Dry-Season Overlap in Activity Patterns, Habitat Use, and Prey Selection by Sympatric African Insectivorous Bats

Abstract

We used mist nets, ultrasonic sensors, light tags, and analysis of feces to examine habitat use, activity patterns, and prey selection of some insectivorous bats during the dry season at the Sengwa Wild Life Research Area (18?10'S; 28013'E) between 7 and 28 June 1977. The results show broad overlap in all parameters investigated for the 13 species present in the area during the dry season. During the wet season, some species of insectivorous bats relied more on beetles than on moths as food, or vice versa. The lack of food partitioning, particularly in the dry season, appears to conflict with theories of niches and competitive exclusion, but it is in accord with predictions based on optimum foraging strategy. We conclude that most insectivorous bats are opportunistic feeders, a strategy which results in a mosaic of specialized and generalized diets and which is compatible with their energetic demands. We report the first record of Tadarida chapini from Zimbabwe (Rhodesia). FOOD AND ROOSTS are two potentially limiting resources which may be partitioned by sympatric species of bats (Tamsitt 1967, Fenton 1970, McNab 1971, Humphrey 1975). Partitioning of food resources can involve habitat (Handley 1967) and/or time (Kunz 1973) and/or types of food (Black 1972, 1974). A variety of studies has shown that bats can be either very selective in their feeding (Buchler 1976a), or generalized in their diets reflecting local abundances of insects (Belwood and Fenton 1976, Anthony and Kunz 1977). Bradbury and Vehrencamp (1976) found broad overlap in prey size between five species of Neotropical emballonurids which showed flexible feeding behavior, and Kunz (1974) reported that Myotis velifer adjusted its foraging patterns to meet local conditions. A reflection of the flexibility in foraging behavior is provided by Fenton and Morris' (1976) experiments which demonstrated that some insectivorous bats were opportunistic, rapidly responding to concentrations of insects. Similar responses to concentrations of food occur for other bats as well (Fleming et al. 1977, Gould 1978). Our purpose here is to report some observations we made of activity patterns, habitat use, and prey selection by some sympatric African insectivorous bats, and to interpret these results in the context of resource partitioning (e.g., Schoener 1974) and optimal foraging strategy (e.g., Pyke et al. 1977). The field data for this study were gathered during the dry season in an African deciduous forest; they are compared to previously published wet-season data from the same locality (Fenton 1975, Fenton et al. 1977). We made our observations in the vicinity of the Hostes Nicolle Institute of Wild Life Research (18?10'S; 28013'E, ca. 820 m above sea level) in the Sengwa Wild Life Research Area, ca. 110 km west of Gokwe, Zimbabwe. Most of our work was done at the same sites used by Fenton (1975) and Fenton et al. (1977), and included miombo and mopane woodland, riparian forest, pans with and without water, and some locations along the Sengwa and Lutope Rivers (for details of vegetation and physiography see Cumming 1975). Several different types of habitats can be identified in the study area based on the composition of plant communities, but most of the area is open woodland, whether miombo or mopane (fig. 1). However, woodlands along the rivers may have higher densities of trees than woodlands away from the rivers, and the Sengwa River is bordered by open, grassy floodplain (fig. 1). While during the wet season some bats were more commonly captured in a specific habitat (Fenton 1975), during the dry season this distinction was much less clear. Furthermore, captures in one habitat or another do not necessarily reflect the use of these habitats by insectivorous bats (see below). MATERIALS AND METHODS We captured bats in 9 or 12 m, 351 mm mesh mist nets or in a Tuttle trap (Tuttle 1974) set at various locations near the Institute. Bats were removed from the nets or trap, sorted by sex and species, and then either released (sometimes after tagging) or held overnight in cloth bags to permit collection of feces. Bats held for collection of feces were those which had fed prior to capture (determined by palpation of the BIOTROPICA 12(2): 81-9