How Animals Hibernate
Aristotle postulated that the swallows and kites of Greece spend the winter in holes in the ground. He further mused about swallows being found without feathers in their cavities. Over time, these myths would build on ancient misconceptions of swallows in the north spending the cold season in the mud at the bottom of lakes. These even more fanciful falsehoods were likely contributions from 16th-century Swedish Archbishop, Olaus Magnus. He observed vast numbers of swallows congregating on ponds in the fall and purported that they sink down into the mud and water, packed like sardines for the winter. There even remain Scandinavian woodblock prints from the Middle Ages of fishermen pulling up swallows out of the water. During a time of strong belief in mythical creatures and magical theories, it seemed hardly a stretch. While science, observation, and technology have since enlightened us beyond far-fetched ideas about birds and their life histories, these seemingly outlandish accounts are not far off from actual wintering strategies of other animals.
The period of dormancy animals use to survive changing environmental conditions is typically referred to as hibernation; however, scientists also use terms such as diapause, brumation, and torpor to describe animals in various types of hibernation. There often seems to be a bit of hand wringing among scientists and naturalists over the exact definitions of these terms and which animals seem to meet them.
Bears are often at the crux of the hibernation debate. When I worked as a seasonal guide in Alaska, bears were a common topic of conversation in the backcountry. Our understanding of hibernation was in the context of what we understood to be “true” hibernators. In Alaska, animals like the arctic ground squirrel undergo an extreme form of dormancy in their winter burrows. The arctic ground squirrel is an incredible hibernator, lowering its body temperature to the freezing point, or sometimes just below freezing (supercooled body fluids!), with little to no brainwave activity and a measured heart rate of 1 beat per minute. It is an astounding form of deep hibernation. If you dug one up and picked it up in this state, it would appear dead and frozen. In the context of this extreme hibernator, the hibernation of bears seemed hardly comparable.
Recent studies have sought to better understand the physiology of bears in the winter by taking measurements during hibernation. The studies showed black bear metabolic processes to be significantly lowered, ensuring maximum energy and fat conservation. During hibernation a bear’s body temperature is slightly depressed, but it is still regulated. A black bear can be aroused from its winter nap, though I wouldn’t recommend trying this! Black bears during this time don’t eat, drink, urinate, or defecate. Their breathing slows, and their heart rate drops significantly from 40-50 bpm to as low as 8 bpm. Due to these observations in black bears and in other recent studies, biologists have redefined the definition of hibernation to a “specialized, seasonal reduction in metabolism concurrent with scarce food and weather.” Black and brown bears, bats, and chipmunks easily meet this broader definition, and one can be fully confident in describing these animals as true hibernators!
Cape May hasn’t seen the influx of black bears that North Jersey has had, but we have experienced a spate of black bear sightings in recent years, including in Dennis Township. With continually growing bear populations in New Jersey and plenty of protected areas in Cape May County, it is plausible the most famous of hibernators may soon be here to stay!
The woodchuck or groundhog is probably the best example in Cape May County of a seasonal hibernator in the traditional sense. Though not quite as common in South Jersey counties, the groundhog is a regular resident in suitable areas from Upper Township to Cape Island. Groundhogs are a type of marmot, large rodents that typically live in rocky and remote mountainous areas. The groundhog has adapted to lowland open areas like valleys, prairies, agricultural areas, and suburban backyards. Groundhogs in New Jersey go into hibernation in October and usually emerge from their burrows in April. According to records on the online animal sighting database iNaturalist, Cape May, Cumberland, and Atlantic counties all see their first groundhogs around March 15th (other than a few oddball sightings in each winter). Males often emerge first followed by the females. By the time of emergence, they may have lost more than half their body weight. This is why the late summer and fall is such a crucial time to pack on the pounds!
Groundhogs have become a bit of a cultural symbol in North America in interpreting seasonal weather predictions. Punxsutawney Phil’s emergence on Groundhog Day is the most famous take on this in North America, and he became an especially popular cultural touchstone after the eponymous 1994 film starring Bill Murray. Groundhog Day has been celebrated since the 1890s but was probably inspired by ancient Celtic and Germanic traditions involving other hibernating animals like hedgehogs and badgers in Europe. The concurrent traditional pagan holiday Imbolc occurs in late winter and was a time of weather divination, watching to see if snakes or badgers came from their winter dens. Phil’s predictions and what seems like silly small-town traditions in Pennsylvania indeed have some deep roots!
Torpor is another definition describing dormancy and is often used in the context of short-term hibernation. Many animals can be said to use torpor, even a few birds! Where the definitions diverge is that this term typically describes a shorter time period when a hibernator goes into a temporary state of lowered metabolism for weeks or days. It is especially relevant in circumstances where the animal enters a state of lower body temperature lasting less than 24 hours. A prime example of this in the winter is bats. Some of the bats that stick around winter roosts in Cape May can be seen flying around Cape May Point on warm mid-winter days, looking for insects that may have been similarly roused by warmer temperatures.
Birds like our Carolina chickadees may also go into a state of torpor each night during particularly cold weather to minimize energy loss and conserve fat reserves. In harsh winters it is particularly amazing that some of our smallest forest birds like chickadees and kinglets can survive extreme mid-winter conditions. The superb 2003 book Winter World, by University of Vermont professor Bernd Heinreich, goes into detail about the strategies of these little birds surviving long winter nights. In the case of golden-crowned kinglets, an inch of thick feather-down helps insulate body heat for a bird that is only 3.5 to 4 inches long and weighs as little as a nickel. They also may congregate in small groups, bunched together on a branch or in a cavity sharing body warmth with their tiny heads tucked deep in fluffed-up feathers. Torpor has not yet actually been shown in kinglets but has been shown in black-capped and Carolina chickadees. On very cold nights, chickadees will go into torpor, significantly reducing their body temperature to conserve calories.
Most well-known among the neotropical migrants for their amazing ability to tolerate the cold are the hummingbirds. The familiar ruby-throated hummingbirds in Cape May leave for warm wintering places like Mexico by September. However, in recent years, the ruby-throated hummingbirds’ western counterparts have been turning up all over the east coast in late fall and winter, especially at backyard feeders left out all season. Hummingbirds have an extremely high metabolism and substantial energy requirements, with a heartbeat of up to 1,200 times a minute! So, a stranded hummingbird needs a lot of calories to keep hovering along. If stuck in bouts of cold weather, they need a way to conserve calories at night. In coastal Alaska I observed an Anna’s hummingbird coming to a feeder day after day about a half hour after sunrise where the overnight temperatures led to a significant frost each morning. This bird seemed more than comfortable singing from perches in the early morning sun after enduring temps that were dipping well below freezing each night. Though they have limits, the secret to this tiny bird’s cold tolerance is called torpor.
Lab studies have correlated observations like this showing that hummingbirds can regulate their metabolism based on their environment. Biologists describe multiple states for infrequent hibernators that use torpor: regular sleep, shallow torpor, and a deep torpor. During regular sleep the body temperature stays the same. In shallow torpor, their body temperature drops by about 20 degrees Fahrenheit. In deep torpor they may further lower their body temperature by another 30 degrees Fahrenheit and slow their heart rate down to as low as 50 beats per minute on freezing nights (from 1000bpm in peak activity periods). For most other animals, including humans, even a drop of three to four degrees in body temperature is considered hypothermic. After enduring a freezing night, hummingbirds may rewarm their bodies up 50 degrees in just a half an hour—what amazing little birds!
When you think of complex winter survival strategies, it is natural to think of mammals or birds. However, insects have some impressive tactics as well. People have been coming to an understanding of the natural cycles of insects since antiquity, but it wasn’t until the early 17th century that specific information and speculation on the hibernation of insects was written down. French naturalist René-Antoine Ferchault de Réamur wrote, “the cold, which stops plant growth, which causes our prairies and fields to lose their flowers, puts the bees into a state in which food ceases to be necessary to them; it holds them in a sort of numbness, during which there is stop sweating or, at least, during which the quantity of what they sweat is not considerable that it may not be repaired by food, without their life running at risk.” In describing the observations of hibernating honeybees, he gave the first known description of an insect’s diapause.
In mild climates insects may be able to burrow underground, into a tree, under bark, and other milder microclimates to wait out cold weather or rare overnight freezes. In harsher environments and areas with predictable cold weather, they go into a dormant state marked by little to no activity if undisturbed, called diapause. Insects do this in a variety of life stages. Some insects like dragonflies overwinter as aquatic larvae, slowing down metabolism and not feeding until the water warms again in the spring. Others may overwinter as an egg or pupa. The largest dragonfly native to Cape May, the swamp darner, lays its eggs in rotting logs in the late summer and fall when water levels are at their lowest in ephemeral wetlands. The eggs survive the winter in diapause. When the pools fill during spring rains, the water acts as a catalyst, the logs are submerged and the eggs hatch into larvae. It’s a pretty neat trick to set up the next generation for their early aquatic life.
To prepare for diapause, insects go through a variety of enzymatic processes initiated by photoperiod (day length) in order to prepare for a winter dormancy. It can be initiated because of a changing photoperiod, temperature changes, or even a lack of moisture in the environment. A species of beetle called Eburia quadrigeminata (say that 10 times fast!) prefers fresh rather than rotting wood, different from most beetles. Sometimes after an infested hardwood tree is cut and milled, the larvae remain trapped inside of the drying wood. Undergoing diapause and waiting for better conditions, this beetle is notorious for emerging as adults from already installed floorboards and furniture. Normally it takes two years for their larvae to emerge, but in a few cases, homeowners have witnessed the emergence of a beetle from 40-year-old furniture and floorboards!
The study of insect biology can seem a bit esoteric, but insects and their arthropod brethren are the most abundant of animals on earth. If you took a shovel full of leaf litter or soil and dumped in on a white sheet you would find quite an abundance of tiny creatures, potentially thousands of wingless arthropods called springtails, among the macro-sized insects we are more familiar with. Estimates of the abundance of springtails in soils is in the tens of thousands to millions per square meter! Such an abundance provides a solid base to the food chain and supports a healthy ecology in all habitats, including our own backyards. Springtails are hard to see, but not hard to find with a magnifying glass, especially in moist areas outdoors. Most people may only take notice of them in the wintertime if they have stumbled across tiny dark hopping critters on the snow surface, referred to as “snow fleas.”
Not all springtails are active in the winter, but the snow flea type springtail uses a different strategy than other arthropods to survive. On warmer winter days they do still feed (on fungal spores and algae) going about their business, but once the temperature starts to tank again, they go into diapause and are protected by special antifreeze compounds in their blood called glycerol. Glycerol isn’t much different than the ethylene glycol in your car radiator, keeping the engine from freezing up in mid-winter
There is one very particular springtail on the coast that inhabitants of the tidal marshes and rock pools of the eastern seaboard called the Seashore Springtail. This collembolid (springtail) lives in the intertidal zone, feeding on algae and other organic material. During the high tides they hide in air pockets along the shoreline until the next tide cycle. These animals adjust their wintering strategy depending on the climate. Those in New England and north lay eggs that will overwinter in diapause without any inner activity until it warms up. The adults will die. Further south the animals can overwinter and survive as adults.
Finding the necessity for their own terminology, herpetologists (biologists who study reptiles and amphibians) have come up with another word to describe dormancy: brumation. Brumation is again analogous to hibernation but has a few different characteristics from our cold-blooded friends. One of the main factors is that brumation is characterized by periods of punctuated activity if environmental conditions change, like a warm spell in early spring. Individuals will then return to brumation if conditions change again.
Brumating reptiles and amphibians are also tolerant of low-oxygen environments like mucky ponds while dormant. Perhaps aquatic turtles were the inspiration for the myths about buried swallows. They do indeed bury themselves in mud at the bottom of ponds and wetlands, surviving in extremely low oxygen conditions. Turtles have a special way of getting the little oxygen they need during these long months of brumation under water. They obviously cannot come up to breathe when locked under ice so they must have another way to maintain minimum oxygen levels. It is often to the delight of children to learn that in the wintertime, instead of breathing into the lungs, turtles breathe out of their butt (or more accurately, the cloaca). Through cloacal respiration turtles move oxygenated water through the cloaca, which is loaded with blood vessels. This allows them to be able to extract oxygen out of the water and support vital body functions during brumation. Other known cloacal breathers include frogs and salamanders.
Nature has certainly been creative when it comes to dealing with seasonal changes. Seasonally cool climates pose a challenge for the animals living in these environments and the nuances of their adaptations are deep and varied. We humans have adapted not with an extraordinary biology but have learned to manipulate the environment around us through clothing, harnessing fire, energy, and technologies. In fact, our influence is so powerful that we have incidentally manipulated the atmosphere, tipping the scales in favor of a rapidly changing and warming climate. Where does this leave hibernators? In certain short-term situations, it may create a more favorable climate for less cold-tolerant and adapted species. However, for many species, freak and frequent warm spells in places all over the world can severely undermine their wintering strategies. What happens to cold-adapted caterpillars like the Wooly Bear when annual freeze cycles are interrupted by extreme warm spells in mid-winter? Often it may mean too many or too early wake up calls, ultimately leading to increased exposure and depletion of their energy reserves normally saved during dormancy. Unforeseen consequences from more frequent erratic climate events may throw out our expectations.
Modern scientific understanding of hibernation is still in a relative infancy, though humans have sought to understand the mysteries of nature since the days of Aristotle. Whether we label it brumation, hibernating, diapause or torpor, these special biological states are even more ancient than myths and legends, evolving with millions of years of changing environmental conditions. The study of hibernation has many important research applications for the future, from long-distance space travel to extraordinary medical applications. The lives of bats, bugs, turtles, and marmots bring up new and stimulating ideas to make one wonder. The next time you are out on a quiet winter walk, think of the millions of creatures still hibernating in their winter seclusion, waiting for the long days of light and warmth to return. ■