Electric communication during courtship and spawning in two sibling species of dwarf stonebasher from southern Africa, Pollimyrus castelnaui and P. marianne (Mormyridae, Teleostei): evidence for a non species-specific communication code?

Abstract

SummaryThe fixed part of the electrocommunication signal, the electric organ discharge (EOD) waveform, is well differentiated in the two vicariant dwarf stonebasher species, Pollimyrus castelnaui and P. marianne. However, differentiation regarding the variable, situation-dependent part, i.e., inter-discharge interval (IDI) patterns, has never been studied in a pair of sibling species of mormyrid fish. We here compare the electrical signalling that accompanies different motor behaviours (such as resting and swimming, territorial agonistic interactions, courtship and spawning) in the two species. Double pulse patterns of regularly alternating short IDIs of 8-11 ms and long ones of 16-100 ms accompanied threat displays in both species. In three pairs of P. marianne and five pairs of P. castelnaui, courtship was characterised by nest building, territory patrolling and acoustic displays (advertisment calls) that were accompanied by long discharge breaks in the male and highly regular IDIs around 50 ms in the female of both species. Nest-tending males showed IDI sequences consisting of regularly alternating double pulse patterns, similar to threat displays. During spawning both sexes generated stereotyped IDI sequences of a low discharge rate. All IDI patterns occurring in one species were also found in the other, and no species-specifity was identified at that level. Playback experiments contrasting conspecific and heterospecific IDI sequences (that had been recorded from nocturnally swimming fish) revealed preferences in none of the six experimental subjects. Double pulse patterns, high discharge rate displays (HD) and regularisations of the IDI sequence accompanying specific behaviours occurred in similar form in Baier & Kramer both dwarf stonebasher species of the present study. Therefore, we conclude that in the speciation of P. castelnaui and P. marianne the fixed part of the EOD, its waveform, was under more differential selection pressure than its variable part, the patterns of IDI.

Sexual selection by female choice prevents speciation reversal in a hybridizing trio of mormyrid fish in southern Africa: evidence from playback experiments of electric organ discharges

“…In fry of the present Pollimyrus species, learning by imprinting on the father fish's EOD waveform is conceivable. After the last spawning bout when the female has left the area for good, the father fish collects the eggs and tends the nest from egg stage until the juveniles disband (Lamml & Kramer, 2005;Baier & Kramer, 2007). This is assumed to occur when the juveniles start to behave aggressively against each other, as has been observed at an age of 50-60 days in P. adspersus (Postner & Kramer, 1995;Kramer & Postner, 1997).…”

“…(Continued. ) 2006; Baier & Kramer, 2007) in a more natural setting were, of course, excluded in this laboratory study that focussed on the EOD as a putative sensory basis for assortative mating. Common developmental and conditional factors are also seen as sources for individual differences in female mate choice, and variation in both response readiness and in female sensory detection of male displays is suggested to depend on factors such as nutrient availability, hormone profiles or age (Ronald et al, 2012).…”

“…Preference for one EOD was measured as the difference in association time (staying in the area defined as 'close' to a dipole stimulator), the frequency of head butts directed at the stimulators, and the number of circles performed around the stimulator. Head butts and circling are typical behaviours seen in courtship and spawning (Baier & Kramer, 2007). One of the two male EODs that were contrasted in a test was a conspecific one, of which there were two representations in order to guard against the possibility of one being immensely attractive or repulsive (Kroodsma, 1989).…”

“…Head butts and circling (additionally tallied in the main tests) are commonly shown behaviours in interacting mates (Lamml & Kramer, 2005Baier & Kramer, 2007). All subjects responded well to the playback stimulation by swimming towards and associating (successively) with both dipoles, and also head-butting it as if it were a fish, but only two subjects discriminated between the two conspecific playback pulses.…”

“…In playback experiments the experimental subjects did not show any preference for one or the other species' IDI patterns. These experiments aimed at detecting hypothetical species-specific features hidden to the human observer but potentially recognised by the experimental subjects, and were not found (Baier & Kramer, 2007).…”