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

Abstract

Weakly electric fish in the genus Apteronotus provide a unique vertebrate model for studying the evolutionary interplay between signal structure, production, and function in complex communication systems. This diverse group of fish produces quasisinusoidal electric organ discharges (EODs) for electrolocation and communication. EODs are relatively simple and are easy to record, analyze and manipulate. These signals can vary in frequency, amplitude and waveform, and therefore reliably convey information about species, sex and individual identity (Hopkins, 1988;Smith, 1999;Zakon and Smith, 2002).In many species, sex differences in EOD frequency are stable enough to unambiguously distinguish males from females. In the brown ghost knifefish Apteronotus leptorhynchus, males produce EODs between 800 and 1000·Hz whereas females produce lower-frequency EODs between 600 and 750·Hz (Hagedorn and Heiligenberg, 1985;Kirschbaum, 1983;Meyer et al., 1987). In the closely related black ghost knifefish Apteronotus albifrons, EOD frequency is also sexually dimorphic, but in the opposite direction. In this species, females discharge at higher frequencies than males (Dunlap et al., 1998;Kolodziejski et al., 2005).EODs in both A. leptorhynchus and A. albifrons can be modulated to produce discrete signals that are often sexually dimorphic and species-specific. EOD modulations known as chirps and rises are transient changes in the frequency and/or amplitude of an otherwise constant-frequency EOD. Chirps are produced most often in response to social stimulation and vary in structure both within and between species. Several studies have proposed that chirps, and to a lesser extent rises, function as intraspecific communication signals (Bastian et al., 2001;Dunlap and Larkins-Ford, 2003;Engler et al., 2000;Hagedorn and Heiligenberg, 1985;Kolodziejski et al., 2005;Tallarovic and Zakon, 2002;Triefenbach and Zakon, 2003;Zupanc and Maler, 1993).Chirps can be classified into categories based on frequency excursion and duration. Although the classification of chirp types differs somewhat across studies, two broad classes of chirps, low-frequency and high-frequency chirps, are consistently described in A. leptorhynchus

Evolution and hormonal regulation of sex differences in the electrocommunication behavior of ghost knifefishes (Apteronotidae)

“…Unlike in A. leptorhynchus, high-frequency chirps in A. albifrons are produced by both males and female in response to EODfs close to the fish's own (i.e. to same-sex fish) (Kolodziejski et al, 2007). In some species with sexually monomorphic EODs (e.g.…”

“…same-sex EODfs) than in response to EODfs that differ more from their own (e.g. opposite-sex EODfs) (Bastian et al, 2001;Bohorquez and Smith, 2011;Cuddy et al, 2012;Dye and Heiligenberg, 1987;Engler and Zupanc, 2001;Kolodziejski et al, 2007;Triefenbach and Zakon, 2003;Zupanc et al, 2006). Low-frequency chirps have been hypothesized to function as agonistic signals (see 'Function of chirps' below).…”

“…Male A. leptorhynchus produce high-frequency chirps, which are hypothesized to function as courtship signals (Hagedorn and Heiligenberg, 1985), more in response to EODfs far from their own than to EODfs similar to their own (Bastian et al, 2001;Kolodziejski et al, 2007;Triefenbach and Zakon, 2003). By producing high-frequency chirps to EODfs 100-200Hz below their own EODf, males may be using EODf to direct these chirps preferentially to females.…”

“…Unlike chirps, rises are often sexually monomorphic Kolodziejski et al, 2005;Zhou and Smith, 2006). Also unlike chirps, EOD playbacks to fish in chirp chambers inhibit rather than stimulate the production of rises in A. leptorhynchus, A. albifrons and A. bonapartii Kolodziejski et al, 2007).…”

“…Additional evidence that these chirps may function in courtship comes from playback studies showing that females very rarely produce high-frequency chirps, and that males produce more high-frequency chirps in response to playbacks of EODfs more than 100Hz away from their own (Bastian et al, 2001;Cuddy et al, 2012;Kolodziejski et al, 2007;Triefenbach and Zakon, 2003). Thus, EODs of females, which are typically 100-300Hz lower in frequency than those of males, are more likely to elicit highfrequency chirps from a male than are the EODs of most other males.…”