Early social environment and the fighting behaviour of young Oreochromis niloticus (Pisces, Cichlidae)

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 gr oups consisting of a normal individual (the test fish) and either mutant co nspecifics lacking the dorsal fin and thereby the ability to perform fin di splays, 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 cons idering their previous group type (mutant or normal) and rank (alpha or bet a). There was no difference between test fish in the rate and sequence of b ehaviour patterns used in fighting. However, test fish that had developed i n 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 rest f ish with normal experience. This finding is attributable to the form of agg ressive behaviour experienced by the test fish during development but not t o existing differences in the amount of aggression previously experienced, nor to previous rank, sex, or size relative to the stimulus fish. The resul ts suggest that early experience influenced decision making by the test fis h during the fight. The involvement of the fin displays and the possible me chanism of this influence are discussed.