Diets and resource use
Food plants used by Yaeyama fruit bats, as previously reported based mostly on anecdotal and sporadic notes, include 16 species of fruits, ten species of flowers, and fibrous stalks of sugarcane, for a total of 25 species (Nakamoto et al., 2007b). This study added another 14 species of plants to this list. Overall, the diet was less diverse than that reported for Orii’s fruit bats on Okinawa (fruit: 53 species, flower: 20 species, leaf: 18 species; Nakamoto et al., 2007a), but broader than that of Erabu (fruit: 19 species, flower: eight species, leaf: 11 species, bark: two species) and Daito fruit bats (fruit: 22 species, flower: nine species, leaf: six species; Funakoshi et al., 1993; Nakamoto et al., 2007b). Compared to other tropical pteropodids (e.g., Marshall, 1985), Funakoshi et al. (1993) attributed the broader diet of Erabu fruit bats to the warm-temperate seasonality in the food supply. This was not fully supported by our study conducted at a lower latitude near the Tropical of Cancer, and equally or more-diverse diets reported more recently in other paleo-tropical fruit bats (e.g., on American Samoa, Banack, 1998; Malaysia, Tan et al., 1998).
Instead, the latitudinal variation in the diet breadth of Ryukyu flying foxes, and the fact that our assessments were conducted only in the prime summer season, suggest a connection between diversity and variation in diets of fruit bats with food availability that is influenced by local phenology and the size of the distribution area where bats occur. This is confirmed by results for other species in the Pacific (e.g., McConkey and Drake, 2007). We observed asynchronous fruiting among individual trees of the same fig species, which also explains the high variation in abundances of bats recorded among transect-nights, and is an indication of local movements (Funakoshi et al., 1993). Food availability to frugivorous bats is often difficult and complicated to accurately estimate (Stashko and Dinerstein, 1988; but see Kalko, 1998), and it has not been quantitatively assessed for most subspecies of Ryukyu flying foxes. Our data provide evidence of a positive correlation between bat abundances and heterogeneity of tree compositions and the density of fruiting trees.
The Moraceae is the single most important family of food plants for Yaeyama fruit bats, and figs are the predominant food type, which was reported in almost all previous studies on diets of flying foxes (Marshall, 1985; reviews in Shanahan et al., 2001) and even other non-pteropodid fruit bats (Kalko et al., 1996; Korine et al., 2000). On Iriomote where the human population is low and mature primary forests remain intact on a large proportion of the land, bats feed mainly on native fruit plants (32 of 39 species of plants, 82.1%). This is a proportion closer to but higher than the one observed in Erabu fruit bats on Kuchinoerabu (22 of 29 species of plants, 75.9%; Funakoshi et al., 1993), and much higher than the one found in Daito fruit bats (15 of 26 species of food plants, 57.7%; Nakamoto et al., 2007b). Nakamoto et al. (2007a) concluded that the diverse diet of Orii’s fruit bats results from adaptation to cultivated plants (30 of 78 species of food plants, 38.5%) in urbanized environments, as also has been noted in grey-headed flying foxes (McDonald-Madden et al., 2005; Williams et al., 2006). Yet, nutrient deficiencies may be the real drive why flying foxes feed on a more diverse group of plants, as high proportions of cultivated plants have replaced native species, especially figs (Nelson et al., 2000). Most of the major fruits identified in samples, i.e., figs and mulberries, contain small but many seeds, which is consistent with feeding behavior described for flying foxes (Richards, 1990). Common garcinia, which contains a few large seeds, is an exception and also a fruit found favored by Daito fruit bats (Kinjo et al. unpubl. data). The dropped fruit remains, retaining seeds, were often located beneath or very close to the trees where bats fed, and the distances the seeds were dispersed by bats appeared greatly reduced.
We found seven species of leaves in the diet, and various animals at a notable frequency (11.9%), but with a minute proportion in mass. These have not been reported previously in the Yaeyama subspecies (Nakamoto et al., 2007b). Leaves appear in an increasing number of species of fruit bats of both the Paleo- and Neotropics (e.g., Kunz and Diaz, 1995; Courts, 1998; Nelson et al., 2000), and support the idea that folivory provides essential nutrients (e.g., carbohydrates, proteins, and calcium) for daily requirements (Rajamani et al., 1999; Nelson et al., 2005). While animal items are not uncommon in diets of pteropodids (e.g., Barclay et al., 2006), they are often attributed to incidental ingestion. It requires further observations or experiments for us to fully verify this possibility or alternatives, such as a rare case of carnivory in megachiropterans (Courts, 1998).