Effect of food and predators on the activity of four larval ranid frogs

When animals are more active they encounter both more food and more predato rs. Thus, activity rates mediate a trade-off between growth rates and preda tion risk. Models of the trade-off generally, but not exclusively, predict reduced activity when resource availability increases or when predation ris k increases. In a laboratory setting, we videotaped larvae of four species of ranid frogs (bullfrog, Rana catesbeiana; green frog, R. clamitans; leopa rd frog, R. pipiens; and wood frog, R. sylvatica). Changes in activity leve l in response to changes in food and predator density were measured. Overal l, species reduced both the proportion of time active and swimming speed wi th increases in resource level and predator density. These effects were add itive. Regardless of food level, additional predators reduced activity leve ls similar amounts in all four species. Larger animals, which are less vuln erable to predation, were more active than smaller animals. Leopard frog an d wood frog larvae, which are characteristic of more temporary waters, resp onded more strongly to variation in food levels than did bullfrog and green frog larvae.