The development of behavioral defenses: a mechanistic analysis of vulnerability in red-eyed tree frog hatchlings

Abstract

I examined the development and effectiveness of behavioral defenses of red-eyed tree frog hatchlings, Agalychnis callidryas, against predatory shrimp Macrobrachium americanum. Arboreal eggs of A. callidryas hatch early if attacked by egg predators and later if undisturbed, producing tadpoles that enter the water at different developmental stages. Older hatchlings survive better than young hatchlings with aquatic predators, including shrimp. Hatchlings respond to shrimp by both increasing activity and avoiding the bottom microhabitat. Older hatchlings are more active and, in the presence of shrimp, avoid the bottom more than young hatchlings. These ontogenetic changes in behavior improve survival. Specifically, the likelihood of fleeing from shrimp increases with hatchling age, and fleeing is an effective defense. In contrast to results from experiments with odonates, immobility does not reduce risk of shrimp attack, thus there is no trade-off between fleeing and motionless crypsis. Shrimp spend most of their time on the bottom, where attacks are more often successful. Avoidance of the bottom microhabitat by tadpoles therefore improves survival. Evasive maneuvers also function in defense, but evasiveness does not change with age. Morphology may limit microhabitat use by younger hatchlings. Failure to flee may reflect unresponsiveness to disturbance, which would reduce unnecessarily early hatching and limit exposure of young A. callidryas to aquatic predators.