Hibernation, torpor, diapause, aestivation, and brumation
Dormancy refers to the state of reduced activity that many organisms enter when exposed, or anticipate exposure, to environmental stress. During the dormant phase, energy is conserved within the organism by reducing metabolic activities. Chemical changes may also occur to protect the organism from freezing, heating, or drying. The five types of dormancy are hibernation, torpor, aestivation, diapause, and brumation.
Many rodents, including chipmunks and groundhogs, as well as mammalian species like hedgehogs and bats use hibernation to survive harsh winters. To prepare for hibernation, food is gorged upon and stored as body fat. Then shelter is found for their winter’s deep sleep, during which heart rates slow to as much as 90%; a chipmunk’s heart rate drops to 4 beats per minute from 350! Hibernation can last for weeks or months. Some hibernators awaken during warming periods and will eat and urinate, but soon will resume their deep sleep.
A champion of hibernation is the Arctic ground squirrel, supercooling its body from 98° to 30° for 8 months…shivering (without waking) every 2-3 weeks to reheat. The severe cold shrivels their brain neurons, which totally recover when they awaken. A protein called tau builds up in their brain cells when hibernating…and in the brain cells of humans with Alzheimer’s. The squirrels scour tau from their brains when they wake up. Studies of the squirrels abilities could provide insights into Alzheimer’s and other neurodegenerative diseases.
And the only bird to hibernate is the common poorwill, close kin to our whip-o-will. Found in the Western United States and Mexico, their body temperature drops from 104° to about 41°. Other birds enter a state of dormancy called torpor.
Some animals, mostly birds and bats, reduce their body temperatures only a few degrees for only a few hours a day, along with lowering activity levels. This dormancy state is called torpor, and is used during cold nights, when food is scarce, and as a general energy saving adaptation. Studies have shown that those few degrees save a significant number of calories from being burned off overnight.
Hummingbirds require huge energy levels with heart rates over 1,200 beats per minute; they use daily torpor to conserve energy even in the tropics, dropping their body temperature over 50%. Chickadees and swifts are other well known users of torpor.
Butterflies are cold-blooded, so when they get too cold they can’t fly, and if too hot, they’ll dry out. To survive these conditions, they’ll enter a state of dormancy called diapause…a spontaneous reaction to certain environmental conditions. This temporarily affects their life functions and development by slowing them down to almost a stop. This state can happen at any time of year, and for varying lengths of time; it mostly affects insects, but also happens with birds, fish, and mammals. Some butterflies use diapause in the middle of a hot summer day for an hour or so, and some insects may enter diapause for months. Diapause in summer is often called aestivation.
To avoid the stresses of excessive heat, estivation is the state of dormancy in which animals in mostly desert and tropical habitats use a summer sleep for survival. The sleep is relatively shallow, allowing the critters to awaken quickly when conditions return to favorable levels. Some animals, notably insects, are common estivators. Salamanders, frogs, and snails conserve water requirements by estivating during dry times. Probably the expert is the African lungfish: it’ll burrow into the mud of a dried up lake, cover its body with mucus which dries into a water-retaining sack…breathing through a small tube. The lungfish is able to thus survive without water for up to 3 years!
Brumation dormancy can be described as torpor done under ground by reptiles. Sometimes during brumating, the reptile will become a bit active during a warm spell; although not requiring food, it searches for needed water to escape hydration. So, like torpor, it’s not a true hibernation phase. Brumation in reptiles may be considered the opposite of aestivation in insects, where the former addressees the heat and the latter, the cold.
Preparing for brumation, eating is increased to not only build up fat reserves, but also to add glycogen, which is a form of sugar and is mostly stored as energy in the muscles. Some glycogen is stored in the blood, which aids with oxygen intake in low-oxygen environments like underwater. Some turtles have areas of high blood vessel concentrations at their cloaca (butt) which can absorb oxygen from the water…a butt snorkel! And snakes will absorb oxygen through their skin. The added fat seems to play more of a reproductive part with reptiles, aiding egg production and post-dormancy mating.
A relatively sheltered place called a hibernaculum is found and often shared with many others of the same species…sometimes hundreds! Crevices in rock piles, leaf litter, caves, burrows…even under water for some…are used. Generally, they enter dormancy in the cooler, shorter days of autumn and emerge from their rest at the arrival of the warmer, longer days of spring.