Amphibian breeding phenology influences offspring size and response to a common wetland contaminant.

BACKGROUND

Increases in temperature variability associated with climate change have critical implications for the phenology of wildlife across the globe. For example, warmer winter temperatures can induce forward shifts in breeding phenology across taxa ("false springs"), which can put organisms at risk of freezing conditions during reproduction or vulnerable early life stages. As human activities continue to encroach on natural ecosystems, it is also important to consider how breeding phenology interacts with other anthropogenic stressors (e.g., pollutants). Using 14 populations of a widespread amphibian (wood frog; Rana sylvatica), we compared 1) growth; 2) tolerance to a common wetland contaminant (NaCl); and 3) the ability of tadpoles to acclimate to lethal NaCl exposure following sublethal exposure earlier in life. We evaluated these metrics across two breeding seasons (2018 and 2019) and across populations of tadpoles whose parents differed in breeding phenology (earlier- versus later-breeding cohorts). In both years, the earlier-breeding cohorts completed breeding activity prior to a winter storm and later-breeding cohorts completed breeding activities after a winter storm. The freezing conditions that later-breeding cohorts were exposed to in 2018 were more severe in both magnitude and duration than those in 2019.

RESULTS

In 2018, offspring of the later-breeding cohort were larger but less tolerant of NaCl compared to offspring of the earlier-breeding cohort. The offspring of the earlier-breeding cohort additionally were able to acclimate to a lethal concentration of NaCl following sublethal exposure earlier in life, while the later-breeding cohort became less tolerant of NaCl following acclimation. Interestingly, in 2019, the warmer of the two breeding seasons, we did not detect the negative effects of later breeding phenology on responses to NaCl.

CONCLUSIONS

These results suggest that phenological shifts that expose breeding amphibians to freezing conditions can have cascading consequences on offspring mass and ability to tolerate future stressors but likely depends on the severity of the freeze event.