Agonistic behaviour and electric signalling in a mormyrid fish, Gnathonemus petersii

1. Agonistic motor behaviour and concurrent electric signalling were studied in individually held, residential Gnathonemus petersii. Aggressive behaviour was elicited by presenting a specimen of a closely related species, Mormyrus rume, for 3 min a day. 2. The principal agonistic motor patterns are described (Fig. 2). Among them head butt, approach and lateral display were further analysed. 3. The electrical activity displayed during agonistic behaviour was found to differ fundamentally both from isolated resting and swimming conditions. The mean discharge rate recorded during aggressive behaviour (31 Hz, Fig. 3 c) is approximately twice the rate observed in an isolated swimming fish (Fig. 3b) and three times the rate displayed by a resting animal (Fig. 3a). An attacking G. petersii exhibits a much greater range of electric organ discharge (EOD) intervals than isolated swimming or resting individuals. EOD-interval histograms recorded from attacking fish show two sharp modes at high discharge rate; there are no intermediate intervals. 4. During the course of an attack, the initially low and variable discharge rate increases fairly linearly as the distance from the attacked fish diminishes (Fig. 9). The EOD rate associated with physical contact (head butt) comprises between 60 and 80 Hz in 24 of 28 attacks analysed; the dominant mode of the distribution is 61 Hz (Fig. 8). 5. During subsequent lateral display, G. petersii emits a high discharge rate pattern consisting of two types of ldquosteady-staterdquo activities which may last up to a few seconds: the first is a fairly regular alternation of approx. 16 and 8 ms intervals (paired pulses); this pattern gives rise to the two peaks of high discharge rate in the interval histogram (Fig. 3c). The second is a regular sequence of either 16 or 8 ms intervals (Fig. 4A). The only female among the animals used in our study showed the same display but did not exhibit the highest possible discharge rate (i.e. a regular sequence or 8 ms intervals; Fig. 4B). The high discharge rate is terminated by a sudden discharge break (Figs. 4A and 6). 6. It is suggested that the attack-associated EOD rate increase is a remnant of an ordinary locomotory pattern which has changed its function to a ritualised aggressive signal that occurs in a socially significant and well-defined context. The high discharge rate might serve three functions: (i) behavioural isolation of closely related, sympatrically living mormyrids (perhaps by character displacement); (ii) recognition of sexes; (iii)_synchronisation of mates during courtship

Optimal movement in the prey strikes of weakly electric fish: a case study of the interplay of body plan and movement capability

Animal behaviour arises through a complex mixture of biomechanical, neuronal, sensory and control constraints. By focusing on a simple, stereotyped movement, the prey capture strike of a weakly electric fish, we show that the trajectory of a strike is one which minimizes effort. Specifically, we model the fish as a rigid ellipsoid moving through a fluid with no viscosity, governed by Kirchhoff's equations. This formulation allows us to exploit methods of discrete mechanics and optimal control to compute idealized fish trajectories that minimize a cost function. We compare these with the measured prey capture strikes of weakly electric fish from a previous study. The fish has certain movement limitations that are not incorporated in the mathematical model, such as not being able to move sideways. Nonetheless, we show quantitatively that the computed least-cost trajectories are remarkably similar to the measured trajectories. Since, in this simplified model, the basic geometry of the idealized fish determines the favourable modes of movement, this suggests a high degree of influence between body shape and movement capability. Simplified minimal models and optimization methods can give significant insight into how body morphology and movement capability are closely attuned in fish locomotion

Electrifying love: electric fish use species-specific discharge for mate recognition

Mate choice is mediated by a range of sensory cues, and assortative mating based on these cues can drive reproductive isolation among diverging populations. A specific feature of mormyrid fish, the electric organ discharge (EOD), is used for electrolocation and intraspecific communication. We hypothesized that the EOD also facilitates assortative mating and ultimately promotes prezygotic reproductive isolation in African weakly electric fishes. Our behavioural experiments using live males as well as EOD playback demonstrated that female mate recognition is influenced by EOD signals and that females are attracted to EOD characteristics of conspecific males. The dual function of the EOD for both foraging and social communication (including mate recognition leading to assortative mating) underlines the importance of electric signal differentiation for the divergence of African weakly electric fishes. Thus, the EOD provides an intriguing mechanism promoting trophic divergence and reproductive isolation between two closely related Campylomormyrus species occurring in sympatry in the lower Congo rapids