Effects of single and combined embryonic exposures to herbicide and conspecific chemical alarm cues on hatching and larval traits in the common frog (Rana temporaria).

Recent studies indicate that amphibian embryos can exhibit hatching plasticity in response to chemical cues indicative of a predation risk. However, data are lacking concerning the possible impacts of waterborne contaminants on such a process. To investigate this impact, we raised eggs of the common frog (Rana temporaria) until hatching in water contaminated with sublethal concentrations (0.01, 0.1, and 1 mg/L) of amitrole, a widely used triazole herbicide, either with or without the presence of chemical alarm cues from crushed conspecific tadpoles. Embryonic exposure to conspecific alarm cues resulted in a delay in hatching, reduced growth, and decreased larval activity, regardless of the amitrole concentration present during the incubation. Conspecific cues also induced morphological changes, but only in individuals incubated in water contaminated with the highest amitrole concentration. The herbicide impacts on hatching time were restricted to embryos incubated in the presence of conspecific cues, with individuals exposed to 0.1 and 1 mg/L showing an extended embryonic period compared to controls in uncontaminated water. Whether tested alone or in combination with conspecific cues, amitrole also induced slight morphological changes but did not affect larval growth or behavioral activity. Thus, depending on the trait considered, both chemical stressors exhibited either single or interactive effects. Furthermore, our data indicate that a stressing factor without apparent impact when tested alone could exert effects when combined with another stressor. Such results highlight the importance of considering multiple environmental factors and biological traits when examining stress-induced phenotypic variability.