Where Does a Snake Spend the Winter? Somewhere It Found Years Ago

A winter refuge must solve a narrow problem, which is staying above freezing without being warm enough to keep the animal awake and burning reserves. It therefore needs to be below the frost line or otherwise insulated, humid enough that the animal does not dry out over months of immobility, stable in temperature rather than fluctuating with the weather, and safe from flooding and from predators that could take a torpid occupant. Very few locations in a landscape satisfy all of those at once, which is the key to everything else about the subject.

Suitable places are opportunistic and varied:

• •Deep rock crevices and old mine workings
• •Caves, for bats above all
• •Mammal burrows appropriated by reptiles and amphibians
• •Root systems of large old trees
• •Compost heaps, log piles and old walls
• •The mud at the bottom of a pond that does not freeze through

Because suitable sites are scarce, the same locations are occupied year after year and frequently by very large numbers of animals at once. Garter snakes in Manitoba gather in tens of thousands in limestone sinkholes, which is among the largest concentrations of reptiles anywhere. Bats return to the same caves across generations. Animals navigate back to a site over considerable distances, and juveniles in some species appear to find one by following adults. The site therefore has a value out of all proportion to its size, since it supports the entire population of a wide area.

The state the animal enters varies more than the single word suggests and the differences matter practically. A true deep sleeper drops its body temperature to near that of its surroundings, slows its heart to a few beats a minute and cannot be roused quickly, which is what ground squirrels and some bats do. Many mammals instead enter a lighter state, waking readily and occasionally feeding, which is what bears and badgers do. Reptiles and amphibians simply cool to the surrounding temperature and become immobile, with some frogs tolerating partial freezing of their own tissues.

Concentration is exactly what makes these places a conservation problem. Destroying one site, by filling a crevice, sealing a mine, clearing a log pile or draining a pond, can remove the overwintering capacity of an entire local population in a single afternoon, and the animals have nowhere else to go. Disturbing hibernating animals is almost as damaging, since each waking burns reserves that were calculated to last until spring and a few disturbances can be fatal. That is why access to bat caves is restricted in winter and why conservation work frequently consists of building artificial refuges rather than protecting habitat generally.

A winter refuge must stay above freezing without being warm, stay humid, stay stable and stay safe, and very few places in a landscape manage all four. Scarcity means the same sites are reused for generations and can hold enormous numbers, so destroying one can end a local population. Disturbance is nearly as bad, since each waking burns reserves meant to last until spring.