Sexual Size Dimorphism in Vespertilionid Bats

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Abstract
Sexual size dimorphism was investigated in 18 spp. of vespertilionid bats [Myotis auriculus, M. evotis, M. keenii, M. leibii, M. nigricans, M. lucifugus, M. thysanodes, M. velifer, M. volans, M. yumanensis, Lasionycteris noctivagans, Eptesicus fuscus, Lasiurus borealis, L. cinereus, Euderma maculatum, Idionycteris phyllotis, Antrozous pallidus and Plecotus townsendii] (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, 2 proportional characters were calculated, HBL/FAL and CCL/MTL. Student''s t-tests were used to determine significant differences (P .ltoreq. 0.05). Females averaged larger than males in 2 or more size dimensions for all 18 spp., 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 found in only 5 spp. Mean lengths of forearm and maxillary toothrow were adjusted by covariance analysis to negate the differences in absolute size between the sexes. Using this procedure, females of 6 spp. had significantly longer forearms than males, and males of 1 sp. 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 the 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 important in the selection of larger size in females, but this hypothesis is not supported by these data.

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