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First published online June 29, 2007
Journal of Experimental Biology 210, 2501-2509 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.005272

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Stimulus frequency differentially affects chirping in two species of weakly electric fish: implications for the evolution of signal structure and function

Johanna A. Kolodziejski^1,2,*, Sara E. Sanford^2,3 and G. Troy Smith^1,2,4

¹ Department of Biology, Indiana University Bloomington, Bloomington, IN 47405, USA
² Center for the Integrative Study of Animal Behavior (CISAB), Indiana University Bloomington, Bloomington, IN 47405, USA
³ Department of Psychology, Emory University, 532 Kilgo Circle, Atlanta, GA 30322, USA
⁴ Program in Neuroscience, Indiana University Bloomington, Bloomington, IN 47405, USA

^* Author for correspondence (e-mail: Hanna{at}alumni.brown.edu)

Accepted 3 April 2007

During social interactions, apteronotid electric fish modulate their electric organ discharges (EODs) to produce transient communication signals known as chirps. Chirps vary widely across species and sex in both number and structure. In Apteronotus leptorhynchus, males chirp far more than females and their chirps have greater frequency modulation than those of females. High-frequency chirps are produced by males most often in response to female-like electric signals. As such, they have been hypothesized to function in courtship. The more common low-frequency chirps, produced by both males and females in response to same-sex signals, are hypothesized to function as aggressive signals. To determine whether the two chirp types in the closely related Apteronotus albifrons have similar functions, we stimulated chirping in male and female A. leptorhynchus and A. albifrons with playbacks simulating the EODs of same-sex versus opposite-sex conspecifics. As in A. leptorhynchus, male and female A. albifrons produced low-frequency chirps most often to same-sex signals. Unlike A. leptorhynchus, however, A. albifrons also produced more high-frequency chirps to same-sex stimuli than to opposite-sex stimuli. This suggests that high-frequency chirps in A. albifrons, unlike those in A. leptorhynchus, may not function as courtship signals and that the function of similar chirp types has diversified in Apteronotus. Examples such as this, in which the function of a communication signal has changed in closely related species, are rare. The electrocommunication signals of apteronotids may thus provide a remarkable opportunity to investigate the evolutionary interactions of signal structure and function.

Key words: communication, chirping, sexual dimorphism, electric fish, signal evolution, Apteronotus, difference frequency

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