Weakly Electric Fish: Behavior, Neurobiology, and Neuroendocrinology

Abstract

Electric fish generate electric organ discharges (EODs) from an electric organ and sense these discharges via sensory cells termed eletroreceptors. Amazingly, electrocommunication evolved independently in two groups of fishes. EODs are used in the same ways as signals in other modalities and are similarly influenced by natural and sexual selection. The EOD waveform conveys information on the species, age, sex, and individual identity of the signaler, and brief modulations of the EOD emission rate transmit information about breeding status, dominance/subordinance, and motivation. As with many sexually dimorphic communication signals, EOD waveform is regulated by gonadal steroids by long-term actions on the morphology and physiology of the neurons and electric organ while short-term modulations of EOD rate signaling motivational states are driven by peptides and amines at the brain stem pacemaker nucleus. EOD waveforms are similar in juveniles and females in many species, reminiscent of the plumage similarities of juvenile and female birds. In species in which males compete for reproductive access to females, male signals are more conspicuous and/or ornate than those of females and carry costs, such as increased risk of predation and/or energetic costs. Similarly, the electrical signals produced by males are more conspicuous both to conspecific females and to potential predators. A striking feature of electrocommunication is the wealth of species diversity including variation in the magnitude and direction of sex differences in body size, EOD properties, and other electrical behaviors; diversity in habitat preferences, sociality, and associated diversity in physiology and patterning of the EOD; and variation in courtship and parental behavior. This diversity is coupled with numerous striking examples of convergence in the behavior and physiology between the two different orders of fish that independently evolved electrocommunication. These features lend themselves to comparative studies of the hormonal control of reproductive and communication behavior and make electric fish excellent model animals for studying how evolutionary lability of neuroendocrine mechanisms that regulate sex differences can lead to species diversity in sexually dimorphic behavior.