Ontogeny of Sexual Dimorphism in Size among Polytocous Mammals: Tests of two Carnivorous Marsupials

Abstract

In many species of carnivorous marsupials (Dasyuridae), adult males are far heavier than females (≤ 100%). The theory of maternal investment predicts that juvenile males should receive greater lactational investment than female littermates, especially if maternal condition is good, because rapid growth disproportionately benefits reproductive fitness of males. I examine the development of size dimorphism between male and female siblings of Phascogale tapoatafa and Dasyurus geoffroii and compare litters of captive mothers with food given ad lib. to litters in the wild. In captivity, sons are significantly heavier and larger than daughters prior to weaning. Wild juveniles are not sexually dimorphic in size until after they begin to forage for themselves. The ontogeny of sexual dimorphism in size among wild P. tapoatafa and D. geoffroii occurs gradually after weaning and is primarily a somatic as opposed to skeletal divergence in size. The somatic energy stores of males are catabolized during the short breeding season and allow a reduction in foraging effort during this critical period of high activity