Predatory Functional Morphology in Raptors: Interdigital Variation in Talon Size Is Related to Prey Restraint and Immobilisation Technique

Abstract

Despite the ubiquity of raptors in terrestrial ecosystems, many aspects of their predatory behaviour remain poorly understood. Surprisingly little is known about the morphology of raptor talons and how they are employed during feeding behaviour. Talon size variation among digits can be used to distinguish families of raptors and is related to different techniques of prey restraint and immobilisation. The hypertrophied talons on digits (D) I and II in Accipitridae have evolved primarily to restrain large struggling prey while they are immobilised by dismemberment. Falconidae have only modest talons on each digit and only slightly enlarged D-I and II. For immobilisation, Falconini rely more strongly on strike impact and breaking the necks of their prey, having evolved a ‘tooth’ on the beak to aid in doing so. Pandionidae have enlarged, highly recurved talons on each digit, an adaptation for piscivory, convergently seen to a lesser extent in fishing eagles. Strigiformes bear enlarged talons with comparatively low curvature on each digit, part of a suite of adaptations to increase constriction efficiency by maximising grip strength, indicative of specialisation on small prey. Restraint and immobilisation strategy change as prey increase in size. Small prey are restrained by containment within the foot and immobilised by constriction and beak attacks. Large prey are restrained by pinning under the bodyweight of the raptor, maintaining grip with the talons, and immobilised by dismemberment (Accipitridae), or severing the spinal cord (Falconini). Within all raptors, physical attributes of the feet trade off against each other to attain great strength, but it is the variable means by which this is achieved that distinguishes them ecologically. Our findings show that interdigital talon morphology varies consistently among raptor families, and that this is directly correlative with variation in their typical prey capture and restraint strategy.

Claw morphometrics in monitor lizards: Variable substrate and habitat use correlate to shape diversity within a predator guild

“…It was not surprising that phylogenetic signaling plays a significant role in varanid claw morphology, considering it also strongly influences claw shape in other lizards (Tulli et al., 2009, 2011, 2012) and birds (Birn‐Jeffery et al., 2012; Fowler et al., 2009). We could not develop rigorous phylogenetic conclusions about Australian varanids as a whole, but tentative inferences may be drawn.…”

“…Claw characteristics in lizards have been correlated with performance variables such as clinging and sprinting (Crandell, Herrel, Sasa, Losos, & Autumn, 2014; Tulli, Abdala, & Cruz, 2011, 2012; Zani, 2000) or habitat/microhabitat preference (Ribas et al., 2004; Teixeira‐Filho, Rocha‐Barbosa, Paes, Ribas, & de Almeida, 2001; Tulli, Cruz, Herrel, Vanhooydonck, & Abdala, 2009). Bird claws have also received significant attention (Hahn, Dimitrov, Rehse, Yohannes, & Jenni, 2014), focusing on curvature and its relationship with habitat (Bock & Miller, 1959; Fowler, Freedman, & Scannella, 2009; Glen & Bennett, 2007; Mosto & Tambussi, 2014; Pike & Maitland, 2004). Large comparative studies of mammals have used claws/unguals to determine locomotor, and in particular fossorial, adaptations (MacLeod & Rose, 1993).…”

In vivo bite and grip forces, morphology and prey-killing behavior of North American accipiters (Accipitridae) and falcons (Falconidae)

“…Furthermore, the morphology of raptor digit flexor tendons and sheaths (Einoder and Richardson, 2006), along with talon size and shape (Einoder and Richardson, 2007a;Fowler et al, 2009), have been suggested to enhance their grasping abilities, and might mitigate any deficiency in force production. Alternatively, perhaps the ability to generate forces rapidly in order to dispatch vigorously struggling prey is paramount, and power (force ϫ velocity), rather than force alone, is the operative metric.…”

“…North American owls (Marti, 1974); American kestrels (Yamamoto, 1994); common buzzards and Eurasian kestrels Csermely and Gaibani, 1998); North American Buteo hawks, American kestrels and owls (Ward et al, 2002)]. Various characteristics of raptor talons play important roles in grasping, such as claw size and shape (Csermely and Rossi, 2006;Einoder and Richardson, 2007a;Fowler et al, 2009), and the digital flexor tendonlocking mechanism of the feet (Ward et al, 2002;Einoder and Richardson, 2006). Although these features are undoubtedly important for transmitting and maintaining grip forces, in the present study we focus on their abilities to generate them.…”

Morphological and functional diversity in therizinosaur claws and the implications for theropod claw evolution

“…In the past, numerous studies have focused on the morphology and geometry of pedal unguals in theropod dinosaurs [14,15], supplemented with or based on observations in extant birds [16,17] or squamates [18], in an attempt to infer function and behaviour from morphology. The manual unguals of theropod dinosaurs in general, and therizinosaurs in particular, however, have rarely been considered, or have been limited to range-ofmotion studies of the forelimbs [19][20][21].…”