Sexual dimorphism in body composition of small mammals

Differences in reproductive roles between the sexes may lead to sexual dimo rphism in body composition. Body size and composition of three species of s mall mammals (bushy-tailed wood rats (Neotoma cinerea Ord), deer mice (Pero myscus maniculatus Wagner), and red-backed voles (Clethrionomys gapperi Vig ors)) were analyzed to test the predictions that (i) males will have more m uscle mass than females and (ii) females will have more fat than males. Res ults supported the first prediction but not the second. For all three speci es, males had more lean dry mass relative to body size than females, but fe males did not have relatively more fat than males. Muscle mass of males may aid in mate-searching and mate-guarding activities, but fat content may no t differ between the sexes because female small mammals depend on increased ingestion rates, rather than fat stores, to support reproduction.