Hibernation Plus! Sleep-Like Experiences

| Hibernation | Waking Up | Bears | Torpor | Dormancy | Aestivation |

Hibernation: Winter Sleep

The exact definition of hibernation is controversial. Therefore, it is hard to say exactly which animals hibernate. Some believe that hibernation must involve a significant decrease in body temperature. During this "true hibernation," (sometimes called "deep hibernation") animals will curl up and feel very cold to the touch. (1) This deep hibernation occurs when the animals "experience a profound reduction in Tb [body temperature] and metabolic rate with Tb dropping to a few degrees above the freezing point of water and metabolic rate experiencing a concurrent decline . . . The animal is capable of rewarming using only self-generated heat . . . . Deep hibernation [occurs when an animal] remained at a [body temperature] of about 5 degrees C for a period of days or weeks." (2) Deep hibernators include: hedgehogs, woodchucks, marmots, dormice, thirteen-lined ground squirrels, Turkish hamsters, some bats. Hibernators generally do not respond to their outside environment. Others believe that hibernation must involve not taking in any food or drink. (3)

Here are some hibernators (broadly defined):

• In the fall, European hedgehogs will eat extra food to store up fat for the winter. During hibernation, they will not eat. (4)
• Woodchucks, also called groundhogs, are famous hibernators. "Groundhog Day," (February 2) is an American holiday. It is then that the groundhog traditionally emerges from its long winter's nap. The legend continues that if the groundhog sees its shadow, spring is coming. Otherwise, it goes back down in its hole for six more weeks, meaning that winter is not yet over. In between late September and early October, groundhogs will seal off the entrance to the lower burrow with soil. Their body temperatures will drop from 36.7 to 3.3 degrees Celsius (98 to 38 degrees Fahrenheit). (5) If the weather gets too cold, they will awaken to warm up, then return to sleep. Occasionally, they will wake up to eat a little of the food stored in their burrow. However, they don't eat nearly as much as they do in the spring and summer. During winter hibernation, a woodchuck will lose one third to one half of its body weight! (5)
• Both chipmunks and dormice occasionally awaken during hibernation to eat food stored in their nest. Chipmunks, instead of eating extra in advance to prepare for winter, will store their food in their burrows. They wake up often during the winter, and will even come outside when the weather is warm enough.
• Some ground squirrels' breathing decreases; their temperature decreases from 32.2 degrees C to 4.4 degrees C (from 90 down to 40 degrees F) and their pulse drops from 150 to 5! Ground squirrels don't hibernate until they've stored enough extra fat in their bodies by eating lots of extra food. They can wake up occasionally to eat more and to relieve themselves. (6)
• Some bats hibernate as well. During hibernation, a little brown bat's pulse decreases from the usual 400-700 beats per minute to a mere 7-10 beats per minute. (5) Just as many other hibernating animals, a bat will put on weight before going into hibernation, doubling or even tripling its weight!
• Skunks and raccoons huddle together in family groups in the winter, even though they are most often solitary animals. Skunks will dig dens 1.83 to 3.66 meters (6 - 12 feet) deep. (5) For both animals, body temperature decreases by only a slight amount. When they awaken depends on external temperature.

See our Hibernation Lesson Plan.

Waking Up (4)

How do these hibernating animals wake up once springtime comes around? In order to awaken, these animals must first warm themselves up. They have special tissue in their bodies called brown fat that produces heat according in response to changes in of the surrounding temperature [thermogenic function]. This allows hibernating animals to survive during their winter sleep as well as to awaken when the time comes. It seems that all mammals have brown fat at birth (even baby humans). In most animals, it has vanished by adulthood. It remains, however in full-grown adults of animals such as bats, woodchucks, hedgehogs, mice, and rats. Euthermia is a term used to describe the "warm-blooded or active state when the animal is asleep or awake," to which the animal slowly returns after waking up from hibernation. (2)

Bears

A Bear's' body temperature will drop from 102 degrees Fahrenheit to 95 degrees during hibernation. (3) Those who do not believe that bears truly hibernate usually call a bear's winter sleep a "torpor," drowsy stupor," or "dormancy." (2,4) "Intermediate type of hibernation . . . TB [body temperature] declined markedly, but not usually below 15 degrees C. The word torpor has often been used to refer to this condition. (2) A mother black bear will give birth to her cubs in a state of dormancy. Bears can go three to six months without any food or drink or without relieving themselves during their "hibernation." They can, however, occasionally emerge from their den. (4) In comparison to sleep during the summer, a bear's winter sleeping heart rate is much lower, its breathing rate is 50%, and its oxygen consumption only 40%. (4) Unlike many "true hibernators," bears can awake quickly at any moment during hibernation.

Torpor

During daily, or shallow, torpor, animals are inactive and their temperature is lowered slightly for several hours within a circadian cycle. Some small mammals and birds (including hummingbirds, to conserve energy) experience this shallow torpor. Dwarf lemurs of Madagascar, small, arboreal, and nocturnal, may be the only "occurrence of torpor in primates." (9)

Dormancy

Fish , amphibians, butterflies , and invertebrates all have time periods when they become less aware of their surroundings, but their brain waves do not change. (7) The animals aren't quite asleep, but they don't seem to be fully awake either.

Aestivation

Aestivation, also called estivation, is a state of torpor or dormancy in which an animal spends time during hot, dry periods, to protect itself from dehydration "in much the same way as animals that hibernate are protected from cold." Some even call this hibernation. (8) Breathing, heartbeat, and metabolism slow down.

A frog's "hibernation" is called aestivation. Some frogs make a hole in the mud to sleep. They may lie there in the mud for months at a time! The frog's outside layer of skin helps keep it moist when the ground is dry. Some turtles will bury themselves in the mud as well! Several amphibians, reptiles, insects, and even fish, aestivate. Some use a cocoon, including some frogs and the lungfish (8). "During the hottest and driest season, the mouse lemur curls up in her grass and leaf-lined nest and goes to sleep. This long, inactive period during warm weather" takes advantage of the fact that the little mammal eats to store up fat in the rainy season. (11)

(1) Lyman, Charles P., and others. Hibernation and Torpor in Mammals and Birds. New York, Academic Press, 1982, p. 30. "The deeply hibernating rodent enters that state from the curled position of sleep, and its TB [body temperature] may decline to a low of 2 degrees to 3 degrees C." and p. 55. "With the exception of bats, all [deep] hibernators are curled up in a ball when in hibernation and usually have the same position in natural sleep. The head is tucked under the abdomen, and the back is usually exposed above the insulating material of the nest. If the tail is long and bushy, it is curved over the back, but species with bare tails usually curl the tail beneath them."

(2) Lyman, Charles P., and others. Hibernation and Torpor in Mammals and Birds. New York, Academic Pres, 1982, p. 2.

(3) Yakutchik, Maryalice. "We're Counting Cubs Before Momma Bears Wake Up." Discovery Communications Inc. c.1998 URL: http://www.learningchannel.com/stories/nature/bears/dispatch5.html#5

(4) Burton, Maurice. "Sleep and Hibernation," Vol. 12 Animal World in Color.Childrens Press, Chicago: 1969. p. 15, 19, 34-35.

(5))Brimner, Larry Dane. "Animals That Hibernate." Franklin Watts, New York: 1991. pp. 18, 22, 24, 29, 37, 55.

(6) "What is Hibernation?" URL: http://geocities.com/Heartland/7134/Shadow/ghhibernation.htm c.1998 Cathy's Picnic

(7) Hartman, Ernest. "Sleep," The Worldbook Encyclopedia (Vol. 17). Chicago: World Book, Inc. 1990, pp. 507-508.

(8) "Estivation," The Worldbook Encyclopedia (Vol. 17). Chicago: World Book, Inc. 1990, pp. 309-310.

(9) Lyman, Charles P., and others. Hibernation and Torpor in Mammals and Birds. New York, Academic Press, 1982, p. 16 (hummingbirds) , 23-24 (lemurs).

(10) Pasanen, Seppo. Seasonal variations in interscapular brown fat in three species of small mammals wintering in an active state.Oulu [Finland] 1971. 46 p. Cover title. "Aquilo, ser. zool. 11."(

(11) Whayne, Suzanne Santco. Night creatures. New York, Simon & Schuster, 1993. p. 33.