The Role of Intersexual Selection on EOD Duration in Weakly Electric Fish

Electric organ discharges (EODs) by weakly electric fish serve multiple purposes. As discussed in class, EODs produce electric fields around the fish by generating a separation of electric charges. The charges are specifically produced by electrocyte cells that are stacked in series within the electric organ of the fish. The electric field that the fish generates can act as a manner of mapping surroundings, and as a mode of communication.

For the South African bulldog fish, Marcusenius pongolensis, which emits a pulse EOD wave form, EODs convey a wealth of information. Previous studies have found that they are able to detect very minor variations in EOD waveforms that allow them to recognize different species, and specific individuals. It has also been determined that EOD duration increases as a function of length in the males of this species. This revelation, along with evidence that females of some gymnotiform species preferentially choose large males with longer EODs, led to this article’s study of the role of female choice in the evolutionary development of male bulldog EODs.

In this study single female bulldog fish were placed in tanks containing a porous pot (shelter), and dipoles located outside the pots, which were used to transmit male EOD playbacks. Females of this species are known for their extremely aggressive behavior towards males prior to spawning. For this reason, once playbacks began, four female behaviors directed toward active dipoles, and associated with readiness to spawn, were measured: 1) head butts 2) circling the dipole without head butts 3) circling with head butts, and 4) association time.

Females were subjected to either a single-dipole experiment, or a double-dipole experiment. During the single-dipole testing the female bulldog fish was exposed to a single stimulus pulse. In the double-dipole test two stimulus pulses were played concurrently. In each of the double-dipole tests, one of the stimuli played was always that of the shortest pulse (320 µs), and the other was one of the four longest pulses.

The EOD recordings were gathered from 18 male bulldog fish. All EOD recordings were biphasic short pulses with head-positive first phase, followed by a negative phase. Each of the 25 males recorded had variations in their pulse duration ranging from 320µs to716 µs. As stated above, the increase in EOD length in adulthood is a sexually dimorphic feature of this species, as it only happens in males.

The results of the experiment showed that females had a stronger response to longer male playbacks. For the single-dipole test females immediately swam out of their shelters to the active dipole, and often head butted it. Head butting behavior increased with pulse duration. While during the double-dipole test females were significantly more likely to show aggressive behavior towards the dipole emitting the larger pulse wave.

In class the question was raised about how a female might selectively choose a mate based on electrosignaling, and this article reveals a possible answer. As males grow larger their EODs increase in duration, and in turn females are selecting their mates, at least in part, based on longer signals. Larger males are most likely better able to gain access to resources, and defend themselves against aggressors. Therefore, if male EOD duration is a function of size, and a true indicator of male quality, it is clear how female choice could, over evolutionary time, drive males to have longer EODs.

Machnik, P., & Kramer, B. (2008). Female choice by electric pulse duration: attractiveness of the males’ communication signal assessed by female bulldog fish, Marcusenius pongolensis (Mormyridae, Teleostei). The Journal of Experimental Biology, 211, 1969-1977.