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 predators. Thus, activity rates mediate a trade-off between growth rates and predation risk. Models of the trade-off generally, but not exclusively, predict reduced activity when resource availability increases or when predation risk increases. In a laboratory setting, we videotaped larvae of four species of ranid frogs (bullfrog, Rana catesbeiana; green frog, R. clamitans; leopard frog, R. pipiens; and wood frog, R. sylvatica). Changes in activity level in response to changes in food and predator density were measured. Overall, species reduced both the proportion of time active and swimming speed with increases in resource level and predator density. These effects were additive. Regardless of food level, additional predators reduced activity levels similar amounts in all four species. Larger animals, which are less vulnerable to predation, were more active than smaller animals. Leopard frog and wood frog larvae, which are characteristic of more temporary waters, responded more strongly to variation in food levels than did bullfrog and green frog larvae.