The Evolution of lateralized foot use in parrots: a phylogenetic approach

Cerebral lateralization refers to the division of cognitive function in either brain hemisphere and may be overtly expressed as behavioral asymmetries, such as handedness. The evolutionary history of laterality is of considerable interest due to its close link with the development of human language. Although considerable research effort has aimed at the proximate explanations of cerebral lateralization, considerably less attention has been paid to ultimate explanations. The extent to which laterality is constrained by phylogeny or shaped by ecological forces through natural selection has received little attention. Here, the foot preference of 23 species of Australian parrots was examined to investigate the link between laterality and body size. The raw data indicated that the strength of laterality was related to body size and an associated foraging mode. The results of the phylogenetic generalized least squares, however, indicated that both the pattern (left, right, or ambidextrous) and strength of laterality showed a high degree of phylogenetic inertia. Regressions based on independent contrasts revealed no relationship between laterality and body size. These results suggest that laterality in Australian parrots has been shaped by just a few events deep in their evolutionary history. We hypothesize that cerebral lateralization may provide a fitness benefit to larger bodied species that extract seeds from seedpods using coordinated foot-beak actions. The secondary loss of laterality in smaller body species may have been associated with a shift to grazing on small seeds and blossoms as Australia became increasingly arid.