Physiological and behavioral responses of anuran larvae to predation cues
Predators are known to cause prey to alter their morphology, life history or behavior in ways that reduce the likelihood of the prey being consumed by the predator. Seldom considered, however, are the consequences of predators on internal morphology (e.g., gut length) or physiology of prey. Such consideration is important because these traits likely affect prey growth and could explain why prey often grow more slowly in the presence of predators. Furthermore, a history of exposure to predators may alter how strongly visual or chemical signals from predators affect prey physiology and behavior. We raised larval frogs in artificial ponds that either lacked or contained a caged fish predator and assessed whether rearing environment affected prey gut length and metabolic rate. We also assessed whether the rearing environment affected the metabolic and behavioral response of larval frogs to either visual or chemical signals from fish by measuring the metabolism and behavior of naïve and conditioned larval frogs when exposed to visual and/or chemical signals from fish.
Tadpoles raised with predators had shorter guts but rearing environment had no effect on the metabolic rate of tadpoles, body mass, or survival. Exposure to chemical cues from predators altered the metabolic rate of naïve tadpoles but not tadpoles with prior exposure to predators. Smaller naïve tadpoles reduced their metabolic rate but larger naïve tadpoles enhanced their metabolic rate in response to chemical cues. Chemical cues caused the metabolic rate of naïve tadpoles to be 24% greater than that observed in tadpoles that were reared with predators. Visual cues did not influence the metabolic rate of tadpoles. Prior exposure to predators did not cause tadpoles to differ in their activity levels or their likelihood to seek a refuge. Exposure to chemical cues increased the likelihood that naïve tadpoles sought a refuge. The combined influence of visual and chemical cues increased refuge use by conditioned tadpoles but neither affected refuge use alone. Our results indicate that prior exposure to predators may compromise the ability of prey to extract resources by causing prey to develop shorter guts. These results further suggest that the greater activity of predator conditioned tadpoles to be a result of a less efficient digestion system requiring increased foraging effort.