Androgen correlates of socially induced changes in the electric organ discharge waveform of a mormyrid fish

Weakly electric fish from the family Mormyridae produce pulsatile electric organ discharges (EODs) for use in communication. For many species, male EO Ds are seasonally longer in duration than those of females, and among males , there are also individual differences in EOD duration. While EOD elongati on can be induced by the administration of exogenous androgens, androgen le vels have never before been assessed under natural or seminatural condition s. By simulating the conditions occurring during the breeding season in the laboratory, we provide evidence of a sex difference in EOD duration as wel l as document levels of circulating androgens in males. In this study, we a nalyzed the nature of social influences on male EOD duration and plasma and rogen levels in Brienomyrus brachyistius. individual males, first housed wi th a single female and then placed into social groups consisting of three m ales and three females, showed status-dependent changes in EOD duration. To p-ranking males experienced a relatively large increase in EOD duration. Se cond-ranking males experienced a more modest increase, and low-ranking male s experienced a decrease in EOD duration. These changes were paralleled by differences in circulating levels of plasma 11-ketotestosterone (11-KT), bu t not testosterone, suggesting that the changes in EOD duration may have be en mediated by changes in plasma 11-KT levels. Thus, it appears that EOD du ration is an accurate indicator of male status, which is under social and h ormonal control, (C) 2000 Academic Press.