Available online at www.sciencedirect.com Sexual communication in cane toads, Chaunus marinus: what cues influence the duration of amplexus? HALEY BOWCOCK, GREGORY P. BROWN & RICHARD SHINE School of Biological Sciences, University of Sydney (Received 16 July 2007; initial acceptance 15 September 2007; final acceptance 11 October 2007; published online 20 February 2008; MS. number: 9458R) Successful reproduction often involves complex communication, but signalling capabilities frequently differ between the sexes. Research on sexual communication in anuran amphibians has focused heavily on the advertisement call made by reproductive males, with less attention paid to the release signal that males (and some females) give in response to misdirected attempts at amplexus. In the cane toad the re- lease signal consists of a mechanical component (vibrations of the body wall, given by both sexes) as well as an auditory call (apparently given by males alone). We investigated the cane toad release signal to ask: (1) do females call in a manner similar to that of males but at a frequency inaudible to human ears? and (2) what stimuli (acoustic or otherwise) induce males to terminate amplexus? Analysis of calls over a wide (4e100 000 Hz) frequency range confirmed that female toads are indeed mute. Acoustic playback experi- ments revealed that males were less likely to dismount if we prevented an otherwise vocal target animal from making a release call. However, adding a release call to an otherwise mute target did not initiate dis- mount. Instead, the cue eliciting male dismount appears to involve synchrony between the release call and some other factor (probably, vibration of the clasped animal). Sexual dimorphism in skin rugosity also may play a role in amplexus termination. Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Keywords: Anura; Bufo marinus; communication systems; honest signalling; multimodal signals; sexual conflict In any sexually reproducing species, determining which individual to combine gametes with is a critical determi- nant of individual fitness and likely to be under strong selection pressure (Patterson 1985; Andersson 1994). Suc- cessful reproduction involves selecting a mate not only of the appropriate species and sex but also of appropriate quality and genetic compatibility (Andersson & Simmons 2006). Thus, reproducing animals use complex communi- cation, across a wide range of sensory modalities, to eval- uate potential partners (e.g. Searcy & Andersson 1986; Greenspan & Ferveur 2000; Endler et al. 2005). Such com- munication systems reflect species-specific aspects of mor- phology, courtship postures and degree of sexual size dimorphism (e.g. Ohtsuka & Huys 2001; Vanhooydonck et al. 2005). Much research has focused on the roles of courtship cues and signals in maintaining genetic separation among sympatric species (e.g. wolf spiders: Stratton & Uetz 1986; drosophilid flies: Blows & Allan 1998; wall lizards: Barbosa et al. 2006) and on the ways in which individuals assess the quality of potential mates (seabirds: Jones & Hunter 1993; fish: Jordan et al. 2003; baboons: Gesquiere et al. 2007). In particular, the role of ‘honest’ (i.e. reliable) sig- nalling of mate quality in animal communication systems has received much theoretical (Zahavi 1975; Schluter & Price 1993; Maynard Smith & Harper 2003) and empirical (e.g. Hauser 1993; LeBas et al. 2003; Kolm 2004) attention. More recently, sexual communication systems have been examined from the viewpoint of sexual conflict, with an emphasis on situations in which individuals of one sex (usually males) force copulations onto individuals of the other sex (usually females) by eliminating opportunities for mate choice in the subordinate sex (see Arnqvist 2005). As in many fields of biology, this extensive literature on sexual communication shows strong biases in the species, sex and communication pathways that have attracted the most study. In anuran amphibians, most research has focused on female responses to advertisement calls of males (see reviews by Ryan 1991 and Gerhardt 1994). The evolution of advertisement calls thus has been attributed Correspondence: R. Shine, School of Biological Sciences A08, University of Sydney, NSW 2006, Australia (email: rics@bio.usyd.edu.au). 1571 0003e 3472/08/$34.00/0 Ó 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2008, 75, 1571e1579 doi:10.1016/j.anbehav.2007.10.011