Estivation is like hibernation in hot weather. Animals that live in deserts or tropical climates practice estivation. It may not occur solely because of food supply issues, as with hibernation, but because the conditions become too hot and dry for the animal to survive. The process typically involves burrowing into the ground, where the temperature stays cool, and reducing metabolic activity in a similar manner to hibernation.
Lungfish are capable of an amazing form of estivation that allows them to live without water for as long as three years. Lungfish are primitive fish that still have lungs, allowing them to breathe air. When a lungfish's lake dries up, the fish burrows into the mud, then secretes mucus until its entire body is covered. The mucus dries into a sack that holds moisture in. Even when the mud dries completely, the lungfish stays moist and breaths through a mucus tube.
Many species of birds use daily torpor to get through colder months. The black-capped chickadee is a good example. Daily torpor (which really ought to be called nightly torpor) is like low-grade short-term hibernation. It only lasts for a few hours, and the reduction in body temperature's just a few degrees. However, studies have shown that those few degrees save a significant number of calories from being burned off overnight.
Hummingbirds have an incredibly high metabolic rate, with a heart rate that can exceed 1,200 beats per minute. Their energy consumption is so great that hummingbirds use daily torpor to conserve energy even in the tropics. Hummingbird torpor is more profound than that of other birds, with a temperature exceeding 50 percent.
For more information on hibernation and animal topics like migration and symbiosis, try the next page.