Effect of natural and synthetic noise on evoked vocal responses in a frog of the temperate austral forest MARIO PENNA, HANS POTTSTOCK & NELSON VELASQUEZ Programa de Fisiologı ´a y Biofı ´sica, Facultad de Medicina, Universidad de Chile (Received 7 June 2004; initial acceptance 19 July 2004; final acceptance 18 November 2004; published online 9 August 2005; MS. number: A9906) Animals using sound communication have developed different strategies to overcome noise interference, but studies have rarely examined animals behaving in their natural environments. Males of the leptodactylid frog Eupsophus calcaratus exposed to natural noises of wind, rain, creek and sea surf and to a band-pass noise encompassing the main spectral components of the conspecific advertisement call increased their call rate in the presence of noises of moderate level, and this effect was particularly strong for the band-pass noise. Frogs exposed to band-pass noise of different intensities increased their call rate in response to exposures of 66–78 dB RMS sound pressure level. Call duration followed similar trends, but the effects of noise exposure on this measure of evoked vocal response were not as strong as those on call rate. The vocal responsiveness of males of this species in the presence of noise denotes adaptations to cope with high interference, in spite of the relatively simple acoustic environment of the austral temperate forest. Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Animals that communicate by means of acoustic signals confront the difficulty of conveying information in environments with different noise levels and qualities. Noise sources of biotic origins are especially important in terrestrial environments of tropical latitudes, where a di- versity of organisms build up a complex sound environ- ment at different phases of the circadian cycle (Narins & Zelick 1988; Owings & Morton 1998). In temperate latitudes, the diversity of taxa is more restricted and sources of abiotic noise, like wind, rain and running water are likely to have a higher importance (e.g. Klump 1996). Vertebrates have developed strategies to overcome noise interference, thus facilitating the transmission of acoustic signals to potential receivers. Various vertebrates, includ- ing humans, increase the amplitude of their vocalizations in the presence of noise (e.g. Sinnot et al. 1975; Cynx et al. 1998; Brumm & Todt 2002; Pytte et al. 2003; Brumm 2004). Some anurans and birds dwelling in environments having high levels of stream noise produce vocalizations containing remarkable frequency modulations and/or high frequencies beyond the noise spectral range (Penna et al. 1983; Dubois & Martens 1984; Penna & Veloso 1990; Feng et al. 2000; Ho ¨dl & Ame ´zquita 2001). Other birds increase the emission rate and duration of their vocalizations to communicate in environments with high wind noise levels (Potash 1972; Lengagne et al. 1999). Acoustic signals are the principal mediators of anuran social behaviour. Male frogs and toads in chorusing aggregations produce advertisement calls that attract females (e.g. Rand 1988). Also, vocal interactions between males serve to defend a territory or calling site that is used to broadcast advertisement calls (e.g. Wells 1988). Interference from biotic noise has been shown to affect the vocal behaviour of males. During vocal interactions, males compete with opponents by increasing the dura- tion, amplitude, complexity and rate of emission of their calls and by modifying their dominant frequency (Lo ´ pez et al. 1988; Wells 1988; Schwartz et al. 2002). Males also typically alter the timing of their calls to avoid overlap with their neighbours (Narins & Zelick 1988; Klump & Gerhardt 1992; Grafe 1996). High-intensity (above 90 dB sound pressure level (SPL)) exposure to playbacks of continuous choruses of sympatric species inhibits calling of males of the Central American treefrog Hyla ebraccata (Schwartz & Wells 1983a, b). Similarly, males of the Caribbean treefrog Eleutherodactylus coqui cease respond- ing to synthesized calls if they are accompanied by intense broadband noise (Narins 1982). However, that study found that noise of moderate level could increase evoked vocal responses. The phonotactic responses of female frogs are also affected in different ways by exposure to synthetic noise imitating the background activity of conspecific choruses. Correspondence: M. Penna, Programa de Fisiologı ´a y Biofı ´sica, Facultad de Medicina, Universidad de Chile, Casilla 70005, Santiago, Chile (email: mpenna@med.uchile.cl). 639 0003–3472/04/$30.00/0 Ó 2005 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. ANIMAL BEHAVIOUR, 2005, 70, 639–651 doi:10.1016/j.anbehav.2004.11.022