Vertical stratification in foraging activity of the wrinkle-lipped free-tailed bat, chaerephon plicatus (buchanan, 1800) in Central Thailand

Abstract

The Wrinkle-lipped free-tailed bat, Chaerephon plicatus, widely distributes in South and Southeast Asia. The species roost in large colony with the estimated number up to several million individuals. Chaerephon plicatus also provides important guano resource. Previous studies have shown the species as important pest suppression agents which predated on rice pests such as White-backed planthopper, Sogatella furcifera, and Brown planthopper, Nilaparvata lugens. It is also suggested that C. plicatus forage at high altitude since substantial number of brown planthopper, a high altitude migratory insects, found in bat diet while no brown planthopper capture in the ground. Yet there is any direct evidence found to confirm this hypothesis. This study concentrates on the foraging activity of C. plicatus on vertical dimension. The bat activity was measured by acoustic sampling using bat detectors mounted on a 3×3m obsolete helium kite balloon. Three altitudinal levels were measured simultaneously including 2m, 100m and 200m. Foraging activity of C. plicatus was highly stratified in vertical dimension. Number of call recorded aloft (100m and 200m) was significantly higher than at the ground. Nocturnal boundary layer was typically formed at the nights when the data were collected. The level of 100m might have been the top of nocturnal boundary layer which is known at the layer of high migratory insect density. The high number of calls recorded at 100m and 200m showed a link between foraging activity of C. plicatus and migratory insects. However, the altitudinal level of high activity of bats, which ca. 100m is also in the swept-area of wind turbine blade. The construction of wind turbine farms in the areas of high number of C. plicatus could be harmful to the species. The vertical stratification in foraging activity of C. plicatus might also reveal a niche partitioning between open space bats in vertical dimension. This study revealed a clue in a shift in whole-night activity pattern of C. plicatus. Whole night pattern at the ground has a peak in the early part of the night, however, gradually decrease until morning. The biomass of insects near ground level also showed at gradual decrease trend during the course of night. Meanwhile, an increasing in activity found at high altitudes after midnight. Chaerephon plicatus might take advantage by foraging near ground level when the insects swarm shortly after dusk. However, as the number of insects decrease, C. plicatus shift their activity to higher altitude to avoid interspecific competition with other edge-open space bats such as Myotis sp. However, this remains a hypothesis and needs to be investigated in more detail since a few data are available in the area. Future study can be conducted to test this hypothesis.