CLUMPED ISOTOPE TEMPERATURES FROM BEAVER TEETH: DO SEMI-AQUATIC MAMMALS RECORD THE TEMPERATURES OF THEIR FRESHWATER ENVIRONMENTS?

Beaver tooth enamel and dentine bioapatite provide a unique geohistorical archive of environmental conditions in freshwater ecosystems. Previous studies show that enamel δ13C and δ18O values can record dietary and hydrologic variation over time because beaver incisors grow continuously. Notably, beaver teeth could provide an archive that is both geographically widespread and extends back centuries in some watersheds, and these records may serve as a novel tool for studying changes in watershed hydrology, regional climate, and land use. Understanding the magnitude and timing of changes in water temperatures is of particular interest since these are important determinants of freshwater ecosystem health. For example, elevated water temperatures in spawning streams are a major barrier to salmon recovery along the Pacific coast.

Here we investigated the utility of clumped isotope thermometry (T(Δ47)) and δ18O of beaver tooth enamel and dentine combined as a proxy for the temperature and δ18O of the water in which the beavers lived. Beavers are semi-aquatic mammals with consistent body temperatures, so tooth-derived T(Δ47) values that deviate from body temperature should reflect environmental temperatures. We analyzed beavers from two locations with disparate hydroclimate conditions: a small creek in northern Washington, and a shallow lake in central Missouri. We analyzed lower incisors from beavers that died in late summer to early fall, and estimated that each tooth reflected the previous 2-4 months of growth. Beavers are highly active in summer with engineering projects and foraging, and they are most aquatic during this season.

We found low variability in enamel δ13C and δ18O values across the length of all incisors, and the average δ18O values of each tooth reflects the predicted δ18O values for the water bodies the beavers inhabited. Given the low isotopic variability and rapid growth rate of the teeth, we determined that the large sample needed for clumped analyses does not average across significant environmental variability, and therefore T(Δ47) reflects only the season before death. We will also discuss T(Δ47) results; if beaver incisors record environmental temperatures, we will use location-specific conditions to interpret how strongly summer water temperature influences clumped values.