Exploring the potency and replenishment of woodfrog disturbance cues, a nonspecific communication system in aquatic species

Abstract

Disturbance cues are chemicals released by disturbed but uninjured prey. These cues elicit increased vigilance when detected by conspecifics, similar to the better-known damaged-released alarm cues. Many studies on predation risk expose prey to cues from whole-body homogenates as an indicator of predation risk. Such homogenates are known to contain alarm cues but presumably also contain disturbance cues, although their contribution to responses elicited by homogenates is unstudied. Here, we first attempted to quantify the relative contribution of disturbance cues to the behavioural response to whole-body homogenates, by exposing larval woodfrogs, Lithobates sylvaticus, to homogenates from conspecifics that were either undisturbed (contained disturbance cues), or that just received a disturbance from a simulated predator chase (which presumably depleted disturbance cues). Both sets of homogenates elicited similar antipredator behaviour (reduced activity) in receivers, suggesting that disturbance cues either made little contribution to the responses or were not depleted by the chase. Responses to homogenates were also stronger than responses to disturbance cues alone, suggesting that tadpole alarm cues were the primary risk indicator in the homogenates, and that alarm cues are more potent than disturbance cues. Although the chemistry and release mechanisms of disturbance cues remain poorly understood, research suggests they are pulses of nitrogenous waste. Hence, in experiment 2, we investigated the physiological limitation of disturbance cue communication. Disturbance cues were collected after each of two chasing events, separated by either a short (5 min) or long (2 h) time delay, giving tadpoles either a short or long opportunity to replenish their disturbance cues. Unexpectedly, tadpoles maintained their response intensity to disturbance cues after the shorter replenishment period and showed a weakened response after the longer period. These findings indicate that disturbance cues in tadpoles are not immediately depleted by a chasing event and instead may be released slowly.