Mammals need more energy in winter than in other seasons 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 several ways that mammals survive in winter. Some will migrate or move to a warmer, more hospitable climate. Others will hibernate, sleeping through the winter to miss the harsh weather. Others will fatten and put aside enough food for winter in the autumn months.
Unfortunately, many mammals die in winter, especially smaller species, as they cannot make it through the month of severe winter with a food shortage.
In this article, I wanted to look at how mammals survive winter.
As bats are the only mammal that can fly, they can migrate in winter. However, as this takes up a lot of energy, they usually sleep in the winter. At the end of autumn, bats will have fed well on flying insects and built a layer of fat covering its body. Building this fat up gives them the necessary energy reserves to support a lower metabolism.
Bats roost from October to April and 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, breathing using about one-hundredth of their normal oxygen usage and their heartbeat lowering from 200-300 to just ten beats per minute.
Bats enter a state of torpor rather than total hibernation. If the temperature reaches 10 degrees celsius, they may wake up and fly.
Squirrels can live through severe weather but only for short periods. Although they do not hibernate, they do enter an idle 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 lives, some whales migrate 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 returning 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.
Have you ever wondered how fish live in freezing winter waters? Please find out more in this article I wrote.
For mammals that do not migrate or hibernate, their chances of survival through winter will depend on various factors. One of the characteristics of mammals is that they have fur or hair. The hair can insulate the body by being fluffed out, creating an insulating layer of air—fat layers built up in autumn act as a shield against cold temperatures.
Many mammals hoard food from the previous season as a precaution against the days or weeks when the snow and rain make it impossible to leave their burrows. Stocked larders can also help them 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, lasting up to 29 hours. The badger may not emerge from its sett for several days.
Many food sources are severely limited in winter. Plants, fruits, berries, and seeds are in great demand by many woodland mammals and can be entirely 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 be put in a store of food to survive the winter months. Moles must consume a lot of food to survive, eating up to their body weight in 24 hours. Moles generally survive on insects, but earthworms make up about 90% of their diet in winter. After a frost, earthworms cannot move through the earth, so the mole can injure the front of the worm and store it later inside its home, which is called a fortress.
Moles live their lives in a secret underground. Please find out more in this article I wrote.
Without enough food and energy to keep warm, many mammals do not survive winter. Small mammals such as shrews, mice, voles, and rabbits can have high death rates in winter.
Very few tiny mammals survive more than one winter. Shrews generally die in the autumn, leaving their young to survive through the winter and the next breeding season between April and September. Shrews live for about 14 to 16 months, surviving the winter after they are born.
Many 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 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 a tough time in winter, as any foliage that has already been grazed will not grow back.
This is highly hazardous to 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 track 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 tree growth rings, one way to age some animals with horns.