Behavioral Evidence for Species Recognition

Observations of mormyrid electric fishes  both in aquaria and in their natural setting suggest that species and sex recognition take place during courtship and mate choice.  While the breeding behavior of most species remains a mystery,  at least one species has been bred in the laboratory and several have been observed in the field.

Mormyrid males appear to defend nest sites and to call to passing females from these defended areas.    In a study of Brienomyrus brachyistius TP,  Hopkins and Bass (1981) demonstrated that males "call" to passing females using an electrical display called a "rasp" consisting of a rapid burst of EODs at about 100 per second.   Typically the males discharge at less than 10 per second or slower.  In this example,  the rasp is a conspicuous calling display that serves to call to the female.  Click on the picture to "hear" rasps from males during courtship.

Males discriminate between passing males and passing females and call only to females.  During the breeding season,  males produce as many as 10 rasps per minute in the presence of females. They do not rasp during the dry season, nor when females are absent.  

The EOD waveforms of males and females differ in duration. Male EODs tend to be 2.5 ms in duration;  female EODs are less than 1.0 ms.  Typical EODs are illustrated here for this species.

Males Discriminate on the Basis of EODs.  Male and female differ in their EODs, but also they differ in the sequence of pulse intervals (SPIs) produced.  To test the relative importance of EODs and SPIs in sex recognition,  Hopkins (1983) played back computer-digitized EODs with sequences of random intervals, or with sequences recorded from males and of females.  Males gave rasps to any playback as long as the EOD was that of the female.  They did not rasp in response to males, nor to EODs of other species that were found in the same habitat.

Males Discriminate EODs on the Basis of Temporal Features.
 Males give courtship rasps to female EODs played to them in the field,  but they do NOT respond to the same EODs played backwards in time.  Nor do they respond to EODs that have had their phase spectrum phase-shifted by varying amounts to produce EODs with identical power spectra but different phase spectra.  Thus,  the fish is attending to the temporal features of the waveform, not the spectral cues.

The vertical axis is the number of rasps produced, the horizontal is the stimulus waveform.  0 degrees is the natural female EOD.

From Hopkins and Bass, 1981.

The Search for Sign Stimuli for Evoking Courtship.  

In Brienomyrus, courtship calling can be evoked if an electrical stimulus is given in the water that mimicks the temporal features of the female EOD.  Thus,  a square wave that has a duration of approximately 0.4 ms, is adequate to evoke singing in the male.  The square wave has electrical transitions separated by the correct temporal interval to imitate the female EOD.  Square waves of 0.1 ms or 1.6 ms do no stimulate singing in the male. Although the spectrum of the square wave is unlike that of the EOD,  the timing of transitions in the waveform is correct.  These experiments strongly suggest that these fish are attending to the temporal features of the waveform.

The square wave is an example of a sign stimulus in the ethological tradition. It carries the essence of the cues necessary for evoking a species-typical response.

From Hopkins and Bass, 1981

In another species, Pollimyrus adspersus, bred successfully in aquaria,  males call to females using acoustic signals rather than electrical (Crawford et al, 1986).  In this species, the sex differences between male and female EODs are subtle,  and males do not appear to differentiate between EODs.  Instead,  it is the pattern of the SPI that evokes calling by the male  (Crawford, 1991,1992).

Return to Species Recognition

Crawford, J. D., Hagedorn, M. and Hopkins, C. D. (1986). Acoustic communication in an electric fish Pollimyrus isidori (Mormyridae). Journal of Comparative Physiology A 159, 297-310.

Crawford, J. (1991). Sex recognition by electric cues in a sound-producing mormyrid fish, Pollimyrus isidori. Brain Behavior and Evolution 38, 20-38.

Crawford, J. D. (1992). Individual and sex specificity in the electric organ discharges of breeding mormyrid fish. J. exp. Biol. 164, 79-102.

Hopkins, C. D. and Bass, A. H. (1981). Temporal coding of species recognition signals in an electric fish. Science 212, 85-87.