Night and day: comparing flight initiation dynamics in two closely related species of true frogs

Abstract

The decision of when to flee a predator depends on the costs and benefits of flight. Here, we used two species of closely related frogs, Lithobates catesbeianus and L. clamitans, to test the effects of several factors on flight initiation distance (FID), the distance between an approaching predator and its prey when the latter flees. We altered costs of flight by approaching frogs from within versus outside ponds, and we tested the influence of broad-scale visibility by approaching frogs during the day and at night. To assess small-scale visibility, we measured percentage of vegetative cover at the point from which a frog fled. To test effects of distance to refuge and body size on FID, we measured distance between each frog and the pond margin when the frog fled, and we estimated frog size. We predicted that FID would be greatest during the day and when frogs were approached from outside the pond. We also predicted that FID would increase with less vegetative cover, increasing distance between frogs and the pond, and increased frog body size. We used an information theoretic approach to contrast alternative models. For L. catesbeianus, the best-fit model included four highly weighted parameters: approach location, day/night, body size and distance to refuge. For L. clamitans, the best-fit model included only two parameters with high weights, day/night and body size. Lithobates catesbeianus and L. clamitans appear to differ in their sensitivity to predators, with L. catesbeianus having longer FIDs than L. clamitans and being strongly affected by more parameters. The differences we observed in FID between the two species may be best explained by differences in body size.