The effect of visual density on the fecundity of the guppy,Poecilia reticulata

Synopsis This paper examines the effect of visual density on the brood size of female guppies,Poecilia reticulata. Previous laboratory studies of guppies showed that the fecundity of females decreases as density increases. In the present study, the effect of visual density was examined under conditions where detrimental factors such as interaction of individuals, waste substances, and dissolved oxygen were constant. The brood size of females at visual densities of 4 or more individuals was significantly larger than at zero visual density. There was a significant positive correlation between brood size and visual density. Sex ratio did not significantly affect brood size. Guppies can recognize surrounding densities and change their own fecundity in response to changes in density.