INTRODUCTION Chaerophon plicatus (family Molossidae, sub- family Molossinae) is a widely distributed bat throughout lowland South Asia and Southeast Asia, from India east to the Philippines and south to Lombok (Heaney et al., 1998; Molur et al., 2002). The species is a facultative cave-dweller, and in suit- able locations forms very large cave colonies. In northern Borneo, spectacularly large colonies are well-known from Simud Hitam Cave (Gomantong Forest Reserve, Sabah — Fig. 1) and Deer Cave (Mulu National Park, Sarawak). A formerly very large colony, now much reduced, occupies Niah Cave (Niah National Park, Sarawak — Medway, 1958; Struebig et al., 2010). Foraging areas have not been explicitly measured for this species, but by analogy to the North American member of the same subfamily — Tadarida brasiliensis (Ammer- man et al., 2012) which has measured nightly foraging ranges averaging 41 km around the Carls- bad Caverns (New Mexico) colony (Best and Geluso, 2003). It can be presumed that major C. plicatus colonies are top insect predators over very large expanses of lowland rainforest and agricultural mosaics. These colonies feed on some nine kg of insect prey per 1,000 bats per night (based on mass scaled feeding rates of T. brasiliensis from Kunz et al., 1995; C. plicatus mean body mass of 15.5 g from Leelapaibul et al., 2005 versus 12.3 g for T. brasiliensis from Wilkins, 1989), and their accumulated guano supports rich subterranean ecosystems (Chapman, 1983) including a number of endemic species (Ng, 1991; McFarlane et al., 2009). Acta Chiropterologica, 17(1): 171–177, 2015 PL ISSN 1508-1109 © Museum and Institute of Zoology PAS doi: 10.3161/15081109ACC2015.17.1.014 Estimating colony size of the wrinkle-lipped bat, Chaerephon plicatus (Chiroptera: Molossidae) at Gomantong, Sabah, by quantitative image analysis DONALD A. MCF ARLANE 1, 5 , GUY V AN RENTERGEM 2 , ANNEMIEKE RUINA 1 , JOYCE LUNDBERG 3 , and KEITH CHRISTENSON 4 1 Keck Science Center, The Claremont Colleges, Claremont, CA 91711, USA 2 NA c9 3803919, Koningin Astridstraat 57, 9800 Deinze, Belgium 3 Department of Geography and Environmental Studies, Carleton University, Ottawa, ON K1S 5B6, Canada 4 2026 Peach Orchard Drive Apt. 23, Falls Church, VA 22043, USA 5 Corresponding author: E-mail: dmcfarlane@kecksci.claremont.edu The Gomantong cave system, Kinabatangan, Sabah (Malaysia) hosts one of the largest bat colonies known from north Borneo. The nightly exodus of Chaerophon plicatus from this site is an economically valuable tourist attraction, and must impose significant controls on the regional ecology. Monitoring ecosystem health requires monitoring bat population size, but no quantitative assessments for the Gomantong colony are available in the literature. Traditional censusing techniques (based on packing density and roost area or on roost exodus counts) yield notoriously unreliable results. Here we have applied innovative image analysis and counting techniques to videographic, photographic and terrestrial laser scanning data obtained in July 2012. The colony exits in a consistent stream along a narrow exit trajectory. The laser scanning of the large cave entrance allowed precise measurement of bat position. Video data provided 0.02-second time resolution. Average flight speed was calculated (10.38 ± 0.85 m/sec), bat exit rate was estimated at three minute intervals (peaking at 14,000 bats/minute) and then summed over the 40 minutes of bat flight. The resultant colony size estimate of 275,625–276,939 (95% confidence interval) individuals of C. plicatus in Gomantong Caves in 2012 demonstrates that the technique is viable, provides a realistic basis for ecosystem management, and can be repeated to monitor ecosystem change. The estimated insect consumption by the colony of this size is 927 ± 191 metric tons of insects per year over an estimated 270 km 2 area, a very important component in maintaining ecosystem stability in the Gomantong Forest Reserve and the Kinabatangan Wildlife Sanctuary, as well as pest control over the large agricultural tracts of the region. Key words: Borneo, Kinabatangan, bats, caves, colony size, image analysis, terrestrial laser scanning