How do bears hibernate?

By John DiTraglia MD


Somewhere near Portsmouth there is probably a bear hibernating. How do bears do that? The first thing to clarify is that hibernating is not sleeping. The brain activity of hibernating animals is like awake brain activity. In fact when they “wake up” from hibernating many mammals show signs of sleep deprivation and may have to sleep more for a few days. These are some things that are known about hibernation in bears.(1)

– For as many as 7 months of hibernation bears go without eating, drinking, defecating or urinating, in contrast to chipmunks and other small mammals that get up to do that stuff every week or so.

– Bears hibernate at or near normal body temperature, in contrast to the those other small mammals who hibernate with a body temperature near ambient, approaching but not falling below 0°C.

– Weight loss in hibernating bears ranges from 16% to 37% of body weight at the start of hibernation. So they will need to put that weight back on before the next hibernation.

– Bears conserve lean body mass during hibernation using almost only fat combustion for energy needs.

– So fat content at the start of hibernation is 36% to 38% of total weight in black bears and 49% in polar bears and declines proportionate to the decline in body weight.

– Blood lipids (cholesterol, triglycerides, and free fatty acids) increase during hibernation, a result of increased combustion and decreased production of fat.

– Ketosis which is a sign of fat burning in starving humans does not occur in hibernating bears.

– Total body water, blood volume, and water content of plasma and red blood cells do not change during hibernation in black bears.

– During hibernation, black bears maintain bone mass and measures of bone formation. In other species undergoing long periods of skeletal inactivity, including humans, substantial bone loss occurs.

– Thyroid function is not reduced during hibernation and thus does not explain the reduced metabolic rate characteristic of hibernation.

– Female bears exhibit the even more amazing phenomena of hibernation. Implantation, fetal growth, and lactation during hibernation can be delayed. Following conception in the summer breeding season, the fertilized egg arrests development at the blastocyst stage (about 300 cells). The unimplanted blastocyst remains dormant in the uterus until implantation in late November or early December. Actual birthing usually occurs in January or February. Lactation can occur in hibernating bears and places great demands on females. The first 2 to 3 months of maternal care often occur during hibernation. Nutrients for neonatal growth must be supplied under conditions of no food and no water. The fat content of polar and black bear milk is the highest known for terrestrial mammals, increasing from about 9% (on a wet-weight basis) immediately after birth to 30% in late lactation in black bears. Fat content of milk from polar bears in the fourth month of lactation can be as high as 36%. Milk protein content ranges from 5.9% to 12.6% in black bears and from 9.1% to 13.2% in polar bears; sugars are < 5% of milk composition. The production of high-fat, low-carbohydrate milk is adaptive to the metabolic economy of hibernation – it conserves maternal protein. A black bear cub requires about 3 gallons of milk (composed of 2 gallons of water, 2/3 gallon of fat, 1/5 gallon of protein, and a tiny bit of sugar) during the first 12 weeks of life to gain an appropriate amount of weight. A female bear nursing a litter of three cubs must therefore produce almost 9 gallons of milk over a 12-week period in the den. These bear mothers can recover most of the water and about half of the nitrogen in protein by recycling (eg, ingesting the excreta of the cubs to put it delicately). So this contradicts to some extent the routine of no eating or drinking during hibernation. Polar bears lose 43% of body mass while lactating, with 93% of the expended energy drawn from fat stores.

One thing that is not well understood is how this metabolic adaptation occurs. The signals that trigger these feats of metabolic magic do not seem to occur across species. Taking blood serum from hibernating bears and injecting it into squirrels does not induce hibernation metabolism. Still a better understanding of how these adaptations occur might help in the treatment of obesity and metabolic syndrome and osteoporosis among other things in humans.

1. Hellgren EC. Physiology of hibernation in bears. Ursus. 1998;10:467-477.


By John DiTraglia MD

John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- or phone-354-6605.

John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- or phone-354-6605.


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