Timing of Cave Emergence and Return in the Dawn Bat (Eonycteris spelaea, Chiroptera: Pteropodidae) in Southern Thailand

Abstract

The time of emergence of bats from their roosts at dusk and their return was originally described as a circadian rhythm entrained by the time of sunset and sunrise (Erkert 1978). Later, various environmental factors such as ambient light conditions, weather (Frick et al. 2012) and moonlight (Elangovan and Marimuthu 2001; Thies et al. 2006) were considered to influence bat activity. Bats emerging before dark or during bright moonlight in­ creases their risk of predation by raptorial birds and they avoid such situations (Welbergen 2006). However insecti­ vorous bats often risk predation to increase foraging opportunities when their prey is available in abundance at dusk (Jones and Rydell 1994; Rydell et al. 1996; Duverge et al. 2000). Besides environmental factors, foraging efforts differ between male and female mammals according to the energetic requirements of reproduction (Bronson 1985). Breeding females allocate more time and energy to for­ aging to meet the energy demands of growing pups (McCracken and Wilkinson 2000; Racey and Entwistle 2000; Barclay and Harder 2003). In contrast, reproduc­ tive males spend time and energy in defending females or resources useful to them in order to achieve mating suc­ cess with multiple females (Clutton­Brock 1989). In fruit bats, roost emergence and return times of adult males often differ compared with other reproductive categories, especially during the breeding period. For example, adult males of Old World tent­making (e.g., Cynopterus sphinx) and cavity roosting fruit bats (e.g., Balionycteris maculata) spend less time away from the roost for­ aging and more time at the roost (Balasingh et al. 1995; Marimuthu et al. 1998; Hodgkison et al. 2003). Adult male Rousettus aegyptiacus and Pteropus tonganus emerge after adult females (Korine et al. 1994; Banack and Grant 2003). Similar patterns were found in neotropical fruit bats such as Artibeus jamaicensis (Morrison and Morrison 1981) and A. lituratus (Munoz­Romo et al. 2008). In Southeast Asia, Eonycteris spelaea roosts in caves and travels over tens of kilometres to access food sources (Start 1974). It is the principal pollinator of economi­ cally important crops like durian (Durio zibethinus), a leguminous canopy tree Parkia spp. and the Indian trumpet flower (Oroxylum indicum) in managed agri­ cultural habitats in southern Thailand (Bumrungsri et al. 2008, 2009; Srithongchuay et al. 2008). Eonycteris spelaea reproduces year round (Beck and Lim 1973; Bhat et al. 1980; Heideman and Utzurrum 2003). In a previous study, a capture survey at a cave entrance suggested a high proportion of mature males return during the early hours of the night (Bumrungsri et al. 2013). We investigated the emergence and return time of the dawn nectar bat, E. spelaea, using radio­telemetry to record the time of cave emergence and return by E. spelaea of known sex and reproductive status. We hy­ pothesized differential foraging durations between adult male E. spelaea and other reproductive categories.