Sexual Dimorphism in Size of Vespertilionid Bats

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Abstract
Sexual dimorphism in body size of mammals is usually explained in terms of competition among members of 1 sex for members of the other or as a means of reducing intraspecific competition for resources such as food. Sexual dimorphism in vespertilionid bats, often characterized by females exceeding males in body size, does not seem to fit either explanation. A 3rd hypothesis, that female body size is influenced by the need to fly with and nourish large fetuses (and occasionally young), is probably unique to bats. If true, it should follow that the greatest degree of sexual dimorphism would occur in those species bearing the greatest fetal or neonatal weight. Examination of data on size (skull and forearm lengths) and reproduction for vespertilionid bats revealed that total weight and modal number of young born per pregnancy per species were strongly positively correlated with degree of sexual dimorphism in wing size. Covariance analysis showed that wings of females of several vespertilionid species were larger than those of males even after statistical adjustment of the data to make the body size of each sex the same. These observations suggest that sexual dimorphism in size of vespertilionid bats is influenced by the demands of large fetuses.