sciences, Philadelphia, the Field Museum of Natural History, the American Museum of Natural History and the Smithsonian Fellowship program. We thank the following institutions for allowing access to their collection and/or logistical help: The Auckland Museum, the Australian Museum, the Bird group of the Natural History Museum at Tring, the Bishop Museum, the Canadian Museum of Nature, the Canterbury Museum, the Carnegie Museum of Natural History, CSIRO Sustainable Ecosystems, Australian National Wildlife Collection, the Department of Ornithology of the American Museum of Natural History, the Field Museum of Natural History, the Florida Museum of Natural History, the McGill University Redpath Museum, le Muséum National d’Histoire Naturelle, the Museum of New Zealand Te Papa Tongarewa, the Museum of Vertebrate Zoology, the University of California Berkeley, Museum Victoria, the Natural History Museum of Los Angeles County, the Ornithology Department of the Museum of Comparative Zoology of Harvard University, the Queen Victoria Museum and Art Gallery, the Queensland Museum, the Royal Alberta Museum, the Royal British Columbia Museum, the Royal Ontario Museum, the Sam Noble Oklahoma Museum of Natural History, the San Diego Natural History Museum, the Smithsonian Institution, the South Australian Museum, the University of Alaska Museum, the University of Kansas Natural History Museum, the University of Michigan Museum of Zoology, the University of Washington Burke Museum, the Western Australian Museum, the Yale University Peabody Museum and the Zoology Museum of the University of Copenhagen. We also thank Raoul Mulder and Devi Stuart-Fox for comments on earlier versions of this manuscript.


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Appendix 1.  Raw data used for the analysis
Sample size (nM and nF) and brain volume (mL) for males and females (BV M and BV F), body mass (g) for males and females (BM M and BM F), testicular mass (g) from fresh mass and linear measurement datasets (TMFM and TMLM), mating strategy (MS: 1, monogamous, no FEPC or no EPP; 2, infrequent FEPC; 3, frequent FEPC; and 4, polygynous or promiscuous), phallus length (cm; Phallus) and length of pair bond (PB: 0, no pair bond; 1, seasonal pair bond; and 2, multi-year pair bond) are listed. Sources for both TM and MS are listed at the foot of the table
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