Spring Peeper
Island Science

Overwintering: How do they do it?

by Dr. Sarah D. Oktay Managing Director UMass Boston Nantucket Field Station

Each year in early spring, we start to see the islands’ plants and animals emerge. This past winter has been difficult, with three major storms (Sandy, Nemo, and Saturn) causing significant erosion and destruction around the island. In addition, this winter has been wetter and colder than last year which was unusually warm. After the third major storm, I started to think about the battering the landscape had endured and wondered many times how various animals and plants were fairing.

Some animals, just like many residents of the island, migrate somewhere warmer in the winter. Butterflies, moths, our few bats, and birds of all kinds migrate. Many of our winter avian visitors are migrants from further north. Other animals (and people) eat more, move more, or sleep more to survive the winter months. The most well known method of surviving periods of cold, snow and low abundance of food is hibernation. Hibernation is a state of inactivity and metabolic depression in endotherms. Going back to our grade school classes briefly (which I have to do all the time, trust me), an endotherm (from the Greek: endon = “within”, thermē = “heat”) is a “warm-blooded” organism that maintains its body temperature internally and through metabolic means instead of relying almost purely on ambient heat. Hibernation refers to a season of heterothermy (where body temperature varies with the environment, kind of a part-time cold bloodedness) that is characterized by low body temperature, slow breathing and heart rate, and low metabolic rate. Although traditionally reserved for “deep” hibernators such as rodents, the term has been redefined to include animals such as bears and is now applied based on active metabolic suppression rather than based on absolute body temperature decline. Hibernation during the summer months is known as aestivation (I am sure some of us can identify with that). Animals hibernate during winter to reduce their need for food and to conserve energy.  Endotherms slow down their metabolism and gradually their body temperature lowers, sometimes a few degrees like bears, sometimes 30 degrees like some reptiles. Hibernation may last several days, weeks, or months depending on the species, ambient temperature, time of year, and individual animal’s body condition. Before entering hibernation, animals need to store enough energy to last the entire winter. Larger species become hyperphagic , utilizing insatiable hunger, otherwise known as teenage boy syndrome, to consume a large amount of food and store the energy in fat deposits. In many small species, food caching replaces eating and becoming fat. Some species of mammals hibernate while gestating young, which are either born while the mother hibernates or shortly afterwards (adapted from http://en.wikipedia.org/wiki/Hibernation accessed April 22, 2013).

So how does everything from snakes and bunnies and plants overwinter? What tactics do they use to ensure that they survive big storms?

Daffodils: Daffodils are blooming partly because of their success in spending time underground avoiding becoming a Popsicle. Daffodils have leaves and flowers above ground only for a few months each spring. During the winter they live as onion-like bulbs in the ground. The bulbs are full of food and are protected from winter frosts by the soil above them. Like most plants, daffodils absorb light energy from the sun through their leaves (through the chlorophyll in the chloroplast). This energy is then used to convert carbon dioxide from the air plus water from the soil into a simple sugar called glucose. Daffodils will convert lots of this sugar into starch, and store some of it in the bulb which will be their winter food store, like potatoes for potato plants. There are many excellent sources of daffodil information out there not the least of which is our own Nantucket Garden Club. From the web site http://www.daffodilusa.org/daffodils/faq.html I learned that daffodils bulbs will last a very long time, coming back up and increasing in number year after year. They certainly were unphased by this past winter which, although severely windy and with heavy precipitation, was not as bitterly cold.


Spring Peeper Spring peepers (Pseudacris crucifer): Hopefully you call hear the peepers peeping out there. A true harbinger of spring, the male peepers start their mating calls in late March, revving up their motors in April and May, and slowly diminishing in volume as spring turns into summer.  Calling is weather dependent and begins as ice melts from the surface of our freshwater ponds, bogs, and vernal pools, and the peepers emerge from their winter hibernation. So how do these spring peepers, who have delicate skin and a very high water content manage to stay alive during these cold winter months?  Spring peepers produce an antifreeze consisting of low molecular weight cryoprotectants that give them the ability to survive freezing events.  Basically, they allow most of the water in their bodies to freeze while circulating glucose throughout their organs to keep them from forming ice crystals. To adapt to desiccation and excess heat, they will burrow under leaves and logs and can survive losing 33% of their water weight before beginning to lose body function.

Snakes: You may remember reading about the snake hibernaculum we found last year when we were installing our new septic system (http://yesterdaysisland.com/waking-snakes-discovery-of-a-nantucket-snake-hibernaculum/). During excavation and archeological digging, we discovered two abandoned and clean underground concrete septic tanks shaped like bee hives with a dozen or so garter and milk snakes staying warm inside. The name for this phenomenon is a hibernaculum. This word comes from Latin word “hibernare” (to spend the winter) or “winter quarters.” Hibernaculum is also used in biology to describe a protective case, covering, or structure, such as a plant bud, in which an organism remains dormant for the winter.  One of the many science facts that seems to linger from our elementary school days is that snakes are “cold blooded” creatures, or ectotherms (from the Greek “ektós” for “outside” and “thermos” for “hot”). Ectotherms control their body temperature through external means from basking in the sun to cooling off in the shade. Lucky for us when we had to become inadvertent snake wranglers, cold-blooded animals are very sluggish in cold environments. Their metabolism decreases significantly when it is cold, allowing them to survive on much less food and water and extending their chances for survival. We were able to leave part of the system of concrete blocks underground in a jumble, and hopefully it still provided the refugia that hibernaculums should provide.

Turtles: You may also recall that our aquatic turtles have become quite good at surviving for many years (sometimes too many if you own one like 50 years!) if they can avoid being run over as they move from winter to spring “stomping” grounds (well as much as turtles might stomp). The Field Station is home to a red eared slider, “Trawalney”. These reptiles don’t  hibernate,  they brumate, which means they becoming less active, lower their heartbeats and need for oxygen and usually settle to the bottom of a water body, occasionally rising for food or air. Brumation can occur in varying degrees. Red-eared sliders brumate over the winter at the bottom of ponds or shallow lakes, or in the case of Trawalney, the bottom of the aquairum. The island’s three aquatic turtles: painted, spotted and snapping usually do the same thing each winter, nestling into mucky, highly vegetated muddy holes (that organic matter is much darker and holds a lot of heat)  or sinking into deep water which is insulated from the colder water above.

White-tailed deer: I went to the Maine fish and wildlife site for deer hunting (http://www.maine.gov/ifw/hunting_trapping/pdfs/deer_yards.pdf  ) to learn how white tailed deer survive winter up in Maine, which is obviously much colder than Nantucket. Some of these same behaviors and methods hold true for our island deer population. “The white-tailed deer found here in the Northeast is one of the three northernmost of 16 subspecies. Northern deer have larger body size than deer further south. This is true of all mammals, in that body size increases as you progress northward. Large body size conserves energy better because of a lower surface to mass ratio. Deer shed their hair coat in the spring and fall. The red summer hair has solid shafts and lacks an undercoat, but the gray-brown winter coat has hollow hair shafts and a dense, wool-like under fur, providing effective insulation. Deer have special muscles that can adjust the angle of their hair shafts to obtain maximum insulation.”

During the fall, deer accumulate and store body fat under their skin and around internal organs just as many other endothermic mammals do. Fat is exactly like fuel and is burned in the body to provide heat. This serves both as insulation and energy reserve for the rigors of the winter ahead. Fat reserves can be up to 30 percent of body mass of adult does in the fall. A deer’s diet in the winter is lower in protein and less digestible than the summer diet, requiring more energy to digest and resulting in fewer calories.  Deer and many mammals go into an involuntary dieting phase each winter, struggling each day to balance their need for warmth and predator avoidance with energy expenditures needed to find food. The stored fat is burned during winter to partially compensate for the lack of energy in the winter diet. Deer normally lose weight during the winter even when fed a free choice, high-protein diet. These adaptations are designed for the conservation of energy. Deer go into the winter with fat reserves that may or may not be enough to survive the winter. Like many other mammals, deer have two or three babies almost every year to help replace the ones that may not have survived the winter.

This same site goes on to give a fascinating summary of deer wintering areas and how they adapt an almost a pack mentality to build areas where they can shelter together (“yarding”), use each other’s trails and body warmth while staying in low snow areas out of the wind.

Rabbits: The little guys are the first ones I think of during a long hard winter. They also adapt by eating more as the days start to get colder. Our Nantucket rabbits are the Eastern Cottontail which was imported long ago. Scientists at the Nantucket Conservation Foundation are working with the U.S. Fish and Wildlife Service (USFWS) to see if they can find the New England Cottontail which was the only native rabbit to exist on island http://ncfscience.wordpress.com/2013/02/08/searching-for-a-certain-some-bunny/ ). Rabbits use a variety of typical endothermic tools to survive the winter. They develop high levels of body fat and grow new and thicker fur during the fall season so that when winter comes in, they will be warm and cozy. They huddle together for warmth and form shallow burrows in dens. Food is hard to find in the winter and the rabbits are a favorite prey of carnivores, so some rabbit’s fur changes to white in order for them to hide much better in the snow.

Not all rabbits migrate but some species of rabbits do migrate during winter. Rabbits do not sleep through the whole winter. They sleep most of the time but from time to time when the weather is warmer they wake up and look for food. Then they return to their burrows or tunnels. They spend the winter days in dormant lifestyles or they remain inactive most of the time to conserve their heat and energy.  In a previous article I detailed how they line the babies nests with fur which is amazingly warm  http://www.yesterdaysisland.com/archives/science/4.php )

Ticks: How do these little suckers survive? Deer ticks (Ixodes scapularis often known locally as Ixodes dammini) have a considerably longer lifespan than many other insects (typically two years). Provided they are able to find adequate sustenance, deer ticks can live up to two years. They primarily feed on animal blood during the three important stages in their lives: transitioning from larvae to nymphs, then nymphs to adults and preparing to lay eggs as adults. Even without a host, deer ticks are able to survive for roughly three months. This is particularly evident during the winter. As the cold weather begins to take hold, deer ticks who have not found a host will take refuge in areas that are high in vegetation. They will remain inactive until late February or early March, at which point they will die if they fail to find a host. A cold winter can drastically reduce the tick population as a deer tick can only survive if the average winter air temperature is 19 degrees or warmer. Unfortunately, global warming is expanding the range northward of the deer tick, as scientists have recently discovered they have managed to survive for the first time as far north as Isle Royale in Lake Superior, located off the coast of northern Minnesota (http://www.startribune.com/lifestyle/health/125511433.html?refer=y accessed April 22, 2013).

Moles voles and shrews: Voles can live either above or below ground and they dig snow tunnels in the winter, which is called subnivean behavior. The most common specimen found on Nantucket is the meadow vole (Microtus pennsylvanicus). The meadow vole does not hibernate and is active year-round, usually at night. They dig underground burrows where they store food for the winter and females give birth to their young. You may recall the term for underground habitat use is fossorial. Bees, wasps, badgers, and many rodents are fossorial. Our native shrews are nervous little carnivores (often insectivores). These very small, non-hibernating mammals have high metabolic rates and may consume up to twice their body weight per day. They do not hibernate during the winter, adjusting the warmth factor by eating more, moving more and will often eat meadow voles. One of the coolest shrews found on Nantucket is a subspecies of the northern short tailed shrew (Blarina brevicauda) which has developed a paralyzing bite. One of their venom components, a peptide called soricidin, is in clinical trials to determine its efficacy as a cancer and pain treatment (http://en.wikipedia.org/wiki/Northern_Short-tailed_Shrew#cite_note-MSA-3). The northern short-tailed shrew survives by increasing its food consumption by 43% in winter to fuel a higher metabolic rate required to maintain its body temperature under the cold conditions. This gorging is piled on top of very high normal food consumption as it eats three times its own weight in food each day! Other winter adaptations include the creation of a lined nest, which helps the shrew conserve heat, caching of food in case of prey shortages, foraging below the leaf litter or snow where the temperature is milder, and decreasing activity levels during cold periods. The caching ability of these shrews is pretty amazing; they definitely display an ant type temperament. One study found it caches 87% of the prey it catches, while 9% is eaten immediately and 4% is left where it was killed (Robinson and Brodie 1982). While venturing off on a tangent and reading more about these creatures, I read that they line their nest with the remains from their favorite food, meadow voles fur! That behavior sounds like the basis for a Grimm’s Fairy tale when compared to gentle rabbits that use their own fur to line warm little dens for their kits. Their voraciousness is only matched by their reproductive tactics, a discussion of which is not appropriate for a family publication. Despite all these tactics, the northern short tailed shrew has a mortality rate of 90% in the winter. So at least they have fun before they go.

This is just a few examples of how animals and plants have managed to conserve their energy and survive the winter. I hope everyone reading this is enjoying spring and ready for Daffodil Festival weekend. Send questions and column ideas to sarah.oktay@umb.edu and come visit the Nantucket Field Station this spring.

Resources used in addition to those listed above: http://weatherspark.com/averages/29566/Nantucket-Massachusetts-United-States

Food Hoarding Behavior in the Short-tailed Shrew Blarina brevicauda Denise E. Robinson and Edmund D. Brodie, Jr. American Midland Naturalist Vol. 108, No. 2 (Oct., 1982), pp. 369-375

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