Age-Mass Relationships for Beavers in Montana

Abstract

Beavers (Castor spp.) are receiving increased attention due to their impact on ecosystems and potential for use in stream restoration. Beaver research and relocation projects are especially common in the western United States, and professionals using live-captured animals for projects will benefit from a reliable technique for aging live-caught beavers. The only reliable technique for aging live beavers without sedation is to estimate age based on mass, but estimates of the age-mass relationship for beavers vary regionally and are not adequately quantified in the western United States. We collected beaver carcasses and skulls from trappers throughout southwest Montana to estimate the age-mass relationship using a robust sample collected from a large geographic area. We weighed beaver carcasses and extracted molar teeth from the mandibles to estimate age by counting cementum annuli on cross-sections of the teeth. We collected 193 beaver carcasses and hanging weights from nine major river drainages in Montana. Multiple regression analysis indicated the top prediction equation was mass = 9.4611 + 8.2234 x log(age) + drainage, indicating drainage-level differences in the average mass of beavers. Beavers from the Ruby, Jefferson, and Yellowstone River drainages were larger than those from the other river drainages in Montana. We could reliably separate beavers into four age classes: kits, yearlings, two-year-olds, and adults (>3 years). Our results are useful for researchers who need to estimate beaver age to understand population dynamics and age-specific life history characteristics, as well as restoration practitioners who need to determine colony compositions and recruitment rates to evaluate the success of restoration projects.