Digging for cold: how temperature and depth determine hibernacula selection for a small mammalian hibernator, the New...

Adaptation of mammals to seasonally changing environments is crucial for survival and fitness. Some mammals migrate or alter their behavior when resources are limited, while others hibernate-a strategy that offers profound seasonal metabolic savings. During hibernation, both body temperature and metabolic rate are reduced, minimizing energy expenditures. However, when hibernacula temperatures decrease to below their body temperature setpoint, mammals must increase thermoregulatory output to maintain body temperature. Thus, the thermal characteristics of hibernacula play a crucial role in determining overwinter metabolic expenditure. We identified hibernacula locations of the endangered New Mexico Jumping Mouse (Zapus luteus), described physical characteristics of the hibernacula, and compared soil temperatures between hibernacula and randomly selected sites. Additionally, we explored the relationship between ambient air and soil temperatures at hibernacula to understand their influence on thermal buffering and the stability of microclimatic conditions. In Arizona and Colorado from 2019 to 2021, we tracked New Mexico jumping mice into hibernation to confirm hibernacula locations. We used radiotelemetry and Passive Integrated Transponder tags with a priori criteria to identify underground locations. We confirmed and characterized hibernacula for 4 females and 7 males (n = 11). Soil temperatures at 3 depths (-10, -30, and -50 cm) were measured near each hibernaculum and compared to random sites. Hibernacula depth averaged 29 ± 2 cm (range 21 to 45 cm). Soil temperatures at hibernacula were colder and warmed later in spring compared to random sites. Female hibernacula were colder ( $\overline{x}$ = 2.1 ± 0.03 °C) than those of males ( $\overline{x}$ = 2.5 ± 0.02 °C). Hibernacula were mostly north-facing, possessed vegetation cover, and located on average 59.6 ± 19.6 m from perennial water. The consistent selection of hibernacula with physical traits that maintain colder, more stable temperatures emphasizes the importance of conserving microclimatic conditions critical for the recovery of this species, especially amid environmental change.