As in other species of poeciliid fish, male Xiphophorus variatus (Pisces, Poeciliidae) virtually cease growth at maturity. The size of an adult male, then, is determined by its average growth rate before it reaches maturity and the age at which this happens. The latter factor is under social control in this species. When I raised juvenile males in groups, they usually started to mature in sequence, largest first, and subsequently maturing males did not usually reach maturity until they were longer than all previously matured males. As a result, the juvenile size order was negatively correlated with adult size order. When the fish in a group were isolated from one another such patterns disappeared indicating that some degree of social interaction was necessary for their maintenance1.
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