Diurnal activity of a trawling insectivorous bat species, Myotis horsfieldii, in Gunung Mulu National Park, Malaysian Borneo

Bats are typically nocturnal. However, daylight activity (ie between one hour after sunrise and one hour before sunset) is known for a small subset of species, often associated with small oceanic islands with no diurnal avian predators. Here, we describe numerous observations of diurnal activity of Horsfield’s bat (Myotis horsfieldii), in Gunung Mulu National Park in Sarawak, Malaysia. Between 2015 and 2020, bats were repetitively observed flying along streams and rivers at 14 localities during various hours of the day. Daylight active bats exhibited a flight pattern concurrent with prey hunting strategies and, through bioacoustics, foraging was confirmed by the presence of feeding buzzes. Despite the occurrence of numerous sympatric avian diurnal predators known to hunt bats (eg bat hawk Macheiramphus alcinus) and avian competitors (eg cave swiftlet Aerodramus spp.), the relatively wide temporal and spatial coverage of the records suggests that diurnal activity of this water-associated bat might be a common phenomenon in the area. We speculate that this atypical behaviour might have arisen to circumvent night-time competition with the high numbers of bats roosting in nearby caves.

蝙蝠通常为夜行性动物。然而，对于一小部分物种而言，日间活动（即日出后一小时至日落前一小时之间）亦有所发现，此类现象通常与缺乏日间鸟类捕食者的偏远海洋岛屿相关联。本研究对马来西亚沙捞越的穆鲁国家公园内，霍斯菲尔德蝙蝠（Myotis horsfieldii）的日间活动进行了多次观察。在2015年至2020年期间，蝙蝠在14个不同地点反复被观察到在溪流和河流上空飞翔，且时间分布于一天中的不同时段。日间活动的蝙蝠表现出与捕食策略一致的飞行模式，并通过生物声学技术，通过捕食时产生的嗡嗡声确认了觅食行为。尽管存在众多已知捕食蝙蝠的日间鸟类捕食者（例如蝙蝠鹰Macheiramphus alcinus）和鸟类竞争者（例如洞穴雨燕Aerodramus spp.），但记录所显示的相对广泛的时空覆盖范围表明，这种与水相关的蝙蝠的日间活动可能在该地区是一种普遍现象。我们推测，这种不寻常的行为可能是为了规避与附近洞穴中大量栖息的蝙蝠在夜间竞争。