INTRODUCTION Although insectivorous bats represent nearly 65% of the bat fauna of the Neotropics (Simmons, 2005), information on their abundance, distribu- tion, behaviour and habitat associations is scarce in comparison to species within other ensembles. The majority of insectivorous species are difficult to capture and most free-flying bats cannot easily be identified visually when in flight (Preatoni et al., 2005), particularly in the tropics where there are many sympatric congeners. All microchiropteran in- sectivorous bats emit ultrasonic signals and process the returning echoes to detect, characterize and lo- calize reflecting objects (Schnitzler and Henson, 1980). Bat detectors allow investigators to hear and record the ultrasonic calls of bats (Fenton, 1988), and therefore represent an important tool in their study. The analysis of echolocation calls can be used to identify species and to determine bat activity acoustically (e.g., Vaughan et al., 1997), although, the accuracy and reliability of this method have sometimes been controversial (Barclay, 1999; O’Farrell et al., 1999). Bat detectors have been used in the Neotropics to describe species’ calls (O’Farrell and Miller, 1997, 1999; Rydell et al., 2002; Vaughan et al., 2004), produce more complete species inventories (Ochoa et al., 2000; Barnett et al., 2006), investigate foraging behaviour (Barclay et al., 1981; Schnitzler et al., 1994; Kalko and Condon, 1998; Thies et al., 1998; Jung et al., 2007) and examine the role of vocalisations in reproduction (Behr and von Helversen, 2004). Fewer studies have used bat detectors to investigate habitat use by insectivor- ous bats, which depends not only on the wing morphology and echolocation characteristics of each species (Norberg and Rayner, 1987; Kalko and Schnitzler, 1998), but also on factors such as distance to clutter (i.e., foliage, water), weather Acta Chiropterologica, 11(1): 139–147, 2009 PL ISSN 1508-1109 © Museum and Institute of Zoology PAS doi: 10.3161/150811009X465758 Insectivorous bat activity at cenotes in the Yucatan Peninsula, Mexico M. CRISTINA MACSWINEY G. 1, 2, 4 , BEATRÍZ BOLÍVAR CIMÉ 3 , FRANK M. CLARKE 1 , and P AUL A. RACEY 1 1 School of Biological Sciences, University of Aberdeen, Aberdeen AB24 2TZ, United Kingdom 2 Centro de Investigaciones Tropicales, Universidad Veracruzana, Ex-Hacienda Lucas Martín, Calle Araucarias s/n, Apartado Postal 525, 91019 Xalapa, Veracruz, México 3 Instituto de Ecología. Km 2.5 Carretera Antigua a Coatepec 351, Congregación El Haya, 91070 Xalapa, Veracruz, México 4 Corresponding author: E-mail: cmacswiney@uv.mx The behaviour and habitat associations of aerial insectivorous bats are poorly understood despite constituting up to 65% of bat species in the Neotropics. In 2003, 2004 and 2005 we quantified the activity of insectivorous bats and their insect prey at pastureland and forest sites with and without cenotes (water-filled sinkholes) in the Yucatan Peninsula, Mexico. We used a time-expansion bat detector to survey each habitat for 24 nights and analysed 2,880 one-minute recorded sequences to determine bat activity. We identified 14 species and five phonic types belonging to four families. Bat activity and the average number of bat species acoustically sampled each night were significantly greater in habitats with cenotes than in those without. Pteronotus personatus and an unidentified molossid were recorded exclusively at cenotes. Peropteryx macrotis showed the highest activity of all bat species. In all habitats insects were more abundant during the rainy season but only in pastureland was bat activity significantly greater during the rainy season. Insect abundance was correlated with bat activity only at cenotes in pastureland. Cenotes are important foraging habitats for insectivorous bats as 16 species, 84% of those revealed by this study, were recorded feeding in these habitats and the number of feeding buzzes was higher in comparison to habitats without cenotes. Protection of cenotes and their surrounding vegetation should be a management priority in order to conserve the high diversity of insectivorous bats associated with these distinctive habitats. Key words: Chiroptera, echolocation, insects, pastureland, time-expansion, water resources, Mexico