A common allometric pattern called Haller's Rule states that small species have relatively larger brains and eyes than larger species of the same taxonomic group. This pattern imposes drastic structural changes and energetic costs on small species to produce and maintain a disproportionate amount of nervous tissue. Indeed, several studies have shown the significant metabolic costs of having relatively larger brains; however, little is known about the structural constraints and adaptations required for housing these relatively larger brains and eyes. Because hummingbirds include the smallest birds, they are ideal for exploring how small species evolve morphological adaptations for housing relatively larger brain and eyes. We here present results from a comparative study of hummingbirds and show that the smallest species have the lowest levels of ossification, the most compact braincases, and relatively larger eye sockets, but lower eye/head proportion, than larger species. In contrast to Passerines, skull ossification in hummingbirds correlates with body and brain size but not with age. Correlation of these skull traits with body size might represent adaptations to facilitate housing relatively larger brain and eyes, rather than just heterochronic effects related to change in body size. These structural changes in skull traits allow small animals to accommodate disproportionately larger brains and eyes without further increasing overall head size.
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