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A Comparative Analysis of Relative Brain Size in Waterfowl (Anseriformes)

Variation in relative brain size was examined in 55 species of waterfowl (Anseriformes). Using both conventional statistics and phylogenetically based comparative methods, the extent of variation in relative brain size and possible relationships with mode of foraging and diet were examined. The results indicate that although brain size does vary considerably between closely related species of waterfowl, it is not reliably related to either foraging mode or diet. There are a number of possible reasons for the lack of relationships between brain size and foraging mode and diet. Firstly, subtle changes in foraging mode and diet may favor relatively large changes in brain size. Secondly, foraging mode and diet could be correlated with the expansion of an individual brain region without affecting overall brain size. Thirdly, other behavioral/ecological traits may be more important with respect to brain size evolution in waterfowl. For example, the relatively large brain of the musk duck (Biziura lobata) and altriciality of their young in comparison to other stiff-tailed ducks (Oxyura spp.) indicates that developmental rate plays a significant role in the evolution of brain size. Given the difference between our results and that reported in inter-order comparisons of brain size in birds, further research is required into other avian orders to assess how brain size and behavior might be related within orders as well as between them.