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Sexual Size Dimorphism in Bonin Flying Foxes Pteropus pselaphon on Chichijima, Ogasawara Islands

Sexual size dimorphism is especially enhanced in mammals with polygamous mating systems in which males engage in intrasexual competition to gain access to receptive females (Clutton-Brock 1989; Weckerly 1998). Many Pteropodidae bats as well as other mammals have polygamous mating systems (McCracken and Wilkison 2000) with sexually dimorphic traits including body size, canine teeth, odor glands, and fur coloration (Hill and Smith 1984; Nowak 1994). For example, in a lekmating Pteropodidae bat, the hummer-headed fruit bat Hypsignathus monstrosus, the males weigh twice as much as the female and have the greatest sexual dimorphism in the entire Chiroptera order (Bradbury 1977). In the genus Pteropus, the majority of known mating systems are polygynous (Grant and Banack 1999; McCracken and Wilkinson 2000; Welbergen 2011; Sugita and Ueda 2013). However, statistical documentation of sexual size dimorphism in Pteropus species has been lacking. Welbergen (2010) tested sexual size dimorphism in wild grey-headed flying foxes, Pteropus poliocephalus, and reported that the body size (forearm, tibia, thumb, and thumb claw) and mass of adult males was 1.8–4.5% larger and 25% heavier than that of adult females, respectively. Bonin flying foxes, Pteropus pselaphon, on Chichijima in the Ogasawara islands form warming clusters, which are grouped by sex and ages in winter colonies: males, females, and subadults. In the roosts, resting animals exhibit a female defense harem mating system (Sugita et al. 2009; Sugita and Ueda 2013). If larger male Bonin flying foxes favor male-male competition for mate monopolization as much as in other mammals, selection of sexual size dimorphism of larger males may be a result of mate guarding. However, only a few reports have been published on measurement of flying foxes. Maeda (1983) and Yoshiyuki (1989) reported measurement values for specimens stored in some museums. Abe et al. (1994) and Suzuki et al. (2008) reported external measurements of five wild individuals on the Chichijima Island and four on the Minami-iwoto Island. In previous studies, no statistical test has been conducted on sexual size dimorphisms of Bonin flying foxes with polygamous mating systems. The purpose of this study is to verify that sexual differences are based on external morphological traits in mature wild Bonin flying foxes with polygamous mating systems on the Chichijima Island. The data were obtained through several capture sessions for ecological investigations over a period of two winter seasons. Additionally, we discussed the relationship between measurement taken and the polygamous mating system of the flying foxes.