Points on the curve: An analysis of methods for assessing the shape of vertebrate claws

Abstract

The form of amniote claws has been extensively investigated, often with inferences about ecological association being drawn from studies of their geometry. Various methods have been used to quantify differences in the geometry of claws, but rarely have the underlying assumptions of such methods been addressed. Here, we use one set of bird claws and apply six methods (five that have been previously used, and a new one) that are tasked with comparing their shape. In doing so, we compare the (1) ability of these methods to represent the shape of the claw; (2) validity of the assumptions made about underlying claw geometry; (3) their ability to be applied unambiguously; and (4) their ability to differentiate between predetermined functional clusters. We find that of the six methods considered only the geometric morphometric approach reveals differences in the shapes of bird claws. Our comparison shows that geometry-based methods can provide a general estimate of the degree of curvature of claw arcs, but are unable to differentiate between shapes. Of all of the geometry-based approaches, we conclude that the adjusted version of the Zani (2000) method is the most useful because it can be applied without ambiguity, and provides a reliable estimate of claw curvature. The three landmarks that define that method (tip and base of the claw arc, plus the intersection between said claw arc and a line drawn perpendicular from the midpoint of tip and claw base) do not all bear biological significance, but relatively clearly circumscribe the length-to-height ratio of the claw, which relates to its curvature. Overall, our comparisons reveal that the shape of avian claws does not differ significantly between climbing and perching birds, and that the utilization of preordained functional clusters in comparative data analysis can hinder the discovery of meaningful differences in claw shape. J. Morphol. 278:150-169, 2017. © 2016 Wiley Periodicals,Inc.

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

“…Although body mass was found to be significantly correlated with claw shape, we support previous studies in asserting that body size is not a substantive predictor of claw shape (Birn‐Jeffery et al, ; Pike & Maitland, ) given that only 3% of total variance is explained by size (Figure b). Tinius and Russell () found a significant relationship between body mass (Dunning, ) and claw geometry and shape, but suggest that this is related to size differences in each ecological cluster (e.g., ground‐dwelling birds are typically larger than perching birds). The relationship between body mass and claw arc has been shown to be complex, wherein the relationship likely varies within ecological categories (Birn‐Jeffery et al, ; Pike & Maitland, ; Tinius & Russell, ).…”

“…Tinius and Russell () found a significant relationship between body mass (Dunning, ) and claw geometry and shape, but suggest that this is related to size differences in each ecological cluster (e.g., ground‐dwelling birds are typically larger than perching birds). The relationship between body mass and claw arc has been shown to be complex, wherein the relationship likely varies within ecological categories (Birn‐Jeffery et al, ; Pike & Maitland, ; Tinius & Russell, ). For example, claw angle increases with body mass in predatory and climbing birds, but decreases with body mass in ground birds, and is not correlated with body mass in perching birds (Pike & Maitland, ).…”

“…Some species were sampled at particularly high rates to assess intraspecific variation in claw shape (e.g., Tinamus major , n = 25). To facilitate phylogenetic comparative analyses, species means were taken for the 145 species for both traditional and geometric morphometric measures (Tinius & Russell, ). The maximum credibility phylogeny for extant birds generated by Jetz, Thomas, Joy, Hartmann, and Mooers () was pruned to include only the species sampled in this analysis and was used for phylogenetic comparative analyses (Figure ).…”

“…These measurements were taken from radiographs in ImageJ (Schneider, Rasband, & Eliceiri, ) following the general scheme set forth by Pike and Maitland (). The ventral arc of the claw was not calculated, given that many claws have a ventral constriction near the claw tip (Pike & Maitland, ) and that the dorsal arc gives a better estimate of claw curvature than the ventral arc (Tinius & Russell, ). Note that these traditional methods assume that both the arcs of the bony core and keratinous sheath inscribe a circle.…”

“…We expand on this recent work using a combination of geometric morphometric techniques and radiographs showing the inner bony core and outer keratinous sheath of a large sample of pedal digit III bird claws from across Aves. Using these data, we further test the findings of Tinius and Russell () that the geometric morphometric approach is the best approximation of claw shape, by comparing a geometric morphometric approach with traditional morphometric arc measurements. Traditional morphometric and geometric morphometric data were extracted from both the bony core shape and keratinous sheath shape of each specimen to assess the following: (a) Is there a relationship between ecology and morphology in avian pedal claws after accounting for phylogenetic relatedness and body size (via a proxy)?…”