With Nantucket’s first real stretch of cold this winter and even some recent snow fall, it leads one to wonder about how wildlife survive constant exposure to wicked coastal New England weather. Even the most cynical wildlife biologist might find themselves “feeling badly” for the critters. How do they survive the bitter cold and windy nights? Where will they find food, water and shelter?
Wildlife have developed three basic evolutionary “choices” to handle winter – migration, hibernation or adaptation. For the Nantucket wildlife that are able to fly or swim to a warmer locale, migration is a great option. Many of our songbirds and summer shorebird populations begin heading south in the late summer through the fall. Our flying mammal species, red, hoary and silver-haired bats, while physiologically capable of hibernating, usually migrate farther south along the eastern seaboard to warmer places where they can continue to feed for the entirety of the winter rather than sleeping off the season. Even insects are known to migrate – of course, perhaps the most famous example is that of the Monarch Butterfly. The butterflies that pupate in the late fall are the only generation to make the 3000+ mile trek to central Mexico to overwinter. One of the most common species of dragonfly found in the US, the Green Darner, completes annual long-distance migrations south to central America as well.
For those species that must remain on Nantucket throughout the winter, hibernation (or a form of hibernation called brumation) is a strategy to survive the long, cold months. Northern long-eared bats are one such species. As their name suggests, their northern distribution does make them somewhat hardier than other bat species and more tolerant of colder temperatures but their activity levels do tend to decline after average temperatures dip below 50°F. However, our stationary acoustic detectors set year-round throughout the island have picked up calls from northern long-eared bats in all months except February even on nights with temps in the 30s. If day time temperatures are warm and winter insects are flying, Northerns will emerge from hibernation for short flights to get a snack and take a drink. They will then return to their hibernacula in a crawl space or basement under a house until their next warm opportunity to emerge arises. Unlike larger mammals with long hibernations such as bears, bats are able to cycle in and out of a state of torpor throughout the winter. While they are torpid, their metabolism slows and body temperatures drop. But they can emerge from torpor to take advantage of periodic warmer weather.
The only other mammal on Nantucket that truly hibernates is the Meadow Jumping Mouse. They enter hibernation in late fall when their metabolism drops drastically and unlike bats, they don’t become active again until early spring. Despite slow wintertime metabolic rates, they still lose significant amounts of body weight and must begin feeding immediately upon emergence from hibernation.
Reptiles and amphibians are another group that must enter a form of hibernation called brumation in order to survive. Our spotted turtles for example, being ectothermic (or as some people call them “cold-blooded”), they cannot regulate their own body temperatures the same way mammals can. Their internal body temperatures are dictated by local climate, water temps and sun exposure. When the temperature drops in the late fall, Nantucket spotted turtles retreat to shrubby wetlands with relatively stable water sources and fairly constant water temperatures. They will often hibernate throughout the winter along with other spotted, painted and snapping turtles. They can survive short duration freezes and we have even come upon spotted turtles suspended in frozen wetlands. Luckily, freezes on Nantucket don’t often last long and a chemical in turtle blood that acts like an “anti-freeze” prevents tissue damage so long as the freeze isn’t prolonged. Turtles remain under water throughout the winter. Because their internal systems are much slower in cold temperatures, they don’t require much oxygen but are able to “breathe” by up-taking oxygen through blood vessels at the surface of their skin. Snakes are also ectothermic and so have a long dormant period as well. They will gather in underground hibernacula sometimes in large numbers. Several old cisterns, basements and crawl spaces on Island have been documented with multiple milk and garter snakes inside. Structures such as these are cold but temperatures are relatively stable, providing a safe place for dormant snakes to spend the winter. Salamanders remain in uplands in our forests but will retreat underground during the coldest months.
So, what about all the other animals that must grin and bear the winter weather? There’s a lot of hardy wildlife out there in the winter and there are many diverse strategies to survive. Let’s start small. While the jumping mice are hibernating, all our other small mammal species remain active. Meadow voles are known for their above and below ground runways through the grass and in the winter, even during periods of heavy snow, are able to maintain these paths and are able to continue to feed on vegetation and seeds. White-footed mice as well as squirrels of course, are known to stash food so they can remain active all winter on their hidden stores. Shrews are interesting beasts – they are well known to have super charged metabolisms and feed voraciously on ground dwelling insects and worms constantly in order to keep that high metabolic rate revved up. Insect treats are harder to find throughout the winter but shrews don’t hibernate and still have that metabolism to accommodate. They get through winter by putting on a thick heavy coat and apparently are able to literally shrink their entire body, internal organs and all! This may seem counter-intuitive as a smaller surface to body mass ratio generally means an animal would lose heat more quickly, but this doesn’t seem to happen in a shrunken shrew! The exact mechanism to explain this isn’t known but while a shrew’s metabolic rate doesn’t change in winter, their smaller winter stature means they overall can consume less energy.
Going up in size a bit, what about all of Nantucket’s bunnies? So many bunnies and they seem unphased by winter. Nantucket’s most abundant rabbit is the eastern cottontail, an introduced species to the island. New England Cottontails are nearly identical to the eastern cottontail but much more secretive and habitat specialized, preferring densely wooded areas and avoid yards and open spaces. Occasionally, snowshoe hares are found here as well. Rabbits also put on their thick fur coats for the winter, especially around their feet. They shift their diet from herbaceous and grassy summer time fodder to more woody vegetation and will often peel bark from shrubs. They aren’t diggers so must find shelter in the burrow of another small animal, or at the base of dense shrubs or under fallen logs or buildings.
Birds have all sorts of remarkable winter adaptations! Libby Buck, NCF’s Ecological Steward and Research Technician, wrote a blog a few winters back about how birds survive the winter and how we can subsidize our feathered Nantucket winter residents. You can follow this link to Libby’s blog to read all about birds in winter!
And then there are the deer, often seen all across the island, trudging through even the worst weather. Their coat seems so thin, how do they stay warm when it’s blowing during a nor’easter!? In preparation for coming winter, deer like other mammals, start shifting their diets and retaining more fat for better insulation. They shed their thin summer coats for a longer, thicker, warmer fur that is also darker, which allows the deer to absorb and retain more heat especially on sunnier days. The hairs of their winter coat are also hollow, allowing air to be trapped against the body which is a huge warming factor. Oil glands in the skin are also more active in the winter, providing for a water-repellant layer for the coat which is useful in the snow, or the typical Nantucket relentless rain.
We hope this overview provides some reassurance that our wildlife on Nantucket are sturdily built to withstand harsh winter weather!
The Nantucket Conservation Foundation is a private, non-profit land trust that depends on contributions from our members and community to support our science projects, conservation property acquisitions and land management efforts. If you are not already a member, please join us now! www.nantucketconservation.org