Sinking depths of black-tailed deer in snow, and their indices

Sinking depths in snow of a captive black-tailed deer (Odocoileus hemionus columbianus) were recorded in old-growth, second-growth, and recently clear-cut forests. Data were collected over a wide range of snow conditions. Snow hardness was extremely variable, even within 30 cm, and only weakly correlated with deer sinking depths (r = 0.52 for upper crust hardness). Snow density in the upper 48 cm of the snowpack was a better predictor of deer sinking depths (r2 = 0.65), but the best prediction was from density and hardness combined (r2 = 0.86). Snow depth alone was a poor predictor, because the deer rarely sank to the bottom of the snowpack (9 of 630 cases). Two indices of sinking depth were evaluated: human sinking depth and Hepburn's index. Human sinking depth was both a simpler and better predictor, especially when snow hardness values >256 g cm−2 (deer static foot loading) were eliminated (r2 = 0.65).