In many mammals large size is more important for male than female fitness, which may select for bimaturation with males maturing later and attaining a larger size. In bats, however, sexual dimorphism in size tends to be reversed, with females being larger. Exceptions to this can be found, notably in the genus Pteropus. I examined growth, maturation, and sexual size dimorphism in free-ranging gray-headed flying foxes (Pteropus poliocephalus) to better understand the links between life history, behavior, and ecology of this large, polygynous Australian fruit bat. Juveniles left the colony independently at dusk when their forearm length and body mass exceeded 130 mm and 301 g, or 79% and 39% of mean adult dimensions, respectively. Mean forearm length and mass of these volant juveniles increased by 0.72 mm and 4.9 g per week, independent of sex. However, significant dimorphism and bimaturation were observed: adult males averaged 1.8–4.5% larger for skeletal measurements and 25% heavier than adult females, but 40% heavier at the start of the breeding season. Males also were sexually mature at a larger skeletal size and at a higher mass (604 g) than females (514 g). Sexual size dimorphism in P. poliocephalus likely results from prolonged male growth and delayed maturation. Intrasexual selection among males for fighting ability is implicated as the cause for dimorphism, together with a release of female size from the constraints posed by carrying heavy young. Bimaturation likely results from the high costs of maintaining mating territories that confer few reproductive benefits for small males.
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Vol. 91 • No. 1