The ontogeny of agonistic behavior and the onset of sexual maturation in the paradise fish, Macropodus opercularis (Linnaeus)1.

Fish 20-100 days of age were observed in brief conflicts with a stimulus fish of similar size and age. Four stages of behavior development were identified, based on quantitative changes in behavior which occurred with increasing age. Relatively little aggressive behavior occurs in the larval stage which includes fish up to about 40 days of age. Juveniles, age 40-70 days, approach, chase, and attack each other much more than do the larva. In the adolescent stage, 70-100 days of age, the incidence of species typical displays increases paralleling the onset of puberal changes in the testis and ovary. No sex differences in behavior were seen in fish up to 100 days of age. Among adult fish of unknown age males performed lateral threat displays more frequently than females as has been previously reported.