Inside the Bear Nursery
This story is featured in Montana Outdoors May-June 2012 issue
You didn’t receive a birth announcement, but near the end of January, as they do every winter, hundreds of pregnant grizzly bears and black bears gave birth in their winter dens.
Each litter averages two cubs, born tiny and helpless like human babies. But that’s where the similarity ends.
A grizzly’s average weight at birth is 1 pound. Black bears average half a pound. The average newborn baby weighs 7.5 pounds.
Cubs are blind and nurse on rich milk that is 20 to 40 percent fat, while babies can see at birth and nurse on human milk with 4 percent fat. The mother bear, still half asleep in partial hibernation, stimulates urination and defecation by licking the cubs’ anal areas (and human mothers think they have it tough).
The mother grizzly wakes from her deep sleep from time to time—to give birth, eat the placenta, and clean up her newborns. Wildlife scientists are still not sure if she ever fully comes out of hibernation until spring.
The cubs never hibernate. Instead, they spend their first winter drinking the mother’s milk and growing.
Right now, in March, mom is still asleep and the cubs are awake.
At emergence, a few months from now, a sow with cubs may have lost 30 to 40 percent of her body weight, while females without cubs may have lost only 15 to 20 percent. Cubs weigh about 8 pounds when they emerge, but will weigh 20 to 30 pounds by summer, depending on their diet.
Spring black bear hunting in most of Montana’s 26 bear management units closes May 31; the rest close May 15. Part of the reason is to help hunters avoid shooting a female with cubs, which is illegal. Females with cubs usually emerge from their dens around the end of May.
Bears do not have a rigid schedule for going into hibernation. Generally, pregnant females are the first to enter their winter dens and adult males are the last to enter theirs.
Denning occurs any time from October through December, depending on weather and availability of food. In years when food is scarce, some bears may den earlier.
When a pregnant bear goes into her den, she is only a little bit pregnant. After bears mate in late spring and early summer, a female’s eggs are fertilized but they do not implant in the uterus. By midsummer the fertilized egg has developed into a multicelled blastocyst (an early stage embryo), but further growth is arrested. The embryo floats freely in the uterus until denning time, later in the fall.
This delayed implantation allows the female bear’s body to assess whether it has sufficient fat reserves to carry, give birth to, and nurse cubs through the sow’s long winter nap.
If fat reserves are present, it’s all systems go. But if a bear can’t gain enough fat, the blastocyst won’t attach to the uterine wall, ensuring that a female in poor condition will not be further stressed by reproduction. Then the bear’s body absorbs the embryo, gaining a bit of nourishment.
Sometimes when comparing humans to other animals, I find it hard to believe that we are the more advanced species. .
Bruce Auchly manages the regional Information and Education Program in Great Falls.
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