Our phylogenetic analyses indicate that the continent effect on relative HC volume found by Lucas et al. (2004)
in corvids and parids is a general phenomenon in birds. It seems that relative HC volume is larger in birds inhabiting the Eurasian subcontinent than in birds from North America. We also found a significant relationship between continental distribution and food hoarding. This result may suggest that differences in food-hoarding specialization between the continents promote differences in relative HC volume. However, when food hoarding is statistically controlled for, the association between continental distribution and relative HC volume is still significant. In addition, the relative volume of the HC is significantly smaller in North America than in Europe when involving typically non-hoarding avian families only (b
). Therefore, continental differences in food-hoarding specialization cannot lead to the phenomenon that Eurasian species have larger HC volumes. This pattern seems to be a general phenomenon among birds and it is not an attribute of food-hoarding families.
Lucas et al. (2004)
suggested that systematic methodological differences between different laboratories in how brain measurements were taken may result in differences in HC volume between continents. However, a laboratory effect may only play a minor role because different laboratories are collaborating; analysing data from different laboratories separately gives similar trends for the HC/hoarding relationship (Lucas et al. 2004
). In addition, the high between-studies repeatability of relative HC volume also argues against this explanation (Garamszegi & Eens 2004
Lucas et al. (2004)
also proposed that the phylogenetic distribution of birds may be responsible for the observed differences in HC volume between continents. However, the phylogenetic analyses presented here and in Garamszegi & Eens (2004)
show that phylogenetic effects are unimportant confounding factors.
As a third possibility, Lucas et al. (2004)
put forward the hypothesis that there truly is some continent-wide difference in an ecological factor or a life history trait that mediates differences in the scaling of brains. However, the role of these potential factors remained undetermined. Here we hypothesized that the wintering conditions of species may be different between Europe and North America and that these differences may select for different behavioural adaptations that are ultimately reflected by differences in relative HC volumes. We found a significantly negative relationship between winter temperatures and relative HC volumes at the raw species level, which was non-significant when we controlled for phylogenetic associations. This may indicate that the effect of wintering temperatures on relative HC volume, if it exists at all, is small and, thus, goes undetected in our small sample when controlling for common ancestry. It is also possible that January temperatures at the measured locations do not reflect the coldest temperatures that wintering species experience. However, we conclude that winter temperatures cannot be the primary basis of the continent effects because wintering temperatures did not differ between the continents. We suggest that differences in the degree of food hoarding and relative HC size in Europe and North America should be sought among other ecological factors and life-history traits that are not mirrored by winter temperatures. A comprehensive analysis of the role of biological factors should include a broad range of ecological or life history traits that can be related to HC volume and continental distribution and, thus, potentially shape the continent effect. Ideally, if the mediator variable is found, the continental effects should drop out of significance in the model.
The continent effect on relative HC volume, which is independent of food hoarding and winter temperatures, implies that other behavioural tasks requiring space in this neural substrate also allow species to adapt to environmental factors that vary across continents. Thus, food hoarding seems to be just a part of a complex cognitive adaptation process governed in part by the HC. The evolutionary role played by the HC may be even more important than previously thought.