Abstract
Cranial morphology in birds is thought to be shaped by adaptive evolution for foraging performance. This understanding of ecomorphological evolution is supported by observations of avian island radiations, such as Darwin's finches, which display rapid evolution of skull shape in response to food resource availability and a strong fit between cranial phenotype and trophic ecology. However, a recent analysis of larger clades has suggested that diet is not necessarily a primary driver of cranial shape and that phylogeny and allometry are more significant factors in skull evolution. We use phenome-scale morphometric data across the breadth of extant bird diversity to test the influence of diet and foraging behaviour in shaping cranial evolution. We demonstrate that these trophic characters are significant but very weak predictors of cranial form at this scale. However, dietary groups exhibit significantly different rates of morphological evolution across multiple cranial regions. Granivores and nectarivores exhibit the highest rates of evolution in the face and cranial vault, whereas terrestrial carnivores evolve the slowest. The basisphenoid, occipital, and jaw joint regions have less extreme differences among dietary groups. These patterns demonstrate that dietary niche shapes the tempo and mode of phenotypic evolution in deep time, despite a weaker than expected form–function relationship across large clades.
Highlights
Observations of avian cranial evolution, and especially the beak, in response to ecology and behaviour are part of the bedrock of evolutionary theory
Using a broad sample that encompasses extant avian diversity (159 of 195 extant families), we investigated the effects of diet and foraging behaviour on cranial morphology in light of recent evidence that the avian skull exhibits high modularity [23]
The goodness of fit is weak (R2, 0.10, table 1), indicating that diet is a poor predictor of cranial morphology
Summary
Observations of avian cranial evolution, and especially the beak, in response to ecology and behaviour are part of the bedrock of evolutionary theory. These examples, in addition to non-avian radiations [10,11,12,13,14,15], support the widespread view that diversification is often the result of adaptive speciation that links morphology to behaviour, ecology, and diet Together, these examples suggest that the skull and beak, as the food acquisition apparatus of birds, evolve to fit with the trophic niche of the lineage and the specific functional demands of diverse diets and foraging behaviours [3,4,16,17]. We tested whether similar patterns are present in birds by quantifying the relative rates of evolution among dietary groups in each cranial module
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