Mormyrid fish communicate by continuously emitting weak electric organ discharges (EODs; for reviews see 123456). Waveform of EODs and rhythm of discharge are both important factors in mormyrid communication. The rhythm of discharge is highly variable, accompanying the moment-to-moment fluctuations of social context (e.g., 78910). Individual waveforms of EOD are usually constant over long periods of time 111213 and signal species identity; they may also vary between the sexes and signal individual identity 14; reviews, 1315.

Although genuinely sexually dimorphic waveforms of EOD are rare (sensu two alternative forms 14), in some genera of mormyrid fish, such as Marcusenius, male EOD waveforms of certain species are more diverse than those of females 16. This is also true in some members of the Marcusenius macrolepidotus, or Bulldog, species complex, such as Marcusenius pongolensis. However, differences between the sexes were less distinct than those observed in an allopatric population of Bulldog fish, M. altisambesi from the Upper Zambezi River (Namibia), the males of which generate EODs of up to 4700 μs, or 11× the female average, in the wet season (the two forms of Bulldog fish are now considered to have differentiated on the species level; 1718). The EOD duration values of M. pongolensis females were very similar amongst each other and did not change over lifetime. Juvenile males' EODs resembled those of females, but, on turning mature, increased statistically significantly with standard length over lifetime, up to 3 fold in the biggest males.

Experimental playback of long-duration EODs of sympatric males via an electric dipole fish model evoked a significant increase in male aggression towards the stimulus dipole in M. pongolensis males 19. This result suggests that a long EOD pulse duration signals size, fighting ability, or reproductive status of males, and may thus confer fitness advantages.

EODs of long duration may not only attract females but also electroreceptive predators. The common sharptooth catfish Clarias gariepinus detects M. altisambesi male EODs with pulse duration greater than 2 ms at considerable distance, but not EODs of females and males with EOD duration as found in the non-reproductive season 2021. Accordingly, Bulldogs were found to represent the main prey of C. gariepinus and certain other catfish in the Okavango/Upper Zambezi River systems during certain periods of the year 2223. In contrast, male M. pongolensis from South Africa studied in the field were not found to form part of these catfishes' diet 24. A possible reason for this difference is that in M. pongolensis EOD duration (hence spectral low-frequency content of EOD pulses) remains well below the critical detection threshold of C. gariepinus 21.

In two M. pongolensis males an EOD duration increase that was limited to the short period of actual spawning has been observed in the laboratory 1018, demonstrating short-term plasticity of EOD waveform within the male sex. Here, we characterise short-term plasticity of male EOD waveform during territorial encounters more clearly, and identify the conditions and limits of an EOD duration increase.

In mormyrid weakly electric fish, individual EOD waveforms are documented as temporally stable in several studies 1213; see also Table 2 for fish M11–M15 that were not exposed to new neighbours). But there is also evidence for EOD waveform plasticity. In captive Brienomyrus brachyistius, alpha males in social groups (three males and three females) increased their total EOD duration by 20% 25. Similar results were found in captive Gnathonemus petersii, the EOD duration of which was influenced by social interactions. Dominant fish increased their EOD duration from about 280 to 340 μs in male-male, female-female and mixed-sex pairs over an observation period of 4 days 26.

In M. pongolensis, a permanent characteristic of male EOD duration to increase with fish size, established in the wild, is superimposed by short-term plasticity evoked by social interaction and reproduction, observed in captivity. The permanent characteristic was seen in field recordings in the non-reproductive dry season, when EOD duration in male M. pongolensis was positively correlated with fish length to up to 818 μs in the largest males. This is in contrast to females where no such dependency was found 1718. A superimposed short-term waveform plasticity (that is the subject of the present study) was first documented during reproduction of two males in captivity. EOD duration increased only for a short period of actual spawning (by around 70% in one male 10; in another male, by 75%: from 511 μs EOD duration as measured in the field to a maximum of 894 μs during spawning in the laboratory 18). A longer-lived short-term plasticity was described in the present study in the context of social interaction. By experimentally manipulating territory and neighbour identity, a surprisingly high degree of short-term plasticity in EOD waveform in male M. pongolensis was evoked. An increase in EOD duration was observed in ten males; in one male by 470% or almost 2500 μs total duration. The EOD pulse duration increase was reversible and reproducible, and was correlated with aggressive interactions with both males and females, accompanied by escalating fights at the territory boundary. Before experimental treatment, EOD duration in males that became dominant was greater by about 30% than that of submissive ones, similar to the results of 26 and 25.

Fluctuation in EOD waveform has been studied in detail in gymnotiform weakly electric fish. In the family Hypopomidae EOD amplitude and duration seem to depend on social status and time of the day 272829. The masculinization of male EOD waveform (an increase in certain EOD parameters) is greatest in dominant males (Brachyhypopomus pinnicaudatus, unpublished data reviewed in 30), and may develop within tens of minutes in response to aggressive contacts (review 31). Similar to the present results, an increase in EOD duration was independent of social status when males were separated by plastic mesh 30. As partitions prevented escalating fights, stronger males probably could not prove their dominance.

In gymnotiform fish three types of plasticity of EOD duration are discussed (reviews 303132). Apart from two rapid mechanisms there exists a mechanism of slow change in EOD waveform that is hormonally triggered. Also in certain mormyrids, EOD waveform duration increased upon androgen administration (e.g., 3334). In the mormyrid B. brachyistius, endogenous 11-ketotestosterone level increased with social status 25. Among other effects, testosterone is known to raise aggression. An increase in testosterone titre evoked by our treatments may have caused the high level of aggression and the elongation of EOD duration.

The strong increase in male EOD duration evoked by social interactions is in line with the assumption that EOD duration in M. pongolensis is an important factor in social context. (1) In adult male fish studied in the wild, EOD duration is positively correlated with standard length 18. Larger males probably enjoy a higher reproductive success, as larger males are more successful in competing for females, have better access to high-quality feeding and spawning sites. Also in many other species, females prefer larger males, as body size may indicate fitness 3536. (2) Playback EODs of long duration evoked stronger aggression from resident male M. pongolensis 19, probably because they signalled a greater threat.

Males may be particularly vulnerable to predation when emitting long-duration EODs because these are detected more easily at lower threshold by electroreceptive predatory catfish 2021. A possible way out of this male dilemma may be short-term plasticity of EOD waveform, that is, to signal long-duration EODs for a limited period of time only, when competing for mates.

To restrict EODs of long duration to the period of reproduction, if not only actual spawning, appears to be a superior adaptation to predator pressure. South African Bulldogs (M. pongolensis) seem to have found a way to signal resource holding potential in male-male interactions without alerting predatory catfish for longer than absolutely necessary.

EOD: electric organ discharge; SL: standard length (from the tip of the mouth to the midbase of the forked tail fin); RM: resident male; SM: stranger male; M: male; F: female; P: head-positive phase of the EOD; N: negative phase.

The authors declare that they have no competing interests.

SH designed and carried out the behavioural studies and waveform measurements. BK provided the animals, carried out the waveform measurements in the field, participated in the experimental design and helped to draft the manuscript. Both authors read and approved the final manuscript.