A mixed regression model to estimate neonatal black bear cub age

Determining ages of neonatal black bear (Ursus americanus) cubs is complicated by difficulty in obtaining accurate birth dates while parturient females are secluded in dens. We used data gathered from 43 cubs from 18 wild female bears held at theVirginia Tech Center for Ursid Research to model neonatal black bear cub growth in relation to age. We determined which morphological measurements were the best indicators of age and used mixed regression to develop a model to estimate cub ages from 1-88 days after birth. Hair length, skull width, total length, and ear length were most strongly correlated (r(2)>0.90, P>0.001) with age. Our mixed regression model employing hair and ear length estimated cub ages to +/-6 days of actual age for 93.6% of 219 observations of <70day-old cubs. Our model provides an accurate means by which to estimate neonatal black bear cub ages and a foundation for future efforts to estimate and model neonatal survival rates and associated covariates.