Hibernation is a specialized survival strategy defined by a state of metabolic depression, allowing certain animals to conserve energy during times of environmental stress. This physiological process is characterized by a dramatic reduction in body temperature, a slow heart rate, and shallow breathing, sometimes lasting for months. Historically triggered by cold and food scarcity, this delicate biological timing is now increasingly disrupted by global climate change. Rising ambient temperatures and increased weather volatility are altering the environmental cues that hibernating species rely on, challenging the survival mechanism that allowed these mammals to endure harsh winters.
Altered Timing of Hibernation
Climate change is causing a misalignment in the seasonal cues that regulate the timing of biological events. Warmer autumns can delay the onset of hibernation, as animals postpone their entry into a den when external temperatures remain mild. This shortens the pre-hibernation foraging period, preventing them from accumulating necessary fat reserves for the long dormancy.
Conversely, warmer late winters and early springs often trigger a premature awakening, particularly in deep hibernators like ground squirrels and marmots. The end of hibernation has advanced by several days per decade in response to rising spring temperatures. This early emergence can be disastrous if it precedes the availability of food sources, such as vegetation or insects, leaving the animal in an energetic deficit. While the length of daylight remains a fixed cue, its influence is being overridden by temperature signals, leading to a dangerous mismatch with the active growing season.
Increased Energy Demands During Torpor
Successful hibernation requires maintaining a deeply suppressed metabolic rate (torpor) to stretch fat reserves across the winter. For many small hibernators, the most energetically efficient ambient temperature for their den or burrow is near 0°C. When ambient temperatures rise above this optimal point, the hibernator is forced to increase its metabolic rate to actively cool its body, thereby burning stored fat much faster than intended.
Fluctuating temperatures also lead to more frequent and costly periodic arousals (IBAs), during which the animal rapidly warms its body. This rewarming process is driven by the rapid, non-shivering thermogenesis of brown adipose tissue (BAT). BAT is specialized for heat generation and can raise the body temperature by over 20°C in under an hour, a process that is extremely metabolically expensive.
Each arousal consumes a significant portion of the animal’s total winter energy budget, and higher den temperatures increase the frequency of these arousals. If the fat reserves are depleted too quickly due to warmer conditions and excessive arousals, the animal will starve before spring arrives, a failure known as “winter-kill.”
Disruption of Critical Food Resources
Hibernator survival depends on the availability of external resources before and after the dormant period. The pre-hibernation phase requires hyperphagia, where animals consume massive amounts of food to build the white fat reserves that fuel hibernation. Changes in rainfall and temperature can shift the timing of plant growth and fruit production, leading to a phenological mismatch with the animal’s active season. If droughts cause vegetation to dry out or senesce earlier than usual, animals cannot acquire the necessary fat stores to survive the winter.
The stability of the hibernaculum is also compromised by climate change, particularly through changes in snow cover. A deep, stable layer of snow acts as an insulating blanket, preventing burrow temperatures from dropping too low or fluctuating wildly. Reduced or inconsistent snowpack forces the hibernator to expend more energy on thermoregulation, increasing the rate at which they burn their reserves. Conversely, increased late-season snowfall in some high-altitude areas can delay spring emergence, shortening the summer foraging window and hindering the animal’s ability to prepare for the subsequent winter.
Long-Term Effects on Survival and Range
The combined effects of altered timing, increased energy expenditure, and food scarcity translate into severe population-level consequences for hibernating species. Increased mortality rates result from animals exhausting their fat stores prematurely and waking up into a resource-poor landscape. This heightened vulnerability is particularly pronounced in juveniles, which have smaller body sizes and lower fat reserves than adults.
Animals that survive the winter but emerge in a weakened state often suffer from reduced reproductive success. Females may be too weak to successfully conceive, carry, or nurse their young, leading to lower birth rates and decreased recruitment into the population. Over generations, the failure to adapt forces some species to shift their geographical ranges, seeking cooler climates, often moving to higher latitudes or elevations. These forced range shifts can fragment populations and reduce genetic diversity, further compromising the long-term viability of hibernating species.