Hypopomus pinnicaudatus, an electric fish, has a marked sexual dimorphism in its tail filament. Sexually mature males have long, 'feathered' tails as compared with females. The sexual dimorphism emerges when a fish reaches about 110 mm total length. Mature males have larger electrocytes which are more widely spaced and more numerous than those in mature females: The biphasic electric organ discharge (EOD) is longer in males than in females. The peak-to-peak amplitude of the male's EOD is weaker than a female's of the same total length. The weaker discharge is unexpected given the increase in size and number of electrocytes. It is suggested that the reduction in EOD amplitude is a consequence of the increase in EOD duration among males. Further, female choice probably played a role in the evolution of long duration EODs among males, and males may have secondarily grown long tails to compensate for the loss in active space that would otherwise accompany a weaker EOD.

Keywords:
Electric fish, Electric organ, Electric communication, Electrocyte morphology, Sexual dimorphism, Sex difference, Electric organ discharge, Sexual selection, Female choice
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© 1990 S. Karger AG, Basel
1990
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