Social competition masculinizes the communication signals of female electric fish

Abstract

Male intrasexual competition and female choice explain the evolution of male ornaments. Except in sex-rolereversed taxa, female ornaments have been regarded as an epiphenomenon of genetic correlation, with no femalespecific function or independently selected basis. Females from species with conventional sex roles may still experience some degree of female-female competition and male choice that could explain the persistence of female ornaments. We studied the effect of female competition on the expression of a sexually dimorphic communication signal. In the electric fish Brachyhypopomus gauderio, both sexes produce an electric signal pulse for communication and electrolocation. Male electric pulses are longer in duration and greater in amplitude than those of females. As competition increases, males further enhance their signals in response to elevated androgen levels. We explored whether females respond to social competition as males do, by enhancing their communication signals and increasing androgen levels. We measured amplitude and duration of the electric signal pulse, and testosterone levels in female B. gauderio in their natural habitat in Uruguay and estimated social competition by calculating population density and adult sex ratio (ASR). In the lab, we manipulated ASR and population density independently to separate these factors and eliminate seasonal confounds. Under both field and lab conditions, signal pulse amplitude increased with population density, while pulse duration increased with female bias in ASR. In the field, but not the lab, androgen levels increased when ASR was female biased. Our findings indicate that the socially mediated mechanism of signal regulation is shared by the sexes, although whether androgens regulate this signal plasticity in females remains unclear.

Behavioral ecology, endocrinology and signal reliability of electric communication

“…Signal plasticity among females was shown to save energy during the day (Salazar and Stoddard, 2008;Stoddard et al, 2007) but social consequences of female signal plasticity were not understood. Recently, we have found that female signals also respond to changes in the social environment in ways similar to males (Gavassa et al, 2012a). Both in field observations and laboratory manipulations, females increase the amplitude of the EOD in response to increases in population density, while the duration of the EOD's second phase increases in response to a greater ratio of females to males.…”

“…Females in the field increase EOD second phase duration and androgen levels in proportion to intrasexual competition (Table2) (Gavassa et al, 2012a). However, in the laboratory, female intrasexual competition increases EOD second phase duration without detectable changes in androgen levels (Gavassa et al, 2012a), although experimental increase of androgen levels reliably and consistently increases EOD duration in both sexes (Allee et al, 2009;Gavassa et al, 2011;Goldina et al, 2011;Pouso et al, 2010;Silva et al, 2002).…”

“…Brachyhypopomus gauderio appears to be annual under field conditions, as only immature fish can be found during the austral winter (Silva et al, 2002). The primary sex ratio is unity (Gavassa et al, 2012a), but reproductive males disappear quickly from the population as the breeding season advances, leaving sex ratios of approximately 1:4 (male:female) after 1month of breeding effort (Gavassa et al, 2012a;Miranda et al, 2008).…”

A taxonomic revision of the Neotropical electric fish genus Brachyhypopomus (Ostariophysi: Gymnotiformes: Hypopomidae), with descriptions of 15 new species

“…Reproductive seasonality, life history, and ecology in B. gauderio are documented by several authors (Gavassa et al, 2012;Giora et al, 2014;Miranda et al, 2008;Quintana et al, 2004;Silva et al, 2003). It breeds during the austral summer, with maturation probably triggered by rising temperature and a change in photoperiod.…”

“…It breeds during the austral summer, with maturation probably triggered by rising temperature and a change in photoperiod. During the breeding season it lives in dense social groups, and exhibits a sex ratio of mature individuals highly skewed towards females (around 3:1, despite observations of a primary sex ratio close to unity in a long-term laboratory population), possibly due to elevated male mortality related to stress from intrasexual competition (Gavassa et al, 2012;Miranda et al, 2008). Males exhibit a wider spacing, and move less than females -consistent with a polygynous or polygynandrous mating strategy resembling exploded lek polygyny .…”