North County winters pose a challenge to animals who choose to stay here, rather than migrate to warmer climates. Food is scarce. Many survive by sleeping. Well… not sleeping exactly. Hibernating.
Hibernation is a life-saving adaptation. Essentially, it’s the ability to reduce one’s energy needs when resources run low or become unavailable. Many warm-blooded animals would die of starvation if it wasn’t for their ability to hibernate.
Very Few Animals are ‘True’ Hibernators
The term hibernation is commonly applied to all types of winter dormancy. But ‘true’ hibernators enter hibernation at the same time every year, regardless of the outside temperature or availability of food. During ‘true’ hibernation, body temperature is lowered to slightly above that of the temperature in the animal’s lair. Respiration is reduced to just a few breaths per minute. Heartbeat becomes barely distinguishable.
Groundhogs (woodchucks (Marmota monax)) are true hibernators. They need to put on a thick layer of fat before entering hibernation, so during late summer and early fall, they spend most of their time eating. They then use the stored energy in their accumulated body-fat to get through the winter. If they’re unable to consume a sufficient amount of food, they’ll starve to death, in a deep sleep.
During hibernation, a groundhog’s body-temperature drops from 99°F to about 38°F. Heart rate decreases from 100 beats-per-minute to just 4 or 5 beats-per-minute. And breathing slows from around 16 breaths-per-minute to as few as 2 breaths-per-minute. A hibernating groundhog may even appear to be dead.
The little brown bat (Myotis lucifigus); the most common bat in northern New York; is another true hibernator. In the fall, as days grow colder and the number of insects diminish, little brown bats retreat to sheltered, overwintering sites called hibernacula; most often caves or empty buildings. Like groundhogs, little brown bats must store enough fat in their bodies to make it through the winter.
While in hibernation, their heart rates fall to as few as 5 beats-per-minute, compared to more than 1,300 beats-per-minute, while in flight. They commonly go for 45 minutes or more without taking a single breath.
White-Nose Syndrome (WNS), a serious fungal disease affecting little brown bats, has been reducing their populations to near-extinction levels in New York. Bats affected by WNS wake from hibernation too soon, which results in them using up their fat reserves and dying of starvation.
Ironically, black bears; by far the most celebrated of all of the hibernators in the northeast; are not true hibernators. That’s because, even though a black bear’s heart rate drops from a normal, active average of about 50 beats-per-minute to as low as 8 beats-per-minute during dormancy, they maintain a near-normal body temperature, while in that dormant state. This allows them to wake up quickly (and grumpily), should the need arise. In fact, pregnant female black bears wake from their deep sleep to give birth; then go back to sleep while their newborn cubs nurse.
To be precise, the energy-conserving deep sleep we call hibernation is essentially a subcategory of the state of lowered physiological activity we call torpidity. In spite of this, the term torpor is more-commonly used to describe shorter, less-dramatic forms of dormancy.
Rather than sleeping for months-on-end, many animals take more-measured approaches to surviving the winter. Chipmunks, for example. They hibernate during the winter. But not in the usual sense. Their normal body temperature of about 100°F falls to as low as 40°F while they sleep, but instead of consuming food and building up body-fat before winter sets in, chipmunks spend much of the summer and early autumn collecting and storing food, so that it can be consumed during the long, cold winter.
They’re hoarders. They cache the collected food (mostly seeds and nuts, which are good keepers) deep within their underground burrows and every few days they wake from sleep. Their body temperatures rise to normal and they snack on bits of their stashed food. They may even venture outside for short periods to forage, should the weather turn mild.
Interestingly, a Fordham University study conducted between 2000 and 2007, found that as winter temperatures heat up because of global warming, chipmunks in warmer areas become less-likely to hibernate. The research indicated that when chipmunks followed normal hibernation procedures, they experienced a winter survival rate of 88%. When they remained active, only 11% survived.
Since their body temperatures match their environment, reptiles (snakes, turtles), amphibians (toads, frogs, salamanders), and insects find sheltered places to spend the winter inactive, or dormant. This isn’t hibernation, but it’s similar. It’s called brumation.
Some critters even undergo chemical changes to prevent their tissues from freezing. For example, wooly bear caterpillars (Isabella tiger moths (Pyrrharctia Isabella)) actually freeze solid. They produce a cryoprotectant, glycerol; a type of sugar that acts like antifreeze in their circulatory fluids, inhibiting damage to cell tissue and allowing them to survive.
Why don’t humans hibernate?
Simply stated, humans don’t hibernate because… well… we don’t need to. In fact, humans go into mild hypothermia when their body temperature drops by a mere 3°F.
I am getting tired, though. My eyelids feel heavy. Yawn. A long winter nap sure sounds good!
Photo at top: Hibernating black bear mother and cub. Photo Credit: University of Minnesota.