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Nature Notes: Dormant

Ed Robinson
January 6, 2024

One definition for the word dormant is being “temporarily in abeyance yet capable of being activated.” That describes how I feel after a hard day of cutting, splitting and stacking firewood followed by a long snooze. It takes the old noggin a few minutes to get up to speed for polite conversation with my long-suffering spouse. Since this is, after all, an article about wildlife we will turn to a second definition; “an animal having normal physical functions suspended or slowed down for a period of time; in or as if in a deep sleep.” Now we are getting somewhere.

Thanks to my friend Jaki for prompting this article by asking me about hibernation, a common question from readers at this time of year. Jaki raised the bar further, however, by asking how hibernation differs from estivation. As often happens, her question caused me to do some research to provide her with a complete answer, and then to share the results with you. I have written about some aspects of dormancy in the past, including articles about woodchucks, snapping turtles, leopard frogs, and black bears. It is time to pull it all together.

Historically we used the term “hibernation” quite broadly to describe the condition whereby any creature lowered its metabolic functions for a long rest during periods of extreme weather. Over time scientists realized that relatively few creatures enter a state of true hibernation, with very low body temperature, minimal respiration and heart rate, and months of deep sleep with limited movements. Depending upon the animal and the environmental conditions, hibernation may last for days, weeks or months. True hibernation applies only to warm-blooded or endothermic creatures, those with the ability to self-regulate their body temperatures. In general, hibernation is triggered by the loss of daylight in the runup to winter and resulting hormonal changes that require the animal to conserve energy in harsh conditions.

woodchuck pokes head out of snowy den

Woodchuck (iStock photo by Brian E Kushner)

The only way an animal can survive extended periods of hibernation, even with the limited use of energy, is by storing significant fat reserves. In our area this means that animals are actively feeding on the richest foods available to them in late summer and autumn, those heavy in carbohydrates and lipids. Animals like the woodchuck will gain 20 to 40 percent in body weight during this period and lose most of it during the winter – talk about binge dieting! While their metabolisms slow considerably during hibernation, to minimize energy usage, animals may rouse briefly from deep sleep to find water and food, and to defecate. That arousal process takes several hours, as the animal recovers its mental faculties, biological systems ramp up and body temperature rises, consuming some of their energy reserve.

The term “torpor” is now used to describe a type of dormancy that is less comprehensive than hibernation, taking place over shorter time frames. Torpor is defined as “a state of decreased physiological activity, marked by a reduced body temperature and metabolic rate” combined with partial or total insensibility. You might view this as a state of lethargy, or “hibernation light.” Bears, raccoons, and skunks are known to experience torpor during the winter.

skunk walks through short grass

Skunk (iStock photo by Coleman515)

Torpor may be experienced on a seasonal basis, such as squirrels and chipmunks sleeping for several days at a time through the winter months but waking up regularly to eat (this serves to keep their body temperature from falling well below freezing). Other creatures like hummingbirds and bats regularly fall into a state of torpor during periods of rest to conserve energy, while returning to normal activity levels during feeding times. Recovery from a state of torpor may require a few minutes up to an hour, with muscle contractions and the shaking of body parts.

Cold-blooded or ectothermic animals also experience the winter months as a time of greatly reduced activity. The challenge for them is more significant than for warm-blooded creatures because ectotherms are dependent upon heat obtained from their environment (the reason you can observe turtles sunning themselves on bright days). With some exceptions, reptiles and amphibians must find ways to survive long periods of dormancy called brumation by keeping their body temperatures above freezing. In warm spells they may move and take nourishment to boost their chances of survival.

striped snake suns on a rock

Garter snake (iStock photo by Mlharing)

Creatures like the garter snake enter underground winter dens in large numbers, gaining heat from the Earth and from neighboring snakes. Salamanders find refuge under rocks, logs or leaves but may not survive harsh conditions if they do not choose a reliably warm shelter. Turtles and frogs take up residence at the bottom of ponds or rivers in the hope that the water will not freeze completely during brutal weather. Some frog species are adapted to survive being completely frozen, absorbing oxygen through their skin and relying upon high levels of blood glucose to prevent cell damage.

There is one more state of dormancy to cover, but it takes place in periods of extreme heat. “Estivation” is the term used to describe the process by which many types of creatures take shelter and lower their metabolic rates to survive unusual environmental conditions. As with the other forms of dormancy described above, estivation involves a slowing of all metabolic functions in the hope of survival by conserving energy (and water, in the case of land-based creatures). Unlike other forms of dormancy, estivation can be reversed quickly, for instance, when a sudden rainstorm drenches a parched desert region.

tortoise shell sticks out of soil

A tortoise burrowed in soil (iStock photo)

As one example of estivation, the tortoise in the attached photo burrowed into the soil to avoid extreme air temperatures and a loss of moisture that may be deadly. Many reptiles and amphibians are faced with similar challenges because they are more affected by extreme heat than warm-blooded creatures. Creatures as large as crocodiles or as small as salamanders may suffer in harsh conditions, and insects such as beetles and mosquitoes are equally susceptible. Other examples include snails and crabs, while Maine’s valuable brook trout have struggled to survive recent summers with extreme water temperatures.

In my research, I came across a reference on Wikipedia to the water-holding frog of Australia. This frog is adapted to live on land and has an amazing ability to survive up to five years of drought conditions. The frog buries itself in the sand, then seals itself in a secreted water-tight mucus membrane to avoid losing critical water reserves in the dry conditions. In another amazing adaptation, Aboriginals long ago learned they could dig up a frog and squeeze it, causing the frog to empty its bladder. The urine was dilute enough to be ingested by the humans, allowing them to survive another day in the harsh environment. The frogs were not so lucky, unless a rainstorm soon appeared to allow them to rehydrate.

Biologically, humans are not well equipped to handle harsh cold or soaring heat, as evidenced by the deaths that occur during periods of extreme weather. We have learned over time to emulate the wild creatures around us to handle the change of seasons. Today we live in insulated homes equipped with heating systems to keep us warm in cold weather, and we rely on ventilation or cooling systems to chill out in the summer. While some of us go into a state of torpor during the NFL playoffs, we can avoid either hibernation or estivation to survive. Fortunately, coastal Maine has a moderate climate, so we are not forced to go underground in winter nor resort to drinking frog’s urine when things really heat up!

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