Combined impacts of warming and methomyl on neurophysiological and behavioral responses in Amazonian frog tadpoles

Amphibian populations are declining globally due to anthropogenic stressors such as climate change and environmental contamination. In Amazonia, where aquatic species already endure conditions near their thermal thresholds, the combined effects of rising temperatures and pesticide exposure pose a threat to their survival and physiological stability. Here, we evaluated the isolated and interactive impacts of elevated temperature and sublethal concentrations of the carbamate pesticide methomyl on behavior, neurophysiology, and endocrine responses in tadpoles of two Amazonian frogs, Osteocephalus taurinus and Scinax ruber. Tadpoles were exposed to the stressors for 96 h, and the subsequent assays measured substrate preference, locomotor and antipredator behavior, acetylcholinesterase (AChE) activity, and whole-body cortisol concentrations. O. taurinus showed significant behavioral impairments under both stressors, including reduced swimming activity and increased immobility on dark substrates, suggesting antipredator strategies. AChE activity decreased under elevated temperature but showed a pronounced increase under combined exposure to methomyl and heat, while cortisol concentration did not change significantly. In contrast, S. ruber maintained stable behavior and enzyme activity, yet exhibited robust endocrine responses, with significant cortisol elevation under both stressors. No significant responses to alarm substances were observed in either species. Our results highlight distinct species-specific responses: O. taurinus displayed greater behavioral sensitivity, whereas S. ruber showed pronounced endocrine reactivity. These divergent strategies likely reflect ecological and physiological adaptations, emphasizing the importance of multi-stressor and multi-species approaches in risk assessment. This study underscores the vulnerability of tropical amphibians to co-occurring climate and pollution stressors and calls for integrative conservation strategies that consider species-specific traits and ecological histories.