Do Snakes Hibernate? How Snakes Survive Winter

Hibernation or brumation?

Snakes are ectotherms, often called cold-blooded: they cannot make enough body heat to keep warm, so their body temperature follows their surroundings. When temperatures fall in autumn, a snake’s muscles, digestion and nervous system slow down, and below about 10 °C (50 °F) most snakes can barely move.

Hibernating mammals such as ground squirrels actively lower their body temperature and metabolism, and they periodically rewarm themselves. Snakes cannot do this, so in 1965 the zoologist Wilbur Mayhew suggested the word brumation for the winter dormancy of reptiles. In everyday language, though, “snakes hibernate” is widely used and not really wrong: both words describe a long, energy-saving winter rest.

Where do snakes go in winter?

To survive, a snake needs a winter shelter, or hibernaculum (plural: hibernacula), that stays above freezing but cold enough to keep its metabolism low. The key is getting below the frost line, the depth to which the ground freezes. Common winter sites include:

• Rock crevices and talus slopes, especially on sunny, south-facing hillsides
• Limestone sinkholes and caves
• Old rodent burrows, crayfish holes and the root channels of rotted stumps
• Ant mounds, banks and earth embankments
• Human structures such as old wells, stone walls, foundations and cellars

Inside a good den, temperatures often stay just a few degrees above freezing all winter. Because such places are scarce in cold regions, many snakes travel back to the same den year after year, sometimes over distances of a kilometer or more.

Communal dens: from a handful to tens of thousands

Some snakes overwinter alone, but in northern areas many share dens. Being packed together slightly reduces water loss and heat loss, and it makes finding a mate easy in spring.

The record belongs to the red-sided garter snake. At the Narcisse dens in Manitoba, Canada, tens of thousands of these snakes spend about eight months a year in limestone sinkholes. When they emerge in spring, they form the famous mating balls of garter snakes. The common garter snake can also tolerate short periods of very cold conditions in its dens better than most snakes.

In the eastern United States, the timber rattlesnake dens on rocky, wooded slopes, often sharing crevices with copperheads and rat snakes. Timber rattlesnakes are very loyal to their dens; young snakes appear to find them by following the scent trails of adults, and the same den may be used for many generations.

How long do snakes stay underground?

The length of winter dormancy depends mainly on climate. Farther north and higher up, the active season is shorter.

The common European adder is a good example of how flexible snakes can be. It is the most northerly snake in the world and lives beyond the Arctic Circle in Scandinavia, where the summer is so short that it may spend most of the year underground.

What happens to a snake’s body in winter

During brumation a snake’s heart rate and breathing slow to a fraction of their summer rate, and it uses very little energy. Several adaptations help it survive:

• An empty stomach. Snakes stop feeding well before winter. Food left in the gut would rot instead of being digested in the cold.
• Fat reserves. Energy stored during summer, mostly as fat bodies in the abdomen, must last until spring.
• Water balance. Dens are usually humid, which limits drying out. Some snakes come to the den entrance on mild days to drink or bask.
• Cold tolerance. Some northern species can briefly survive body temperatures near or slightly below freezing, but long, hard freezes are usually fatal.

Winter is still dangerous. In Britain, studies of adders suggest that roughly 15% of adults and 30–40% of young snakes die during hibernation. A sudden early freeze can also catch snakes before they reach their dens, as happened at Narcisse in 1999.

Spring emergence

As soil temperatures rise, snakes move toward the den entrance and begin basking on warm days. In many species, including garter snakes and adders, males come out first. Male adders often bask for weeks near the den, shed their skin and then begin searching for females by scent, sometimes wrestling rivals in the so-called “dance of the adders.”

Females usually emerge a little later, mate and then move to summer feeding areas. In autumn the journey happens in reverse, with many snakes returning to the exact same den.

Snakes that do not hibernate

Not all snakes brumate. In the tropics, temperatures stay warm enough for snakes to remain active all year. In hot, dry places, some snakes do the opposite and enter a summer dormancy called aestivation to escape heat and drought. In mild regions such as Florida or southern Europe, snakes may be active on any warm day in winter and only rest during cold spells.

Respecting snake dens

Winter dens are vital to snake populations because they are hard to replace. Disturbing a den, collecting snakes there or blocking entrances can wipe out a local population, and many dens of rattlesnakes and adders are protected by law. The best way to enjoy spring emergence is from a respectful distance, on official trails, without handling the snakes.