Behav Ecol Sociobiol (2006) 60: 19–25 DOI 10.1007/s00265-005-0133-5 ORIGINAL ARTICLE Irena Rot-Nikcevic . Christopher N. Taylor . Richard J. Wassersug The role of images of conspecifics as visual cues in the development and behavior of larval anurans Received: 7 September 2005 / Revised: 7 November 2005 / Accepted: 9 November 2005 / Published online: 1 March 2006 # Springer-Verlag 2006 Abstract Tadpoles can alter their behavior, morphology, and life history in response to habitat change. Although chemical signals from conspecifics or predators play an important role in tadpole habitat assessment, little is known about the role of visual cues and the extent to which tadpoles rely on their vision for intraspecific social assessment. The aim of our experiments was to determine whether larval anurans use visual images of other tadpoles as indicators of density and to analyze how, and to what extent, images of conspecifics alone affect tadpole development, growth, and behavior. To assess this, we raised both Rana sylvatica and Bufo americanus tadpoles in aquaria with either quarter- or half-mirrored walls. Both physically increased density and increased density simulated with mirrors decreased tadpole growth and developmental rates, and increased activity in Rana tadpoles. Bufo tadpoles did not significantly alter their growth and development in response to visually increased density. Only true, i.e., physically, increased density had an effect on growth and activity in Bufo tadpoles. Our data show that images of conspecifics are used as visual cues by Rana tadpoles and can induce phenotypically plastic changes in several traits. This response to visual cues is taxon-specific. Keywords Tadpoles . Visual Cues . Development . Growth . Behavior Introduction Anuran larvae are capable of using vision, chemoreception (olfaction), and mechanoreception (see references in Lannoo 1999) to assess features of their habitat. Of the three types of cues (i.e., chemical, visual, and tactile), chemical cues have been analyzed the most. Several studies have demonstrated that isolated chemicals from predators such as fish induce a response in tadpoles similar to their response to the predator itself (Petranka et al. 1987; Kats et al. 1988; Semlitsch and Reyer 1992). Also, larvae of some anuran species use chem- ical cues to distinguish kin from nonkin (Blaustein and O' ’Hara 1982; Waldman 1986; Blaustein and Walls 1995). However, the relative importance of other cues is not well established. For example, the extent to which tadpoles rely on visual stimuli to inform them of environment quality is unknown (vS. Hoff et al. 1999). Tadpoles of many species form social swarms whose main advantage is to reduce the collective risk of predation (Hamilton 1971; Wassersug 1973; Hobson 1979). It has been shown that, although kin recognition in tadpoles is mainly mediated by chemical cues, visual cues are important for the formation of those aggregates (Blaustein and Waldman 1992; Wassersug and Hessler 1971; Wassersug et al. 1981). Bufo woodhousei larvae orient parallel their neighbors more precisely in the light than in the dark, suggesting that there is a visual component to their schooling behavior (Wassersug et al. 1981). Visual awareness by tadpoles of the presence of other tadpoles has been shown in experiments with aggre- gates of Rana clamitans and Bufo americanus tadpoles. When an electric pulse is used to startle these tadpoles, they avoid the position occupied by their neighbors the instant before they scatter (vS. Hoff 1988). The objectives of our study were to determine the extent to which tadpoles of Rana sylvatica (family Ranidae) and B. americanus (family Bufonidae) use images of conspe- cifics as visual cues and to examine how these cues affect tadpole behavior and development. We used mirrors to simulate increased intraspecific density without altering chemical or tactile cues, and we tried to elucidate whether images of conspecifics might indicate increased density. In most amphibian species, crowding slows larval growth and development (Smith-Gill and Berven 1979; Semlitsch and Caldwell 1982; Relyea and Hoverman 2003). Larval competitors have also been shown to induce high activity (Smith and Van Buskirk 1995; Relyea and Werner 2000) as Communicated by J. Christensen-Dalsgaard I. Rot-Nikcevic (*) . C. N. Taylor . R. J. Wassersug Department of Anatomy and Neurobiology, Dalhousie University, 5850 College St., Halifax, B3H 1X5 Nova Scotia, Canada e-mail: nikcevic@dal.ca Tel.: +1-902-4942244 Fax: +1-902-4941212