Synchronized maturation and breeding in natural populations of Xiphophorus variatus (Poeciliidae)

Reproductive events tend to be synchronized in natural populations of Xiphophorus variatus. Males physically resembling one another tend to mature at the same rate and time. This coordinate development has also been seen in the laboratory. We hypothesize that juveniles resembling each other experience fewer aggressive responses from males than do juveniles without 'look-alikes', because of habituation of aggression. This would decrease social inhibition and permit their synchronous development. Females have an ovarian cycle. In the laboratory, crowding stress causes a suspension of breeding while a return to uncrowded conditions permits its resumption. Females released simultaneously from crowding tend to be synchronized in subsequent broods, although the synchrony deteriorates with time. Synchronizing events are shown to occur in natural populations. The consequences of reproductive synchrony in males and females are briefly discussed.