Hibernation in Mammals

1. What they did

George Johnson, who had studied thirteen-lined ground squirrels in the laboratory for years, brought together what was known about hibernation in mammals in 1931. He reviewed more than a century of work, much of it European, and added his own observations and experiments on ground squirrels, plus short studies of prairie dogs, opossums and pocket gophers.

He defined hibernation narrowly: a state in which metabolism falls so low that body temperature is only a little above that of the surroundings. In this sense it is not ordinary sleep, and it is not tied to winter. He then covered where and how long mammals hibernate, their posture, body temperature, breathing, heart rate, weight loss, how they wake and how they go into torpor, and the many possible causes of hibernation.

2. What they found

Which mammals hibernate, and where

• Hibernators come from many unrelated groups: monotremes, hedgehogs, bats and several rodents (woodchucks, dormice, ground squirrels, prairie dogs, some chipmunks and jumping and pocket mice). What most share is that they live in the ground and cannot reach their usual food while the ground is frozen.
• Writers disagreed about bears, raccoons, skunks and badgers. Because bears and raccoons are easily roused, the author thought they probably do not become truly torpid.
• Thirteen-lined ground squirrels hibernate in nests of dry vegetation in underground chambers. In 24 nests studied, the nests were 4 to 10 inches across and about 3 to 27 inches below the surface. The burrow entrance was always plugged with soil before hibernation. Prairie dog nests lie 10 or more feet down, and their entrances are not closed.
• One ground squirrel had stored about 24,000 oat kernels in one chamber and about 4,000 in another, but food storing was not found in every animal.

Summer hibernation

• In a refrigerator, ground squirrels hibernated every summer, and hibernation was more common in summer than in spring. No hibernation was seen at typical summer room temperatures of about 24°C or above.
• In the wild, Columbian ground squirrels had almost all gone underground by early August. In outdoor pens, 9 of 22 thirteen-lined ground squirrels plugged their burrows between May and September.

Length of hibernation and waking bouts

• Hibernators wake at intervals. In one experiment of over four months, eight ground squirrels stayed torpid for 1 to 18 days at a time (about 6 days on average), then stayed awake about a day. Oat grains placed on the back of a torpid animal showed whether it had moved.
• When awake, the animals almost always passed urine and faeces, sometimes 20 or 25 faecal masses. The author supported the idea that a full bladder and rectum help trigger these waking bouts.
• Old ground squirrels and females go underground first, and females emerge later in spring. Columbian ground squirrels spent on average 220 days (females) and 204 days (males) underground.

Posture

Ground squirrels hibernate rolled up nearly into a ball, nose tucked against the belly, which likely saves body heat.

Body temperature, breathing and heart rate

• Even when awake, ground squirrels held their body temperature poorly (about 32°C to 41°C). The author saw this as an old trait shared with the egg-laying mammals.
• Torpid ground squirrels were usually about 1°C to 3°C warmer than their surroundings. Near freezing, some died; one at 0.2°C was almost dead.
• Deeply torpid ground squirrels breathed about one-half to four times a minute. The heart beat as slowly as 5 times a minute in deep torpor and as fast as 506 times a minute in a struggling animal.
• Death in hibernation seemed to come from breathing stopping rather than the heart stopping.

Responsiveness

As body temperature falls, ground squirrels respond less and less. Below about 5°C they usually show no response even to strong stimuli, yet strong disturbance wakes them in time.

Weight loss

Ground squirrels lost about 0.3 to 0.6% of their weight a day in hibernation, mostly fat. The author estimated that they commonly lose 30 to 45% of their weight in five months of hibernation in nature.

Waking and going into hibernation

• Disturbed animals woke faster (63 minutes on average) and with trembling and shaking; undisturbed animals woke more gradually (114 minutes on average), which the author thought is probably typical in nature.
• In a very warm room, pouch temperature rose about 20°C in 35 minutes. Body temperature fell more slowly when going into hibernation, often 1°C to 5°C an hour.

Possible causes

• Cool rooms (about 5°C to 10°C) gave far more hibernation than warm ones. Nightly precooling, starvation, dry food and confined air in closed cans all made ground squirrels hibernate sooner. In 19 experiments, 13% of fed animals hibernated compared with 42.4% of starved animals.
• Fat animals hibernated more than lean ones. Very thin animals sometimes went torpid sooner but often died after two or three days.
• Light had no effect. Thyroid and posterior pituitary treatments did not stop hibernation, but implanted pieces of pituitary did hold it back. Castrated animals hibernated more than normal animals in the breeding season.

3. What they concluded

• Hibernation should mean true torpor, whether it happens in winter or summer. The term “aestivation” seemed unsuitable for summer torpor in mammals, since all known cases were produced by cool conditions; the author suggested calling these partial hibernation.
• A poorly developed heat-regulating system is a requirement for hibernation. Because this is an ancient trait, hibernation is something retained, not newly gained, by hibernating mammals.
• Hibernation conserves life through the cold season when food cannot be found, and weight loss is much smaller than in starvation.
• There is no single cause. Many outside and inside conditions influence it: cool temperatures, previous cooling, lack of food, dry food, confined air and fatness favour it, while breeding-season activity of the sex glands and pituitary holds it back.
• Earlier body temperature records were often unreliable because workers did not take proper precautions; the author listed the conditions needed for accurate readings.