Morphological adaptations for relatively larger brains in hummingbird skulls.

Diego Ocampo,Gilbert Barrantes,J Albert C Uy

doi:10.1002/ece3.4513

Diego Ocampo, Gilbert Barrantes + Show 1 more

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https://doi.org/10.1002/ece3.4513

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Abstract

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.

Highlights

Relative to body size, brain size is proportionately larger in small species than in large species, a widespread pattern known as Haller’s Rule (Rensch, 1948)
The current evidence suggests a significant increase in energetic costs associated with more nervous tissues (e.g., Kotrschal et al, 2013; Niven & Laughlin, 2008); little is known about the structural changes that have evolved to deal with housing relatively larger brains and eyes in small animals (e.g., Niven & Farris, 2012)
The scaling pattern of relative brain size on skull morphological traits and body size is consistent with an evolutionary framework of direct and indirect selection acting on skull morphology

Summary

| INTRODUCTION

Brain size is proportionately larger in small species than in large species, a widespread pattern known as Haller’s Rule (Rensch, 1948) This allometric pattern is found in a wide range of vertebrate and invertebrate taxa (Eberhard & Wcislo, 2011; Huxley, 1932; Striedter, 2005). Small salamanders have lost some skull bones, reduced skull ossification, and adjusted the overall skull morphology to accommodate relatively larger brains (Hanken, 1983, 1984). Hummingbirds include the smallest species of birds and have likely been under strong selection to modify their skull morphology to accommodate larger brains and eyes without enlarging overall head size. We predict that small-bodied hummingbirds, which are known to have relatively larger brains, will have less skull ossification (i.e., single-layered skull), more compact braincases, and relatively larger eye sockets than large species

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Findings

| DISCUSSION

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