Reproductive Behaviour

It was only after Pollimyrus isidori successfully reproduced in aquaria (Birkholz 1969, 1970; Kirschbaum 1975, 1987) that the reproductive behaviour of a mormyrid could be studied (Crawford et al. 1986); a breakthrough for the scientific enquiry of electrocommunication (Bratton and Kramer 1989).

60 • Communication by Electric Organ Discharges: Strategies

Throughout the year captive P. isidori males tend to become territorial. Even in a big aquarium there is usually a single, dominant male which chases away all other fish from the aquarium bottom. The only place where the other fish are relatively secure from the high aggression of the "tyrant" is high up in the water column where plants, tubes, etc. are readily accepted for shelter by the subdominant fish.

The aquarium bottom is the place where the territorial male constructs a nest which is made from filamentous plant material anchored between rocks or stones. After dark and even before nest-building, the male begins to sing (Crawford et al. 1986;

Bratton and Kramer 1989). In a quiet room, humans may hear the song without any electronic equipment. These songs, composed of elements called grunts, growls, and moans, are thought to attract females from a distance in the wild (advertisement call).

growl

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Time(s)

Fig. 5.13: Courtship vocalization of a male Pollimyrus isidorU as recorded with a hydrophone. A sonagraphic, B oscillogram representations of the same song. The ordinate in A is sound frequency in kHz, in B sound pressure. Note the tonal quality of the "moan" (consisting of a fundamental frequency of about 220 Hz and its harmonic of twice that frequency) which contrasts strongly with the much more broad-band "grunt" and "growl" signals. (From Crawford et al. 1986)

The reason why in P. isidori acoustic rather than electric signals seem to take over that role may be biophysical - a longer reach. If we consider the singing fish a point source, sound intensity is inversely proportional to the square of the distance between source and receiver (Hassall and Zaveri 1979), while the electrical field gradient (a vector, measured in μ^cm) generated by a dipole falls off with the inverse cube of the distance (Knudsen 1974). In addition, P. isidori's very short EOD is weaker than that of B. niger of similar size, in which the electrical signal's reach, or communication distance, was determined as about 1.6 m at a conductivity of 52 μS/cm, commonly encountered in tropical waters (Squire and Moller 1982; MoUer et

Elephantfishes - Mormyroidei • 61

al. 1989). Therefore, P. isidori^ song probably propagates farther than its particularly weak EOD.

Early on a spawning night, a female signals her readiness to mate by a unique sequence of EOD intervals of low rate and constant duration (histogram peaks between 80 and 140 ms) which is completely devoid of the short bursts which are so characteristic for all other sustained EOD patterns of P. isidori (Bratton and Kramer 1989). This constant discharge pattern of low rate seems to attenuate the male's aggression which is 10-20 attacks of high intensity in the first hour after darkness, and less than 4 per hour afterwards. The reduced aggression allows the female to advance, without discharging, into the bottom region repeatedly for brief periods, at first without contact to the male.

Fig. 5.14: Nocturnal courtship (a-f) and spawning (g) behaviour in P. isidori, as drawn from infrared video recordings. During courtship the male approaches the female and (a) Head-to-tail Circling (HQ occurs, b The male arrives alongside of the stationary female, c becomes coupled Vent-to-Vent (W), d then turns laterally, e as both fish pivot around each other, f in one complete rotation (RO). The male then separates and the female swims away. Courtship bouts may be repeated for about 2 hrs on a spawning night, at a rate of 1-2 per minute. When spawning (OP) begins the rotation is deleted and (g) follows directly on (d). After each spawning bout the male quickly picks up the eggs (ET) in his mouth and (h) places them into the nest. (From Bratton and Kramer 1989)

P. isidori^,s courtship behaviour is one of the most complex among fish (Fig. 5.14).

The female swims to near the male's hiding place (or even into it; for example, a tube). There she waits from 1-3 s. Even before the female's arrival in his territory, the male switches from a high sporadic rate discharge pattern (mean rate of about 18/s,

62 • CommunJcation by Electric Organ Discharges: Strategies

containing many bursts) to a medium uniform rate (MUR pattern of 3.3-12.5/s, locking into the same discharge pattern of constant intervals at very low rate) which is displayed by the female (Fig. 5.15). In an antiparallel position, the two fish circle around each other very rapidly, discharging at surprisingly low rates. After several head-to-tail-circling (HC) episodes, interrupted by the female's quick retreat, often provoking courtship attacks and intense singing by the male, he is able to approach the female from behind and to position himself in parallel to the female while continuing his MUR discharge pattern. The female, however, displays a low to medium sporadic rate of still lower discharge rate, characterized by many discharge breaks. The male pitches head downward 20-40° to the female and rolls sideward after coupling itself tightly to the female by their anal fins. While remaining coupled ventrally, both fish, especially the male, move their caudal fins which cause them to perform one full rotation, resembling a slow (3-4 s) somersault in tandem. On completing the rotation the female returns to her hiding place, usually without discharging; the male immediately switches back from his MUR pattern to the high sporadic rate he displayed before the female's visit. Her brief visits recur every 30-60 s.

Fig. 5.15: Concurrent electric signalling during two nocturnal spawning bouts of (a) a male and (b) female Pollimyrus isidori. Ordinate inter-discharge interval duration (ms); Abscissa time in s. Record begins with the female just returning to her remote shelter (r) and waiting (FW), and the male patrolling within his territory (territory patrolling, TP, ET). Note that shortly before the female returns to the male's territory (FS) the male switches to a regular, low-rate medium uniform rate (MUR) contrasting with its high sporadic rate as shown during territory patrolling (TP) when the female is away. The male's MUR pattern is similar to the pattern the female displays throughout a whole courtship and spawning night until the last egg has been laid. During their times of close contact both fish stop discharging briefly in the male (discharge break, DBR), and for longer times in the female (discharge arrest, DAR). Other abbreviations see Fig. 5.14. (From Bratton and Kramer 1989)

Elephantfishes - Mormyroidei • 63

During the courtship period which typically lasts from 2 to 5 h after dark the male's singing activity wanes; he is completely silent during the later stages of courtship and the whole subsequent spawning period, except for rare courtship attacks.

The behaviour preliminary to spawning is essentially an abridged version of courtship. On the female's arrival at the spawning site, the male immediately positions himself laterally, stimulating her anal fin region with a quivering motion of his anal fin, followed by oviposition (a few eggs per visit), fertilizing and egg transport to the nest. Head-to-tail-circling and the elaborate somersault part are omitted.

The male swims away first, while the female returns to her hiding place. The male returns quickly to pick up the eggs (as many as 4 at a time), which he transfers to the nest. The average time interval between spawning bouts is 72 s while the average duration of a spawning bout is 13 s (Bratton and Kramer 1989).

Spawning continues for 2-6 h during which period 50-192 eggs are laid. Male aggression is very low; during the whole spawning period the male attacks the female less than 5 times (1-3 per h).

On having released all eggs the female stops returning to the male's territory; this is the time when her discharge pattern changes abruptly from a MUR to a RAL pattern which is a regular alternation of high and low rate, with short bursts of high rate occurring at 2/s. The male's aggression towards the female returns immediately (over 12 attacks per h); this is accompanied by intense singing.

The male's main postspawning activity is tending to the nest, retrieving eggs, and covering the front with plant material. The male remains near the nest also during daylight and constantly checks the eggs during the first two weeks (nest nudging).

Males may care for the eggs of more than one spawning or maintain two nests simultaneously. Over a period of 384 days the mean spawning cycle was 24 days; the shortest period between spawnings by the same female was 6 days (Bratton and Kramer 1989).

The male invests heavily into reproduction (nest-building, territorial defence, brood-care), and holds important resources (territory, nest); most likely, the male is polygynous. Therefore, a mating system called "resource defence polygyny" seems the most appropriate term for P. isidori (see Clutton-Brock 1991; Alcock 1993).