The costs and benefits of frog chorusing behavior

Summary

• 1. A number of predators, including a bat (Trachops cirrhosus), a frog (Leptodactylus pentadactylus), an opossum (Philander opossum), and a crab (Potamocarcinus richmondi), prey on the neotropical frog Physalaemus pustulosus, which calls in choruses on Barro Colorado Island, Panama.

• 2. Predation rate (no. of frogs eaten/h of observation) and predation risk to individuals (predation rate/chorus size) were determined for choruses of various sizes. There was no correlation between chorus size and predation rate, but there was a significant negative correlation between chorus size and predation risk.

• 3. There was a significant correlation between the number of females present and chorus size (i.e., number of males). A second order regression indicates that the proportion of females to males, the operational sex ratio, tends to increase with chorus size; thus, males have a higher probability of mating in larger choruses.

• 4. We suggest that the benefits of lower predation risk and higher mating probabilities associated with larger choruses were responsible for the evolution of communal sexual display in Physalaemus pustulosus.

• 5. A cost-benefit model predicts that the size of males that join choruses is influenced by the asymmetric benefits related to male size and the behavior of other males in the population.