Quantifying shape, integration, and ecology in avian pedal claws: comparing geometric morphometric and traditional metric approaches

Terrestrial tetrapods use their claws to interact with their environments in a plethora of ways. Birds in particular have developed a diversity of claw shapes since they are not bound to terrestrial locomotion and have heterogeneous body masses ranging several orders of magnitude. Numerous previous studies have hypothesized a connection between pedal claw shape and ecological mode in birds, but have generated conflicting results, spanning from clear ecological groupings based on claw shape to a complete overlap of ecological modes. These studies have relied on traditional morphometric arc measurements of keratinous sheaths and have variably accounted for likely confounding factors such as body mass and phylogenetic relatedness. To better address the hypothesized relationship between ecology and claw shape in birds, 580 radiographs were collected allowing visualization of the bony core and keratinous sheath shape spanning 21 avian orders. A new method was used to quantify claw shape using geometric morphometrics and was compared to results using traditional arc measurements. Neither traditional nor geometric morphometrics are capable of significantly separating bird claws into coarse ecological categories after integrating body size and phylogenetic relatedness. Further, the bony claw core and keratinous sheath are significantly integrated with one another, suggesting that they function as a single unit. Therefore, it is likely possible to compare fossil bony cores with extant keratinous sheaths after applying corrections. Finally, traditional metrics and geometric morphometric shape are significantly, yet loosely correlated, and geometric morphometric data better distinguish ecological groups in morphospace than is possible with traditional metrics. Based on these results, future workers are encouraged to use geometric morphometric approaches to study claw geometry and account for confounding factors such as body size, phylogeny, and individual variation prior to predicting ecology in fossil taxa.