Quadrupedal Bats: Form, Function, and Evolution

Abstract Although bats (Chiroptera) are unique among mammals because of their ability to fly, they also exhibit quadrupedal locomotion during a variety of nonflight behaviors. This chapter concerns the evolution, morphology, and behavior associated with quadrupedal locomotion in bats. Although many bat families contain members that exhibit some degree of quadrupedal locomotion, we concentrate on three groups of bats that exhibit extensive quadrupedal behavior: vampire bats (Phyllostomidae: Desmodontinae); naked bulldog bats, Cheiromeles (Molossidae); and the New Zealand short-tailed bat, Mystacina tuberculata (Mystacinidae). Numerous morphological traits that apparently facilitate quadrupedal locomotion have evolved independently in these three lineages. INTRODUCTION When bats are not flying or resting in their roosts, they often exhibit some type of quadrupedal behavior while moving around on various substrates. The awkward appearance of quadrupedal locomotion in most bats is primarily a reflection of the fact that chiropteran limbs are markedly different from those of a typical quadrupedal mammal. Obviously, the forelimbs of bats have evolved into wings. Less obvious, but just as important, are the changes in the morphology and orientation that have evolved in their hindlimbs. Vaughan (1970: 135) divided bats into three groups based on ‘‘variation in the posture and proportions of the hind limbs.’’ Such variation was thought to be ‘‘related principally to the great differences that occur in roosting habits and in modes of terrestrial locomotion.’’ One group of bats, typified by phyllostomids such as Glossophaga and Macrotis, have hindlimbs that are rotated approximately 180o from the condition in a typical quadrupedal mammal. In these forms, which we refer to as ‘‘Type 1’’ bats, the thin femora project caudally and slightly dorsad, and the head of the femur is in line with the long axis of the bone. The distal section of the leg (the shank) is directed caudally and ventrad. The feet are directed caudally (figure 8.1A).