Adaptive Mechanisms of the Raptor Pelvic Limb

Abstract

THIS study presents some adaptations in the pelvic limb of six North American raptorial species. Bock and von Wahlert (1965) point out the term adaptation needs to be defined when used. I studied evolutionary adaptations as I am interested in those structural features that enable raptors to sustain a predatory role within a given environment. I am not concerned with evolutionary pathways. In this regard, I studied closely related and diverse species having similar habits. I investigated the raptorial species from both orders and four of the five families in North America (the nonpredaceous New World vultures, the Cathartidae, were omitted). The species studied were the Cooper's Hawk (Accipiter cooperii), Red-tailed Hawk (Buteo jamaicensis), Prairie Falcon (Falco mexicanus), Sparrow Hawk (Falco sparverius), Screech Owl (Otus asio), Burrowing Owl (Speotyto canicularia), and the Barn Owl (Tyto alba). I compared the raptors to one another, as well as to a nonraptorial species. A generalized species is difficult to define, but one that does not use its legs and feet for any particular activity other than walking or standing seems appropriate. I selected the domestic pigeon (Columba livia). The functional aspects of the anatomy of the birds of prey have been little studied. Garrod (1875) described the vinculum connecting the tendons of the flexor digitorum longus and flexor hallicus longus for several genera of Falconiformes. Subsequent workers (Shufeldt, 1887; Hudson, 1937; Ruggeberg, 1960) further discussed the fusion of these tendons. Shufeldt (1909) presented a very general account of the osteology of several species including A. cooperii, B. jamaicensis, and F. mexicanus. Peczely (1964) compared the thoracic vertebrae of several genera of raptors including Buteo, Accipiter, and Falco. Where possible, he related anatomical differences to the habits of the birds. Studies of the pelvic region of various genera of Falconiformes and Strigiformes were made by Hudson (1937, 1948) and Berlin (1963). Accurate determination of muscle forces has long presented a problem to morphologists. Present knowledge of muscle structure and physiology limit severely the traditional techniques of comparing muscle volumes and weights (for a review, see Gans and Bock, 1965). Eccles and Sherrington (1930) demonstrated the dangers of using the weight (and thus volume) of muscles as an indication of tension. They showed that though the medial gastrocnemius of the cat weighs 21/2 to 3 times as much as the soleus, it develops about 6 times as much tetanic tension