Early stress effects on growth and adult behavior in Poecilia reticulata

Small mammals subjected to early stress show decreased aggression and emotionality, accelerated rates of maturation, and increased sexual activity, affiliation, and size. The purpose of this experiment was to test the phyletic generality of this phenomenon using fish (Poecilia reticulata) and, also, to control for the sensitivity of the measures, to determine if controls for both water quality and rearing‐testing conditions might produce different effects than have previously been reported for fish, and to see in what respects the effects of isolation and crowding are similar to those generally found in mammals. Early‐stressed guppies showed reduced aggression, but did not differ from controls on any other measure. Isolated and crowded guppies differed from each other and from the controls on 4 of the 6 measures. Isolated fish matured more rapidly and attained greater adult length than did the controls; the reverse was true for crowded fish. Crowded fish showed higher levels of emotionality than controls, whereas isolates showed lower levels than controls. No differences were noted in affiliation, although crowded fish spent more time in the vicinity of empty stimulus boxes. Crowded fish did not differ from controls in levels of sexual activity or display, but isolated fish showed low levels of sexual activity and high levels of display. These findings suggest that at least part of the early stress phenomenon shown in small mammals may be demonstrated in fish.