Evidence of the peptide identity of the epidermal alarm cue in tadpoles of the toad Rhinella arenarum

Chemical cues associated with predation attempts allow prey to trigger defensive behaviours. Accordingly, tadpoles of several species of anurans display strong behavioural responses to chemical cues of injured conspecifics. As part of the antipredator response, tadpoles show rapid and sustained inhibition of activity when exposed to chemical cues of predation. Although the ability to respond to cues of conspecifics has been confirmed in a wide variety of anuran species, studies about the tissue source and the chemical aspects of the molecules involved are scarce and contradictory. In the present work, we analysed the chemical characteristics, tissue source and release mechanism of the chemical alarm cue in Rhinella arenarum tadpoles. Our results support the hypothesis that a peptide of epidermal origin in mediates amphibian tadpole communication.

Behavioral avoidance of injured conspecific and predatory chemical stimuli by larvae of the aquatic caddisfly Hesperophylax occidentalis

Prey animals often encounter situations that hinder their ability to conduct normal fitness-enhancing behaviors. Mating and foraging are frequently interrupted by predator vigilance and avoidance, and antipredator behavior. Many caddisfly larvae build protective cases that are carried with them throughout the aquatic life cycle. However, they are still vulnerable to predation, yet it is unknown the extent caddisflies use chemical cues for predator recognition and avoidance. We exposed larval caddisfly Hesperophylax occidentalis (Banks, 1908) to predatory, conspecific, and heterospecific chemical cues to determine if caddisfly larvae can use chemical stimuli alone for predator recognition and avoidance. Exposure to predator and injured conspecific chemicals elicited significant decreases in activity, while exposure to injured and uninjured heterospecific chemicals yielded no significant change in activity. The extended latency to move following exposure to predator kairomones indicates larval caddisflies utilize chemical cues for predator recognition and avoidance, and a similar decrease in movement associated with exposure to injured conspecifics suggests the presence of a chemical alarm cue.

Active space of chemical alarm cue in natural fish populations

Chemical cues released from injured fish skin during a predator attack provide reliable information about the presence of predation risk. Here, I report estimates of the area avoided by littoral fishes after experimental release of chemical alarm cues in two small lakes in northern Minnesota. Minnow traps were labeled chemically with either water (control) or skin extract (chemical alarm cue) made from 2 cm2 of cyprinid skin (redbelly dace in experiment 1, fathead minnows in experiment 2). Traps labeled with water were placed 1, 2, or 8 m from traps labeled with alarm cue. After 2 h, water-traps that were either 1 or 2 m distant from an alarm-trap caught significantly fewer fish than water-traps 8 m distant from alarm-traps. Conspecific and heterospecific skin extract produced similar area avoidance by fathead minnows. Redbelly dace showed a larger active space in response to conspecific than heterospecific alarm cues. Brook stickleback showed reduced catches within 2 m of skin extract of fathead minnows. Overall, the radius of active space was between 2 and 8 m under lake conditions with average subsurface currents of 0.82 cm/s. These data are the first field estimates of active space of ostariophysan chemical alarm cues.

Response of juvenile rainbow trout to varying concentrations of chemical alarm cue: response thresholds and survival during encounters with predators

Prey animals may mediate the intensity of their behavioural responses to predators to reflect their risk of predation. However, in the absence of an overt (observable) behavioural response to a particular predation-risk cue, we need to ask whether or not prey animals are still using the cue to assess predation risk. Behavioural responses that are not readily observable within the time frame of the experiment are considered covert. In this study we exposed juvenile rainbow trout, Oncorhynchus mykiss, to varying concentrations of conspecific chemical alarm cue to determine their observable response threshold. In a subsequent experiment we exposed the trout to alarm-cue concentrations above and below their behavioural-response threshold and allowed them to interact with an unknown predator (northern pike, Esox lucius). Trout exposed to concentrations below the observable response threshold were able to evade the predator equally as well as trout exposed to alarm-cue concentrations above the observable response threshold. This study illustrates the sophistication with which prey animals employ chemosensory risk assessment. We must use caution when relying on overt behavioural responses for assessing whether prey are utilizing specific cues to mediate their risk of predation.