For anyone who loves Polar Bears, seeing a mother bear and her cubs is a very special and endearing treat. Polar Bear cubs are undeniably cute, and little ones, struggling to keep up with mom as she strides across a snow field or weaves around pools of frigid water on a melting ice sheet, can tug at your heart. For the mother bear, parenthood is a daunting challenge, as the bear must not only find food for herself and her growing young, but must also remain vigilant for a male bear that might kill her cubs which would, in time, free her to mate with a male much sooner than the average two-and-a-half-year span involved in her raising cubs.
Pregnant Polar Bears face the greatest challenges. In some portions of their range, a female may have nothing to eat from the time the sea ice melts and deposits her on land, as early as late June in some locales, until she emerges from her den and returns to the winter sea ice in mid-March. That may entail a fast of as long as eight months. In order to survive that period, when she has to have enough stored fat to produce milk and nurture new cubs, she must put on hundreds of pounds of fat beforehand. Females normally stay with their cubs until they are approximately two-and-a-half years old, and at least during their second year together the nearly adult-size cubs can contribute to the family’s success by occasionally catching a seal.
In the spring of the cubs’ third year, their mother or a male who attempts to court her drives off her cubs, which may remain together for months before they separate to hunt the ice alone. Should the female successfully mate, her fertilized egg will not implant itself into the uterus lining until October or November, when development resumes. This delayed implantation and subsequent development of the fertilized egg results in the birth of a one- to one-and-a-half-pound cub sixty days later, in the dead of winter. The cub is tiny compared to the size of its mother and very undeveloped. However, by the time the cub and its mother emerge from the den, the cub will be large enough, at twenty to thirty-five pounds, to follow their mother to the sea ice. For a male cub, entering the ice for the first time may be the very last time he ever sets foot on land. For the next two years, he and his mother may hunt the pack ice, and he may continue to do so throughout his life.
ON THE ALERT
A mother Polar Bear stands alert and on the lookout for any bear that might be attracted by the smell of blood. A male Polar Bear might not only steal her hard-won meal, but also kill her cub.
MESSY EATER
Although this cub is only five or six months old, it is already eating meat. Cubs may remain with their mother for over two years, and may still nurse when they’re nearly as large as their mother.
BUMP!
A mother bear pauses in mid-stride, causing a collision with her cub that was following behind. Cubs of the year, often termed COYs, almost always follow behind their mother, while cubs in their second or third year often travel ahead.
NEW MOTHER
Photo: Katherine Pierce/CureUs Designs
Litter size varies, usually between one and three cubs. Bears with their first litters usually have one cub, as do older mothers who are nearing the end of their reproductive life. Single cubs, however, generally are larger and heavier than cubs that are twins or triplets.
“BEARS WITH THEIR FIRST LITTERS USUALLY HAVE ONE CUB, AS DO OLDER MOTHERS WHO ARE NEARING THE END OF THEIR REPRODUCTIVE LIFE.”
TWINS
Photo: Katherine Pierce/CureUs Designs
The average litter size of the Polar Bear is two. In litters composed of both sexes, male cubs grow faster than the females, and the size difference is obvious in only a few months. At two years of age, a male cub may be nearly as large as his mother.
TRIPLETS
Photo: Katherine Pierce/CureUs Designs
Mother Polar Bears in their prime may have three cubs, although it is likely that not all will survive. Prior to the 1990s, litters of triplets were not uncommon in western Hudson Bay, averaging about 12 percent of the cubs produced each year. This is no longer true, as climate change has shortened the seal-hunting season, resulting in pregnant females entering their dens weighing less than they did a few decades ago.
BIRTHING DEN
Photo: Adam Rheborg
In Svalbard, Polar Bear mothers typically den in snowbanks on mountain slopes where the cubs are less likely to be threatened by wandering male bears. Male bears will kill cubs, freeing a female to mate again, as nursing mothers reject the advances of an amorous male.
A CHURCHILL DEN
Photo: Katherine Pierce/CureUs Designs
In Wapsuk National Park, Manitoba, Polar Bears often make their nursery dens thirty or more miles from the Hudson Bay, their hunting grounds. Here, bears excavate cave-like dens in the permafrost sometime during the fall. In winter, these dens are snow covered and well insulated.
DENNING UP
Photo: Richard Day
Polar Bears give birth in late December or January. In denning areas like Wapsuk, a mother bear may have fasted since mid-July, when the last ice melts in Hudson Bay. She will not eat until she leaves the den with her cubs, usually in March. For some bears, that is a fast of eight months!
WEIGHT LOSS
Photo: Katherine Pierce/CureUs Designs
A mother bear may lose over 50 percent of her body weight between mid-July and March, when she leaves the den with her cubs, as she will fast the entire time she is denning. Cubs born to thin mothers may not survive, dying while still in the den or during the arduous march across the tundra to the sea where their mother will hunt once again.
WAIT FOR ME!
Photo: Katherine Pierce/CureUs Designs
A mother bear and her cubs may remain several days around the birth den, giving the cubs time to strengthen muscles and become accustomed to the cold. Cubs may hitch a ride for a short time, but in their journey to the sea ice, cubs will travel under their own power. In Wapsuk, dens may be thirty miles from Hudson Bay.
“A MOTHER BEAR AND HER CUBS MAY REMAIN SEVERAL DAYS AROUND THE BIRTH DEN, GIVING THE CUBS TIME TO STRENGTHEN MUSCLES AND BECOME ACCUSTOMED TO THE COLD.”
KEY TO SUCCESS
Photo: Katherine Pierce/CureUs Designs
A fat, pregnant Polar Bear has the best chance of successfully raising her cubs through the first year of life. Pregnant females may gain as much as triple their body weight, giving the mother enough energy not only to give birth to as many as three cubs, but to successfully nurse the cubs for as long as three or four months before she has her first meal.
TINY BABIES
Polar Bear cubs average about one-and-a-half pounds at birth, perhaps the greatest difference in size between a newborn and an adult mammal. A large male Polar Bear may exceed 1,500 pounds—an increase of 1000 times over its birth weight!
DELAYED IMPLANTATION
Polar Bears mate in spring, between late April and early June, but the active gestation period for the bear is only about two months long. After mating, the fertilized egg divides a few times and then stops until the bear enters her den when the egg, now known as a blastocyst, implants into the mother’s uterus and continues to develop. This process is known as delayed implantation and occurs in seals as well.
VULNERABLE TO THE COLD
Photo: Katherine Pierce/CureUs Designs
Young Polar Bear cubs are very vulnerable to the cold. The surface area of their body is great compared to their actual body mass, so heat can quickly radiate away, especially if a cub is wet. Adult bears, in contrast, have a much greater body mass in relation to their surface area, and consequently are less vulnerable to the cold.
THE LONG MARCH
Photo: Katherine Pierce/CureUs Designs
In Wapsuk National Park, a mother and her cubs may travel for miles as she returns to Hudson Bay and the sea ice where she will resume hunting seals. Thin mothers produce thin cubs, and if her milk production is low, her cubs may die along the journey. Since the duration of the sea ice in Hudson Bay has shortened, pregnant females weigh less entering their dens in fall than they did thirty years ago. Litters of triplets are now quite uncommon.
HITCHING A RIDE
Photo: Katherine Pierce/CureUs Designs Polar Bear cubs occasionally ride on their mother’s back as she travels through the snow, but young cubs often do so when their mother swims through polar seas. Young cubs lack the insulation needed when immersed in frigid seas, and mothers avoid swimming when they can.
STAYING DRY
Photo: Sue Altenburg
Two COYs try their best to stay dry as they follow their mother, hunting Ringed Seals across a landscape of ice floes. On a sunny day, a wet cub will dry off quickly by shaking its fur and rubbing against the snow.
FIRST SWIMS
Photo: Hugh Rose/hughrosephotography.com
By late summer, the polar seas are as warm as they’ll ever be and COYs are big enough to tolerate the cold, either to play or to follow their mother if she swims among ice floes. With the expanse of polar ice shrinking each year, there is more danger of big waves developing in the open water when storms develop—a condition cubs cannot survive.
PLAY TIME
Photos: Hugh Rose/hughrosephotography.com
Bears seem to be naturally playful, and this is most evident when a mother bear and cub take to the water. Older cubs may spend hours in shallow water, wrestling with one another or their mother, at least while her patience lasts.
DANGEROUS JOURNEY
A mother bear marooned on land in Svalbard faces the possibility of an enforced fast for months. If ice floes are nearby, she may hunt for Bearded Seals, swimming from floe to floe with her cub following behind. Swimming burns calories, both for the mother and her cub, that may be needed to sustain both until freeze-up in the fall when the bear can hunt for seals more effectively.
BARREN GROUND
A bear can survive the three months or so she is marooned on land before the ice returns, and a fat mother may be able to nurse her cub during this time as well. Adult bears can undergo a walking hibernation physiological state during which their metabolism slows, even though they are still moving about and nursing their cubs—provided she has enough stored fat and energy to nurse!
TRACKS
Converging Polar Bear tracks may indicate where a mother bear and her nearly full-grown cub reunited or perhaps where an amorous male caught the scent of a female he is now following. Bears can easily travel twenty miles in a day, measured as a straight line from one point to another, but may actually cover three times that distance as they meander through the ice, hunting seals.
FALL APPROACHES
Photo: Katherine Pierce/CureUs Designs
In Churchill, Manitoba, winter ice usually returns by mid-November. This mother’s cubs are now nearly half her size and will remain with her throughout the next year.
PROTECTIVE EYES
Photo: Richard Day
A mother Polar Bear must always remain vigilant. Not only will male bears kill cubs to provide themselves with mating opportunities, but they also will cannibalize other bears when food is scarce. Adult females will attempt to intimidate threatening males, giving her young time to run off to safety, only to return after the danger departs.
NURSING
Photo: Hugh Rose/hughrosephotography.com
This mother Polar Bear nurses her young on the flat fast ice on the northern coast of Alaska. Bears may nurse either by sitting down like this female, or by lying down on her back. Female Polar Bears have four nipples, giving the average litter of two cubs two teats to nurse from.
NURSING BED
A mother Polar Bear may excavate a depression in a snow bank to make a comfortable back rest for herself while she nurses. A single cub, like this one, has a greater chance of survival, as the cub has access to all of its mother’s milk. Single cubs are typically larger than cubs of the same age from litters of two or three.
WHICH ONE IS MOM?
Photos: Hugh Rose/hughrosephotography.com
A mother Polar Bear may nurse her cubs until she finally drives them away for a life of their own, usually when the cubs are two-and-a-half years old. Cubs may be as large as their mother, and only when these two nursed was it readily apparent which one was the mother.
CLOSE TIES
Photo: Tom Wester
While the mother Polar Bear is likely to make most of the kills, her two-year-old cubs are now sufficiently schooled that they can hunt on their own, although with limited success. When the cubs are finally driven off, they will face their greatest challenge as they learn to provide for themselves. In areas where seals are common, these subadults may survive by scavenging the kills of large males that eat only the blubber of their seal kills and leave the rest behind.
SPRING HUNTING
Photo: Tom Wester
This mother Polar Bear and cub hunt the pack ice in spring, searching for the nursery den of Ringed Seals. In spring, when hunting is easiest, survival is still a challenge, and a bear averages about one seal per every three days of hunting. Pups, surprised in their den, are the most frequently captured prey, but provide little nourishment for a mother and cub.
FALL HUNTING
Photo: Richard Day
Bears hunt the smooth and flat fast ice for Ringed Seals at their breathing holes, and by fall, cubs are disciplined enough to stay quiet and still, allowing their mother to ambush a seal should it surface. If the cubs move, either in play or simply from boredom, sounds will travel through the ice and alert the seal to the potential danger.
SHAKING DRY
After a short swim, a mother Polar Bear shakes herself dry while her cub looks on.
PLAY DAY
Photo: Hugh Rose/hughrosephotography.com
Well fed from a summer feasting on the remains of a butchered Bowhead Whale, this cub and mother have enough energy to play in the last days of summer before a new season of hunting.
LONG SHADOWS
Photo: Richard Day
As freeze-up occurs along the southwestern shore of Hudson Bay, a mother Polar Bear and her cub head out onto the ice. Located below the Arctic Circle, the sun still sets during the summer solstice and rises above the horizon during the dead of winter.
DISTINCT POPULATIONS
Photo: Hugh Rose/hughrosephotography.com
A mother bear and her two-year-old cub head out onto the ice toward the Arctic Ocean. Polar Bear scientists have divided the world’s Polar Bears into nineteen subpopulations, like the Barents Sea bears seen here. Most members stay within their own subpopulations, but some bears wander widely, covering thousands of miles in a year.
“POLAR BEAR SCIENTISTS HAVE DIVIDED THE WORLD’S POLAR BEARS INTO 19 SUBPOPULATIONS, LIKE THE BARENTS SEA BEARS SEEN HERE.”