Ecology, 88(10), 2007, pp. 2462–2467 Ó 2007 by the Ecological Society of America PHYLOGENETIC RELATEDNESS AND ECOLOGICAL INTERACTIONS DETERMINE ANTIPREDATOR BEHAVIOR SARAH DALESMAN, 1 SIMON D. RUNDLE,DAVID T. BILTON, AND PETER A. COTTON Marine Biology and Ecology Research Centre, School of Biological Sciences, University of Plymouth, Plymouth PL4 8AA Unkited Kingdom Abstract. Interspecific recognition of alarm cues among guild members through ‘‘eavesdropping’’ may allow prey to fine-tune antipredator responses. This process may be linked to taxonomic relatedness but might also be influenced by local adaptation to recognize alarm cues from sympatric species. We tested this hypothesis using antipredator responses of a freshwater gastropod Lymnaea stagnalis (L.) to alarm cues from damaged conspecific and 10 heterospecific gastropod species. As predicted, the magnitude of antipredator response decreased significantly with increasing phylogenetic distance, but increased when species were naturally sympatric (defined as species cohabiting in the same water body) with the source population of L. stagnalis. The responses to sympatric species were higher overall, and the relationship between genetic distance and alarm cue response was stronger when tested with sympatric species. This is the first study to demonstrate that population sympatry influences innate antipredator responses to alarm cues from intraguild members and suggests that responses based on phylogenetic relationships can be modified through local adaptation. Such adaptation to heterospecific alarm cues suggests that species could be at a disadvantage when they encounter novel intraguild members resulting from species invasion or range expansion due to a reduction in the presence of reliable information about predation risk. Key words: alarm cue; chemical communication; gastropod; induced defenses; local adaptation; phylogeny. INTRODUCTION Biological signals produced by organisms are gener- ally intended to communicate only with conspecific individuals conferring benefit to the signaler, but may be intercepted by heterospecific individuals through ‘‘eaves- dropping’’ (Stowe et al. 1995, Bradbury and Vehren- camp 1998). Eavesdropping implies no benefit to the signaler, but may be of benefit to the receiver, for example in locating resources or identifying potential competitors, and has been found to be important in a wide variety of taxa (Catchpole and Leisler 1986, Stowe et al. 1995, Taga and Bassler 2003, Symonds and Wertheim 2005, Runyon et al. 2006). Eavesdropping on alarm cues from heterospecific intraguild members has been found in several species and may be used to provide additional information about potential predation risk (Stenzler and Atema 1977, Mirza and Chivers 2001, Hazlett and McLay 2005, Schoeppner and Relyea 2005). In communication systems where it is beneficial to both signaler and receiver to respond to related heterospecifics, phylogenetic relationships generally ap- pear to explain the patterns observed, for example, the similarity of chemical aggregation signals in closely related Drosophila spp. (Symonds and Wertheim 2005) or attraction of Aplysia spp. to heterospecifics during egg laying (Cummins et al. 2005). In these cases, there is a benefit to all individuals to oviposit in the same site, irrespective of their species. The interaction between evolutionary history and local adaptation in shaping interspecific communication has also been investigated in relation to sexual signals, where it has been shown that differences between signaling systems are more pronounced in sympatric species pairs (Ryan and Rand 1995, Symonds and Elgar 2004). Phylogeny is not a good predictor of the degree of divergence seen between sexual signals when species are found sympatrically, though exceptions have been found (Smith and Flor- entino 2004). The importance of sympatry in shaping sexual signals suggests that local adaptation to avoid heterospecific interactions is acting on this signaling system, and in many cases counteracting the effects of phylogenetic relatedness. In predator–prey systems, recognition of alarm cues from closely related species is not considered communi- cation as there is no benefit to the signaler (Bradbury and Vehrencamp 1998), however it may be of benefit to the receiver to assess predation risk accurately and, hence, may reduce the high fitness costs associated with predation (Lima and Dill 1990). There is considerable evidence that alarm cues are conserved within phyloge- netic groups (Pfeiffer 1977), with responses to alarm cues from closely related species generally being stronger than from those more distantly related (Snyder 1967, Manuscript received 9 March 2007; revised 18 May 2007; accepted 29 May 2007. Corresponding Editor: J. F. Bruno. 1 E-mail: sdalesman@plymouth.ac.uk 2462 REPORTS