INTRODUCTION Bat echolocation calls evolved under strong se- lective pressure for orientation and object detection and thus have largely been shaped by ecology (Jones and Teeling, 2006; Jones and Holderied, 2007; re- viewed in Schnitzler et al., 2003). Call characteris- tics in the frequency domain (e.g., peak frequency, bandwidth) or time domain (e.g., pulse duration, in- terval between pulses) are directly related to the ex- ploitation of particular ecological niches (reviewed in Jones and Holderied, 2007). Although mainly shaped by ecology, evidence for an additional com- municative potential of echolocation calls is accu- mulating (reviewed in Fenton, 1985; Fenton, 2003; Jones and Siemers, 2011). Bats are calling out at very high amplitudes to be able to generate perceiv- able echoes returning from tiny insects over some distance (Surlykke and Kalko, 2008). Inevitably, they thereby make themselves conspicuous to con- and heterospecific bats in their surround- ing, and over much larger distances than over which they can detect insects (Jones and Siemers, 2011). Any listening bat can potentially extract informa- tion on the caller’s current location, behaviour and identity from the echolocation signals. When this is not ‘intended’ by the caller, we term this information-extraction ‘eavesdropping’. In case the information is ‘intended’ for the receiver, it is com- munication sensu stricto (Bradbury and Vehren- camp, 1998). What is the nature of the information encoded in bat echolocation calls? Detailed studies of call pa- rameters have revealed information of species, colony, body size, age and sex in echolocation calls (reviewed in Jones and Siemers, 2011). Behavioural evidence demonstrating that the bats can indeed de- code and use this information is much more scarce, but urgently needed for a better understanding of the communicative function of echolocation. Four stud- ies have shown that bats can recognise the following from echolocation calls: species identity (Schuch- mann and Siemers, 2010), group membership (Voigt-Heucke et al., 2010) and individual identity of the calling bat (Kazial et al., 2008; Yovel et al., 2009). A fifth study provided evidence for the recog- nition of the caller’s gender from echolocation calls (Kazial and Masters, 2004). During and after play- back of echolocation pulses from an unfamiliar bat, the vocalization rates of the test bats, Eptesicus fuscus, a species that uses frequency-modulated (FM) calls, changed depending on the sex of the bat whose pulses were used as a playback stimulus (Kazial and Masters, 2004). It remained unclear, Acta Chiropterologica, 14(1): 161–166, 2012 PL ISSN 1508-1109 © Museum and Institute of Zoology PAS doi: 10.3161/150811012X654376 Horseshoe bats recognise the sex of conspecifics from their echolocation calls MAIKE SCHUCHMANN 1, 3 , SÉBASTIEN J. PUECHMAILLE 1, 2 , and BJÖRN M. SIEMERS 1 1 Sensory Ecology Group, Max Planck Institute for Ornithology, 82319 Seewiesen, Germany 2 School of Biology & Environmental Science, University College Dublin, Belfield, Dublin, Ireland 3 Corresponding author: E-mail: m-schuchmann@gmx.de Echolocation calls produced by bats contain information on the emitter that could be used in social interactions. Nevertheless, little is known about the whether such information actually is being used by bats. Here we tested the ability of two horseshoe bat species (Rhinolophus mehelyi and R. euryale; n = 20 each) to discriminate conspecific male from conspecific female echolocation calls and vice versa. Behavioural data from a habituation-discrimination paradigm revealed that both species were able to recognize the sex of conspecifics from their calls, providing evidence that cues on the sex of the emitter are encoded in echolocation signals. While our data support that the sex-specific acoustic cues are not (just) the horseshoe bats’ constant-frequency call part, their nature remains enigmatic. This is the second study to show that bats can recognize the caller’s gender from echolocation calls and the first to do so for ‘high-duty-cycle bats’; those that use calls with a prominent constant-frequency (CF) component. Key words: bat, communication, habituation-discrimination, echolocation, gender, recognition, Rhinolophus mehelyi, R. euryale