Early Social Environment and the Fighting Behaviour of Young Oreochromis Niloticus (Pisces, Cichlidae)

EARLY SOCIAL ENVIRONMENT AND THE FIGHTING BEHAVIOUR OF YOUNG OREOCHROMIS NILOTICUS (PISCES, CICHLIDAE)

l 2 ASSAF BARKI ) and GILSON L. VOLPAT0 ) (Departmento de Fisiologia, IB, Universidade Estadual Paulista-UNESP, 18618-000 Botucatu SP, Brazil)

(Acc. IO-YII-1998)

Summary

This study examines the influence of early experience with different forms of aggressive behaviour on the fighting behaviour of young fish. Fry of the cichlid fish, Oreochromis niloticus, were raised from hatching in small groups consisting of a normal individual (the test fish) and either mutant conspecifics lacking the dorsal fin and thereby the ability to perform fin displays, or normal ones. Following a 63-day period of development in groups the test fish were confronted in their home tanks with an unfamiliar normal fish for 10 min. The fighting behaviour of the test fish was analyzed considering their previous group type (mutant or normal) and rank (0: or ß). There was no difference between test fish in the rate and sequence of behaviour patterns used in fighting. However, test fish that had developed in mutant groups were rarely the first to bite in contests and had a longer latency to biting following the first bite of the stimulus fish than test fish with normal experience. This finding is attributable to the form of aggressive behaviour experienced by the test fish during development but not to existing differences in the amount of aggression previously experienced, nor to previous rank, sex, or size relative to the stimulus fish. The results suggest that early experience influenced decision making by the test fish during the fight. The involvement of the fin displays and the possible mechanism of this influence are discussed. Keywords: cichlid fish, Oreochromis niloticus, early experience, fighting behaviour.

I) Corresponding author, current address: Department of Aquaculture, Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet dagan 50250, Israel; e-mail: vlaqua @volcani.agri.gov.il 2) We thank Ilan Karplus and Boaz Yuval for helpful comments on earlier versions of the manuscript. This study was funded by FAPESP and CNPq.

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Introduction

Although genetically programmed to develop specific behaviours, fish may modify their behaviour through feedback from experience (Huntingford, 1986). The effect of past experience has often been demonstrated with re- gard to fish aggressive behaviour. Since aggression is usually elicited by stimuli from conspecifics in a competitive context, the social environment where the fish developed may be important in the expression of its aggressive behaviour. Studies on the effect of social experience on aggression in fish have usually concentrated on social status or winning/losing of fights, and focused on their influence on aggressive motivation and the ability to win future fights (McDonald et aI., 1968; Francis, 1983, 1987; Beacham & Newman, 1987; Frank & Ribowski, 1987, 1989; Beacham, 1988; Bakker et al., 1989; Beaugrand et aI., 1991). Such studies are not concerned with details of the experience but only in the end result (e.g. dominant and subordinate experience). Several studies have indicated that fish may modify their aggressive displays by learning. Hollis (1984) trained blue gourami males (Trichogaster trichopterus) to display at a conditional stimulus predicting the appearance of an opponent, and this conditional response enhanced the ability of trained males to defend their territory against untrained intruders in signaled encounters. This classical conditioning of aggressive behaviour had also long term consequence for territorial males since the advantage in the first signaled encounter carried over to subsequent encounters where the appearance of an opponent was not signaled (Hollis et al., 1995). The behaviour of the fish may also be shaped by its experience with the conse- quences of an aggressive act (Losey, 1993). Male Siamese fighting fish (Betta splendens) learned to withhold displaying to their mirror image (apparently a rewarding stimulus) when displaying led to the disappear- ance of the image (Baeninger, 1984). Using operant conditioning, Adler & Hogan (1967) suppressed aggressive displays in male Siamese fighting fish by punishing any performance of the display with an electric shock, but Losey & Sevenster (1991) demonstrated constraint against learning not to display by punishment in threespine sticklebacks (Gasterosteus aculea- tus). However, rewarding the performance of that display increased its use (Losey & Sevenster, 1995). This line of evidences indicates that the ag-