When conditions turn extreme and food and water get scarce, many creatures shut down their metabolism and go into suspended animation.

This is much different than sleep. Body temperature drops to near the ambient temperature. Bodily functions slow, with some stopping altogether.

This dormancy comes in several varieties:

Some species go into daily torpor in the heat of the day. Hummingbirds, for instance, will find a shady place to shut down, and drop their heart rate to just 5% of normal.

Depending on the severity of winter, bears, skunks and other light hibernators will reduce their heart rate to around 50% and become dormant for a few days to a few months.

So they can wake more easily to respond to danger, they remain somewhat responsive to outside stimuli.

True hibernators are usually small species weighing just a few ounces, like the dormouse.

In years when food is scarce, they’ll put on as much fat as possible, skip reproduction and hibernate till next year brings a better crop of seeds.

Many water and desert species enter what’s called estivation, brought on by drought or heat. Some fish burrow into the mud till the rains return. Snails may seal themselves in their shells for up to a year.

Dormancy of all kinds helps animals conserve energy and hide from predators—and many species of mammal, bird, reptile and insect have been doing it for more than 250 million years.