Claws are found in a huge range of vertebrates, ranging from reptiles and birds to mammals. These pointed structures at the tips of digits, usually covered by a sheath of keratin, come in a wide variety of shapes and sizes that can vary considerably between species and even between limbs or digits of an individual. Claws can be used in many ways, such as climbing, grasping prey or digging. The morphological form of a structure (or rather, the shape of a structure) is often used as a guide to help understand its biomechanical function, but the large diversity of claw shapes has made it difficult to understand the link between the shape of the claws and how they are used. While previous studies have attempted to link claw shape to where the animal lives in its habitat, the link between shape and function has proven trickier. Tracy Thomson and Ryosuke Motani, from University of California, Davis, USA, sought to clarify the links between claw shape and claw use by expanding the scope of measurements used to detail the shape and by categorizing the tasks for which animals use their claws in more detail.
After accessing the skeletons of 80 animals, a mix of mammals, birds and one reptile, the authors analysed 18 features of the claws that reflected their shape. Some of the features that they measured included the claw angles, how deep claws are at the base, the size of the areas where muscles attach to claws and how quickly a claw tapers down to a point. The authors searched the literature to put each claw into one of eight possible functional categories. For example, cursorial claws are used during running or jumping (e.g. cheetahs or ostriches), scansorial claws are used for climbing (e.g. squirrels), and amplectorial claws are used for grasping (e.g. birds of prey).
The authors then used several statistical analyses and found a method that can be used to reveal claw function based on their shape alone 96% of the time. They revealed that cursorial claws used for running and jumping tend to have a blunt sheath and rounded tip to help an animal to push off of the ground and provide friction. Scansorial claws should have greater curves to help the animals get a grip while climbing. These claws tend to have a thin and elongated cross-section with well-developed areas for muscle attachment for quick extension while ascending trees or bushes. Amplectorial claws found in predators such as kestrels and other birds that grasp onto prey typically have an enlarged muscle attachment site for powerful gripping. Those claws usually have a high level of curvature and a wide cross-section, which could help to steady struggling prey.
By taking care to choose accurate claw function categories and function-informed shape measurements, Thomson and Motani have shown common shape themes in claws that are used for specific functions. Their methods could help researchers to understand how extinct creatures that we only know from fossils used their claws, or to predict how animals that are difficult to observe may use their claws. This study is a promising look into the opportunities afforded by a better understanding of the bridge between the shape of a structure and its function.