In late October I had my first ever Louisiana Black Bear sighting in Newellton, Louisiana. It was mid-afternoon and this Tensas Parish bear was seriously chowing down on pecans, oblivious to anything else. In the middle of the pecan orchard, gorging on pecans (shells and all), this bear instinctively knew it was time to fatten up. Winter was right around the corner.
Hibernation is a complex subject, and other terms are used interchangeably: torpor, brumation, and estivation. Each of these terms describes a specific adaptation to winter’s harsh conditions, and they are often confused with one another.
Warm-blooded animals hibernate when the days grow shorter and the weather gets colder. Hibernation is defined as a state of inactivity which allows the animal to conserve energy with a lowered body temperature and slower heartbeats. During this stage the animal does not require additional food or water, thus making hibernation a great adaptation to winter’s cold temperatures, when available food sources are scarce.
Animals preparing for hibernation usually eat large amounts of food to build up the fat reserves that will allow them to live through winter’s scarcity. Other animals prepare by caching reserves of food that will be at the ready when they wake up from their long winter nap. Animals will also prepare a place to hibernate.
True hibernation occurs only in small mammals, such as bats, rodents, woodchucks, and a few birds, like the common poorwill. This bird species, found in the west, is the only bird that truly hibernates. During the winter, the poorwill drops its body temperature up to sixty degrees Fahrenheit and can live upwards of one hundred days on only ten grams of stored body fat.
Louisiana Black Bears, which we think of as the classic hibernating animal, are actually just deep sleepers, falling into what is called “carnivoran lethargy.” While inactive, these bears do not significantly lower their metabolism and body temperature enough to technically be hibernating. Female bears even give birth to cubs during this time. (In mild climates such as Louisiana’s, it’s worth noting, bears can remain active throughout the winter.)
During deep sleep, a bear’s body breaks down fat, which then turns into urea. Instead of excreting the uric waste, which can be toxic at high levels, the bears recycle it. Nitrogen from the urea is used by the bear to build protein, which allows it to maintain muscle mass and organ tissues during the months in its winter den. Interestingly enough, bears don’t get dehydrated either. While bears don’t drink liquids during deep sleep, they are able to absorb enough water from their own body fat to stay hydrated.
Opossums, another deeply sleeping species, also deliver their newborns while the female sleeps; the babies make their way into the mother’s pouch where they nurse for the remainder of the hibernation period.
Torpor is used to describe short-term periods of reduced temperature. Hummingbirds enter a state of torpor at night to conserve energy, which is expended at alarmingly fast rates during their waking hours. Hummingbirds lack the insulating downy feathers that some birds have and eat two to three times their body weight daily. With such high metabolic demands, torpor helps conserve energy when the birds are not feeding.
Brumation is the cold-blooded animals’ version of hibernation—think turtles and snakes. Rattlesnakes, for instance, gather in large numbers underground in dens to escape the cold and live off glycogen reserves.
Hibernation for animals that live in deserts or tropical climates is called estivation. Estivation is defined as “dormancy in some animals during the summer or during months of drought,” which can result from a decreased food supply or from potentially fatal hot and dry conditions. Estivation typically involves burrowing into the ground, where the temperature stays cool and metabolic activity is reduced, as with certain species of fish.
During periods of severe drought, Lungfish, ancient fish that still breathe air with lungs, secrete enough mucus to cover their entire bodies and then breathe through a mucus tube. The mucus seals in moisture and the lungfish can live without water for up to three years.
Lord knows, it’s hard to climb out of a warm cocoon of covers on cold winter mornings, making it feel like humans might have hibernating tendencies themselves. The deep sleep of humans does share some similarities with torpor, including reduced heart and breathing rates and lowered body temperature, but these changes are minimal compared to those experienced by hibernating animals. Also, sleep in humans reflects changes in brain activity. Hibernating animals don’t demonstrate much change in brain wave activity during hibernation; and when an animal comes out of hibernation, it will show signs of sleep deprivation, requiring a lot of sleep over the next few days to recover.
Torpor or not, when it gets really cold outside, it sure is nice to curl up into a blanket on the couch with a good book and a cup of hot tea.
