A Kodiak bear cub fetus develops for only two-and-one-half months, so the cubs are very underdeveloped when they are born. No other mammals except marsupials have such immature offspring at birth. The cubs weigh 1/400 to 1/1000 of what they will weigh as adults. If the same were true for humans, a grown man might weigh as much as 8000 lbs. Cubs are born at such a premature stage of development because the mother must provide nutrients for her unborn young while she is in hibernation and not eating. She provides these nutrients by breaking down her body protein, which causes her to lose muscle mass. If she carried the cubs longer, she would lose too much muscle mass and would not be able to move by the end of hibernation. While a shorter gestation period produces underdeveloped cubs, the mother maintains enough physical strength to be able to care for her offspring. Cubs continue to develop after they are born.
At birth, brown bear cubs are nearly helpless. They can detect temperature changes and move closer to their mother to seek warmth, and they are also able to find the sow’s nipples to nurse. They weigh about one pound (.5 kg) and are blind, deaf, and unable to smell. They are covered by a fine hair and are toothless, weak, and uncoordinated.
A brown bear sow has three pairs of nipples. She may nurse on her side in the den but normally nurses in a sitting or partially-reclined position after emerging from the den. A bear’s milk contains an average of 33% fat, as compared to human milk which contains 3.5% fat. Bear’s milk consists of 11 to 15% protein and 0.3 to 0.6% carbohydrates. Due to this diet of rich milk, brown bear cubs grow rapidly.
A cub’s eyes open about four weeks after birth, and he begins to walk at six weeks. A Kodiak bear cub’s weight doubles every two months during the first year. The cubs are completely dependent on nursing for 24 weeks and may continue to nurse for as long as 82 weeks.
On Kodiak, most cubs stay with their mothers for three years, and nearly half of all Kodiak bear cubs die before they leave their mothers. Causes of death range from starvation, accidental separation from their mother, deliberate abandonment by their mother, fights with other bears, accidents, and infanticide, most often, but not always, by large boars.
Researchers have postulated that the reason for infanticide, the killing of the young of one’s species, is so the male can eliminate the offspring of another male, bring the female into estrus, mate with her, and pass along his genes to the next generation. While this theory might prove true for some species, it doesn’t make sense for bears. For one thing, bears are seasonal breeders in the spring, but boars often kill cubs in the summer when the female cannot go into estrus again. During the mating season, a boar would have to kill all the cubs in the litter for the mother to go into estrus, and even then, the female would not become sexually receptive for several weeks. Unless he waited around for the sow to be ready to mate again, the boar who killed the cubs probably would not be the bear who eventually mates with the mother. Also, females are sometimes the perpetrators of infanticide.
A friend of mine saw a large male bear walk up to a den, stick his head in the den, pull out a cub, shake it to death, and continue on his way. It is difficult to coordinate the actions of that boar with any biological theory. We humans often feel the need to understand the purpose behind every animal behavior, yet we do many things with little or no purpose in mind.
Next week I’ll write more about bear biology and behavior. Please let me know if you have any questions or comments.
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