Diel and seasonal cycles of snow hardness predict changes in activity patterns for roe deer but not mountain hares

The resistance of the snow surface to penetration, or snow hardness, is an important factor affecting wildlife movement. Snow hardness can change dramatically over the course of the day, but it is unknown how animals respond to temporal changes in snow hardness. Using 260 hourly sinking depth measurements as indices of surface hardness along a latitudinal gradient in Norway, we quantified diel cycles of snow hardness in relation to environmental variables. Time relative to sunrise and sunset, temperature, canopy cover, and snow density were the best predictors of surface hardness (R2 = 0.57). We matched diel cycles of the predicted surface hardness index to diel activity in roe deer (Capreolus capreolus Linneaus, 1758) and mountain hares (Lepus timidus Linneaus, 1758), using data from the large-scale Scandcam camera-trap network in Norway from 2017 – 2023 (n = 698 cameras). Roe deer increased morning activity when the snow was harder and deeper, and hares did not show diel shifts in relation to the snow hardness index and preferred softer snow at higher snow depths. As snow conditions and wintertime temperatures continue to change, this study highlights the importance of diel activity adaptations to environmental conditions.