Quantifying shape and ecology in avian pedal claws: The relationship between the bony core and keratinous sheath.

Lindsay E. Zanno,Samantha A. Cordero,Peter Dodson,Christopher Noto,Brandon P. Hedrick

doi:10.1002/ece3.5507

Abstract

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 often 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, yet have generated conflicting results, spanning from clear ecological groupings based on claw shape to a complete overlap of ecological modes. The majority of 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, we collected 580 radiographs allowing visualization of the bony core and keratinous sheath shape in 21 avian orders. Geometric morphometrics was used to quantify bony core and keratinous sheath shape and was compared to results using traditional arc measurements. Neither approach significantly separates bird claws into coarse ecological categories after integrating body size and phylogenetic relatedness; however, some separation between ecological groups is evident and we find a gradual shift from the claw shape of ground‐dwelling birds to those of predatory birds. Further, the bony claw core and keratinous sheath are significantly correlated, and the degree of functional integration does not differ across ecological groups. 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. 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.

Highlights

Claws are important tools that vertebrates use to interact with their environments and are used for a variety of purposes, including loco‐ motion, clinging to surfaces, food gathering, burrowing, and in inter‐ and intraspecific combat (Cartmill, 1974; Manning, Payne, Pennicott, Barrett, & Ennos, 2005; Reichman & Smith, 1990)
The relationship between claw shape and ecological mode has been examined in birds (Feduccia, 1993) and lizards (D'Amore, Clulow, Doody, Rhind, & McHenry, 2018; Tulli, Abdala, & Cruz, 2011; Tulli, Cruz, Herrel, Vanhooydonck, & Abdala, 2009; Zani, 2000), this hy‐ pothesized relationship has most often been used for predicting the ecology of extinct taxa using the claw morphology of extant taxa (Birn‐Jeffery, Miller, Naish, Rayfield, & Hone, 2012; Feduccia, 1993; Fowler, Freedman, & Scannella, 2009; Fowler, Freedman, Scannella, & Kambic, 2011; Glen & Bennett, 2007)
After accounting for body size and phylogenetic relatedness, nei‐ ther traditional morphometrics nor geometric morphometrics are capable of significantly separating birds into a priori ecological categories

Summary

Introduction

Claws are important tools that vertebrates use to interact with their environments and are used for a variety of purposes, including loco‐ motion, clinging to surfaces, food gathering, burrowing, and in inter‐ and intraspecific combat (Cartmill, 1974; Manning, Payne, Pennicott, Barrett, & Ennos, 2005; Reichman & Smith, 1990). Studies that have examined extant bird taxa find conflicting levels of correlation between claw shape and ecology, with different ecological modes often having large amounts of overlap in shape (Birn‐Jeffery et al, 2012; Pike & Maitland, 2004). Aves has high body size disparity, ranging from the bee hummingbird (2.2 g) to the ostrich (111,000 g; Dunning, 1993), which may generate different constraints on pedal claw shape. It would be expected that Aves would have high claw disparity, likely driven by different factors in different ecological groups, and that body mass would have a large impact on shape

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Discussion

Conclusion