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Sexual Dimorphism in Size of Vespertilionid Bats

Sexual dimorphism in birds and mammals is a phenomenon that has provoked intense interest. Two explanations (not mutually exclusive) are most commonly given for selection favoring dimorphism. The first, put forth by Charles Darwin, is based on the competition of one sex for the other and the different selective pressures resulting on the competitor and the object of competition (Darwin 1859; see Trivers 1972 and Ralls 1976 for reviews). Trivers argues that the sex contributing most to parental care will be in short supply and hence will be the object of competition. Thus, in most mammalian species, males compete for females. Large size is often advantageous in intraspecific competition, so males tend to be larger than females when competition occurs. The second explanation of sexual dimorphism is that differences between males and females are a means of reducing intraspecific competition for resources, especially food (Amadon 1959; Selander 1966; Storer 1966; Schoener 1967; Earhart and Johnson 1970). Vespertilionid bats tend to be sexually dimorphic in size. The differences are slight but are found in many species (see below). Females are larger in all cases where significant differences have been found. Bats are unique among mammals because they fly. Flight places strict limitations on the size and proportional relationships of different parts of the body; coefficients of variation for wing measurements of bats are lower than those for limb measurements of terrestrial mammals (Long 1968). Bats have further undergone a reduction in the size of the extreme anterior and posterior parts of the body, resulting in the reduction of weight and the concentration of mass in the pectoral region. Yet, despite the apparent importance of size, female bats are capable of carrying large young. Near-term embryos commonly weigh 20%-30% of the female's body weight after birth (table 1). This is a considerable load, and its effect on flight is increased because it is located well posterior to the point of attachment of the wings. Furthermore, females may carry their newborn young to new roosting sites if disturbed after parturition (Davis 1970). Based on these considerations, a different selective basis for the sexual dimorphism of vespertilionid bats has been suggested (Phillips 1966; Jones 1967; Fenton 1972; Baker et al. 1972; Kunz 1974b). Body size is determined by a compromise between many requirements. In the case of male bats, feeding and roosting habits are probably paramount. For females these same factors are important, but in addition an increase in body size reduces the proportionate load of the fetus or newborn young. Wing size may increase even more, resulting in a reduction of wing loading (grams carried per square centimeter of wing area). In a similar manner, the relative cost of producing milk (measured, for example, as calories per gram female weight) is reduced when females are larger. Further, lactating females consume more food than males (Kunz 1974a). Large size may permit females to carry a larger quantity of insects in their stomachs. Thus,