The Hindlimb Musculature of the Mousebirds (Coliiformes)

-The hindlimbs of two species of mousebirds (Colius striatus and C. leucocephalus) were dissected in order to provide myological descriptions, to elucidate the mechanism of digit rotation and other characteristic locomotor habits, and possibly to yield clues to the systematic position of the order Coliiformes. Among the myological peculiarities, the unusually large M. iliofemoralis extemus, the accessory belly of M. pubo-ischio-femoralis, and the extensive femoral insertion of M. flexor cruris lateralis and origin of M. flexor hallucis longus are features related to the hanging postures of the Coliiformes. Digit rotation is accomplished primarily by enlargement and slight changes in the insertions of four intrinsic foot muscles. Two small intrinsic foot muscles, not previously described in birds, also appear to be associated with digit rotation. In the species dissected, the extensor digitorum longus tendon sends a branch to the hallux as well as to the three forward digits. This condition has previously been described only in the Psittaciformes, and may possibly suggest a common ancestry for the two orders. Received 16 March 1981, accepted 21 July 1981. THE Coliiformes is an order of birds inhabiting scrub and lightly wooded areas of subSaharan Africa. The single genus Colius includes six living species commonly called colies or mousebirds. They are differentiated primarily on the basis of plumage (Sclater 1903, Friedmann 1930, Chapin 1939, MackworthPraed and Grant 1962). Whereas the systematics within the order are not a problem, because of certain peculiarities in their habits and morphology the relationship of the Coliiformes to other avian orders has always been uncertain. At various times and for various anatomical and behavioral reasons, affinities to the Passeriformes, Piciformes, Psittaciformes, Coraciiformes, Apodiformes, and Caprimulgiformes have been suggested (Murie 1872, Garrod 1896, Pycraft 1907, Lowe 1948, Verheyen 1956). Ballman (1969: 192-195) studied fossil mousebirds from the Miocene of France. He briefly described several skeletal elements, and corrected errors of family allocation made by earlier workers. The most conspicuous feature distinguishing the Coliiformes from other birds is their ability to rotate both the first and fourth hindlimb digits to either a cranial or caudal posiI Present address: Department of Biology, College of the Holy Cross, Worcester, Massachusetts 01610 USA. tion. This gives them the option of being anisodactyl, zygodactyl, or pamprodactyl (Fig. 1). Our extensive observations of living mousebirds reveal no correlation between any digit configuration and any particular activity. Whatever position is most effective for the object being grasped is the one put to use. As indicated in Fig. 1, the same configuration is not necessarily most appropriate for both feet at the same time. In addition to the peculiarities of their feet, mousebirds are capable of a variety of very odd movements and postures, which are accompanied by some unlikely contortions of the hind limb. They creep rapidly through dense vegetation in a rodent-like manner, with the longitudinal axis of the body parallel to the branch they are grasping. They are seldom seen perching in a conventional position. When resting, even for a moment, there is usually a tendency for the body to drop below the feet, so that the bird is hanging to some degree (Fig. 1). Mousebirds use their hindlimbs extensively in feeding. Small food items are held in one foot in a parrot-like manner, while the bird hangs or supports itself by the other (Fig. 1). The foot has considerable manipulative ability when used in this way, the digit configuration being modified as the item is consumed and changes shape. Finally, our observations of the use of the 41 The Auk 99: 41-57. January 1982 This content downloaded from 207.46.13.148 on Sun, 11 Sep 2016 04:24:46 UTC All use subject to http://about.jstor.org/terms 42 BERMAN AND RAIKOW [Auk, Vol. 99