Estivation: The Survial Siesta

As autumn descends through North American latitudes, many mammals, including raccoons, skunks, bears, chipmunks, and bats, are fattening up so they can sleep away the cold winter months in a protected den instead of trying to stay warm and find food when conditions outside are harsh. In some parts of North America, summer can be just as harsh on wildlife. Long periods of extreme drought and heat prompt animals to estivate - the summer equivalent of hibernation. The animal goes into a dormant state in a sheltered den insulated from the harmful effects of sun and heat. Metabolic rate slows, but not as much as in hibernating animals. An estivating animal can arouse more quickly from its torpor

Estivating [or aestivating, from the Latin aestas, summer] is not as common as hibernating. No mammals were believed to estivate until the discovery in 2004 of estivating Madagascan fat-tailed dwarf lemurs. It is most prevalent in reptiles and amphibians. Animals that estivate include some species of:

Salamanders
Lizards
Turtles and tortoises
Frogs and toads
Snails
Land crabs
Bees
Earthworms

Reptiles, being cold blooded, maintain the same body temperature as the ambient air. Cool air makes them sluggish, but scorching, dry heat can be dangerous. For the North American desert tortoise(Gopherus agassizii), conditions in their hot, dry desert habitat often exceed 100 degrees, and estivation may be their only viable survival strategy. These long-lived (80-100 years) animals frequently find shelter in the shade of a rock, but if temperatures get too high they will use their tough claws to burrow underground to find cooler conditions to wait out the heat.

Conditions don’t have to be desert-like to cause an animal to estivate. Spotted turtles (Clemmys guttata) will estivate to ride out the worst of the summer heat in the Great Lakes and eastern North America. They form a nest, or “form”, under leaf litter, brush, and vegetation in upland sites. How long a turtle estivates varies individually and depends on the availability and distance to suitable food and shelter as well as heat and drought conditions.

On the other hand, the summer siesta forCalifornia Tiger Salamaders (Ambystoma californiense) begins at the start of the dry season and lasts nine months.

By estivating, the California red-legged frog (Rana aurora draytonii) conserves energy when food supplies are low, and drought and heat become unbearable. They store energy in small fat sacs which they slowly expend while tucked in their cool damp underground refuge waiting for cooler, moister weather. It isn’t much, as their energy needs drop as they slow their metabolism down. Some frog species will shed a layer of skin in their holes. This protective layer wrapped around their body inhibits moisture loss.

Animals have developed numerous strategies for dealing with the extremes of seasons. Some migrate to a more favorable climate. Others sleep away the season. If it’s too cold, they hibernate. Estivating is a good strategy if they can’t take the heat.

Many of these animals need conservation protection, and cannot complete their life cycle in a landscapes that does not provide suitable habitat for estivation.