Switching in Predatory Fish

Guppies (Poecilia reticulatus) were given two prey: Drosophila (on the water surface) and tubificid worms (on the aquarium bottom). The densities of the two prey were varied gradually over 12 days, from 4:1 to 1:4, and we measured the predators' diets, time of each meal, and time spent at the top, middle, and bottom of the aquarium. The guppies switched, attacking disproportionately whichever of the two prey was more abundant. Switching resulted when the fish increased the fraction of time spent at the surface as the fraction of Drosophila available increased. They did this in response to changing reward rates. Switching caused density—dependent mortality. Differences between two groups of fish show that switching behavior is more efficient and should be selected for. The result confirms previous generalizations about switching.