Sexual Size Dimorphism in Vespertilionid Bats

Sexual size dimorphism was investigated in 18 species of vespertilionid bats (Chiroptera: Vespertilionidae). The characters representing size were length of head and body (HBL), length of forearm (FAL), condylocanine length (CCL) and length of maxillary toothrow ( MTL ). From these variables, two proportional characters were calculated, HBL/FAL and CCL/MTL. Student's t-tests were used to determine significant differences (P ^ 0.05). Females averaged larger than males in two or more size dimensions for all 18 species, with significant differences being noted in 15 of the species. In no cases were males significantly larger than females. Proportional differences were fewer, with significant differences being found in only five species. Mean lengths of forearm and maxillary toothrow were adjusted by covariance analysis in order to negate the differences in absolute size between the sexes. Using this procedure, females of six species had significantly longer forearms than males, and males of one species had significantly longer forearms than females. There were no significant correlations between the number of young per pregnancy and the degree of dimorphism exhibited by the species tested. The general patterns in these data, when coupled with life history information, suggest that increased energy demands during pregnancy may be the primary factor in the selection for larger size in females. Larger females can maintain homeothermy, and hence the timing of birth, more efficiently, can store more fat, and have a greater size array of prey available to them. Increased weight loading of pregnant females is probably also important in the selection of larger size in females, but this hypothesis is not supported by these data.