Effects of body mass and legacy of pesticide contamination on oxidative stress biomarkers in larval Rana sylvatica under baseline and NaCl-contaminated conditions.

Abstract

Monitoring biomarkers of physiological stress (e.g., oxidative stress) in sensitive wildlife populations can allow conservationists to identify, quantify, and make predictions about the impacts of global change. However, interpretation of stress responses can be complicated by multiple interacting factors (e.g., individual development, evolved physiological tolerance to stressors) which alter biomarker expression. To better understand the relative influences of these factors, we used wood frog (Rana sylvatica) populations with known variation in ontogenetic and contaminant tolerance traits. We examined how both individual ontogenetic traits and population-level tolerance traits influence oxidative stress responses under baseline and NaCl-contaminated environmental conditions. We exposed tadpoles from six noninteracting populations with known variation in ontogeny, pesticide tolerance, and NaCl tolerance to either baseline or NaCl-contaminated conditions and evaluated five biomarkers of oxidative stress. We found that individual body mass was a significant predictor of two oxidative stress biomarkers (catalase and glutathione reductase) in baseline conditions only, such that greater mass predicted lower enzyme activity. Separately, population pesticide tolerance was a significant predictor of one oxidative stress biomarker (glutathione peroxidase) in NaCl-contaminated conditions only, such that higher pesticide tolerance predicted higher enzyme activity. Our results demonstrate that both individual traits (mass) and population history (selection for pesticide tolerance) can explain some variation in oxidative stress biomarkers. However, these associations are largely dependent upon the environmental conditions experienced. Our findings demonstrate that individual development and population history influence stress responses. This underscores the need for future applications of oxidative stress biomarkers to consider both historical and contemporary environmental contexts to improve their use as indicators of change.