Effects of metolachlor and fish cues on tadpole antipredator behaviors

Antipredator behaviors are often crucial for animal survival. In environments impacted by humans, animals may be exposed to a wide array of chemical contaminants due to agriculture, industry, and other activities. Among other effects, these chemical contaminants may influence behaviors prey animals use to cope with predation risk, likely reducing survival by suppressing or even maladaptively reversing normal behavioral responses. Changes in behavioral responses could be the result of impaired detection of cues that indicate predator presence or altered behavioral plasticity in response to predator cues. In a laboratory experiment, we tested the effects of the commonly used herbicide metolachlor on the behavior of tadpoles of the American bullfrog (Lithobates catesbeianus) in both the absence and presence of water conditioned with cues (odor) of fish. We observed several behaviors in the tadpoles reflective of predator avoidance when exposed to combinations of metolachlor (0 or 80 ppb) and the presence or absence of fish cues. We found that metolachlor had size-dependent effects on tadpole activity levels, with larger tadpoles being much less active in the absence of metolachlor but similar in activity to small tadpoles when metolachlor was present. Fish cues similarly had size-dependent effects on multiple tadpole behaviors. Finally, hiding behavior and zone change (movement) behavior of tadpoles in response to fish cues varied depending on the presence of metolachlor and, in the case of zone changes, tadpole size. Our findings indicate that the presence of chemical contaminants has the potential to alter important antipredator behaviors. However, these effects are complex, and size may mediate the nature of the response to both chemical contaminants and predators. The herbicide metolachlor alters tadpole behavior, sometimes interactively with fish cues. Body size of tadpoles influenced the effects of metolachlor and fish.