J Comp Physiol A (2009) 195:501–514 DOI 10.1007/s00359-009-0427-6 123 ORIGINAL PAPER Social electric signals in freely moving dyads of Brachyhypopomus pinnicaudatus Rossana Perrone · Omar Macadar · Ana Silva Received: 30 October 2008 / Revised: 4 February 2009 / Accepted: 18 February 2009 / Published online: 10 March 2009  Springer-Verlag 2009 Abstract Brachyhypopomus pinnicaudatus (pulse-type weakly electric Wsh) is a gregarious species that displays reproductive behavior and agonistic encounters between males only during the breeding season. During social inter- actions, in addition to its basal electric organ discharge (EOD), Wsh emit social electric signals (SESs) in the con- texts of reproduction and intrasexual aggression. We repro- duced natural behavior in laboratory settings: SESs recorded in the Weld are indistinguishable from those observed in our experimental setup. SESs are nocturnal, change seasonally and exhibit sexual dimorphism. This study provides an exhaustive characterization and classiW- cation of SESs produced by males and females during the breeding season. In male–female dyads, males produce accelerations and chirps while females interrupt their EODs. The same SESs are observed in male–male dyads. We present a novel, thorough classiWcation of male chirps into four independent types (A, B, C, and M) based on their duration and internal structure. The type M chirp is only observed in male–male dyads. Chirps and interruptions, both in male–female and male–male dyads, are emitted in bouts, which are also grouped throughout the night. Our data suggest the existence of a sophisticated electric dialog during reproductive and aggressive interaction whose pre- cise timing and behavioral signiWcance are being investi- gated. Keywords Electric Wsh · Electric communication · Courtship · Aggression · Chirp List of abbreviations EOD Electric organ discharge SESs Social electric signals Introduction Electric Wsh have evolved a sophisticated sensory modality, active electroreception, by which they communicate and sense their surroundings (Lissmann 1958). All known behaviors among electric Wsh (agonistic, reproductive, parental care, feeding) include electric displays in addition to locomotor ones. Changes to the waveform and temporal pattern of the electric organ discharge (EOD) carry infor- mation that signals species, physiological and behavioral states throughout the individual’s lifetime (Kramer 1990; Lorenzo et al. 2006). During breeding, the communicative role of the EOD is encoded in at least two ways: sexual dimorphism in the waveform, and sexually dimorphic rate modulations given during courtship and agonistic encoun- ters (Hagedorn and Carr 1985; Hagedorn and Heiligenberg 1985; Hopkins et al. 1990; Hopkins 1991; Mills et al. 1992; Lorenzo et al. 2006). Both males and females produce com- plex, transient electric signals during courtship and spawn- ing (Hopkins 1974a, b; Hagedorn and Heiligenberg 1985; Hagedorn 1988; Kawasaki and Heiligenberg 1989; Bastian et al. 2001; Zupanc 2002; Hupé and Lewis 2008). For example, a Wsh may silence its EOD suddenly, or interrupt R. Perrone · O. Macadar · A. Silva (&) Departamento de NeuroWsiología, Instituto de Investigaciones Biológicas Clemente Estable, Avda Italia 3318, 11600 Montevideo, Uruguay e-mail: asilva@iibce.edu.uy R. Perrone e-mail: rperrone@iibce.edu.uy A. Silva Laboratorio de Neurociencias, Facultad de Ciencias, Universidad de la República, Montevideo, Uruguay