Mammals need to use more energy in winter than any other season to keep warm. Food is less plentiful and there are fewer daylight hours for hunting. Plants are also not producing the nourishing green shoots required for food and there are no flying insects around. For a mammal to survive to spring, they have to be fit and strong. They also have to be able to adapt to the cold weather.
There are a number of ways that mammals survive winter. Some will migrate or move to a warmer, more hospitable climate. Others will hibernate, sleeping through the winter in order to miss the harsh weather. Others will use the autumn months to fatten up and put aside enough food to get through the winter.
Unfortunately, many mammals die in winter, especially smaller species, as they are unable to make it through the month of severe winter with a shortage of food.
In this article I wanted to look at how mammals survive winter.
As bats are the only mammal that can fly, they have the ability to migrate in winter. However, as this takes up a lot of energy they usually sleep through the winter. At the end of autumn, bats will have fed well on flying insects and build up a layer of fat that covers its body. By building this fat up they have the necessary energy reserves to support a lower metabolism.
Bats will roost from October to April and will hibernate in buildings, caves, mines, or other structures. They hang upside down from the wall or the roof with their wings folded around them to keep them warm.
Hibernating bats save a lot of energy, with their breathing using about one-hundredth of their normal oxygen usage, and their heartbeat lowering from 200-300 to just 10 beats per minute.
Bats enter a state of torpor rather than full hibernation. If the temperature reaches 10 degrees celsius then they may wake up and fly.
Squirrels are able to live through severe weather but only for short periods. Although they do not hibernate, they do enter a lethargic state. Squirrels will stay in this state when there is heavy rain, strong winds, and mist.
Although most mammals live in the same territory for their entire lives, some whales make long migrations to warmer climates. The migration of whales is linked to two things; breeding and feeding.
Humpback whales will travel from their feeding grounds in Alaska to Hawaii in the winter to breed, before traveling back to Alaska in the summer to feed. Gray whales travel to the warmer waters of Baja California in December before returning to the Bering sea in April.
For mammals that do not migrate or hibernate, their chances of survival through winter will depend on a variety of factors. One of the characteristics of mammals is that they have fur or hair. The hair can be used to insulate the body by being fluffed out, creating an insulating layer of air. Fat layers built up in autumn act as a shield against the cold temperatures.
Many species of mammals will hoard food from the previous season as a precautionary measure against the days or weeks when the snow and rain make it impossible to leave their burrows. A stocked larder can also help them to slow down their bodies and metabolism as they do not have to forage for food.
The American badger will spend more time in its sett in winter. The badger builds up a layer of fat from feeding in autumn and will enter a state of torpor which can last up to 29 hours. The badger may not emerge from its sett for several days at a time.
The red fox is an opportunist and can be seen in winter at almost any time of the day or night. However, in the other seasons, the red fox can be mainly seen at dusk.
Many food sources are severely limited in winter. Plants, fruits, berries, and seeds are in great demand by many woodland mammals and can be completely hidden by snow in winter.
Deer will move down from mountainous regions in winter to lower ground where the land is more sheltered from snow and high winds. They may also move into denser parts of woodlands during the colder months for protection from the elements.
Seeds are essential food for the ground-feeders such as small rodents. Many small rodents will set aside a store of seeds, berries, and roots to survive the winter in their burrows.
Moles will also put by a store of food to survive the winter months. Moles must consume a lot of food to survive, eating up to their own body weight in 24 hours. Moles generally survive on a diet of insects, but in winter earthworms make up about 90% of their diet. After a frost earthworms cannot move through the earth and so the mole is able to injure the front of the worm and store it for later inside its home which is called a fortress.
Without enough food to survive and enough energy to keep warm many mammals do not survive winter. Small mammals such as shrews, mice, voles, and rabbits can have especially high death rates in winter.
Very few small mammals survive more than one winter. Shrews generally die in the autumn, leaving their young to survive through the winter and through to the next breeding season between April and September. Shrews live for about 14 to 16 months, surviving the winter after they are born.
A lot of plants die off during winter as their leaves cannot live through severe conditions. Once their leaves are shed they do not provide cover for any animals that used them before.
This is one of the many reasons that small mammals do not survive through the winter as there is a lack of cover. This exposes them to predators such as owls. Owls thrive in winter as they can kill large numbers of small mammals such as voles and mice due to the lack of plant cover.
The vegetation will not grow again until March at the earliest, so deer and rabbits have an extremely hard time in winter as any vegetation that has already been grazed will not grow back.
This is extremely hazardous for male deer as they have just exhausted themselves from the mating season known as the rut. With a lack of food, sometimes the most dominant stags in the rut do not make it through the following winter.
Bighorn sheep survive winter but winter can still interrupt their growth causing growth lines. Growth lines can be seen in their horns, similar to the growth rings in trees. This is one way to age some animals with horns.