If you're bottle-feeding a bear cub, be very careful when you run out of milk.
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Sean Farley has raised bear cubs and said they get really aggressive when they're nursing. That can mean trouble for the mother bear.
"They'd like to nurse a lot more than she can produce," he said. "If you're bottle feeding a cub, as you run out of milk you need to walk away. They get wild and bite. Sometimes you see those mother bears, they're pretty chewed up."
Farley, a wildlife biologist with the Alaska Department of Fish and Game, also has milked bears. He's studied the needs of growing bear cubs, and their mothers' ability to satisfy those needs.
The peak lactation is happening now, starting in June and continuing this month. At this point, a black bear cub consumes about 30 ounces of milk a day, and a brown bear cub takes about 45 ounces. That's per cub, so a brown bear with three cubs is producing about a gallon of milk a day - a difficult metabolic feat to sustain.
Another feat can seem even more difficult. Bear cubs are born in winter, and the mother bear nurses them in her sleep as they are hibernating.
This was not what Farley was expecting.
"That she can lactate while not eating or drinking is pretty phenomenal - nobody else does that," said Farley, a wildlife biologist with the Alaska Department of Fish and Game. "Everything comes from what she's got stored in her body. We thought it would be a significant drain on her."
But the source of the drain was found elsewhere, Farley said.
"What we found was interesting. The big drain on her comes when she's first out of the den in the spring and there's not a lot to eat." The cubs are much more active, and they're growing fast.
Bears mate in the spring, but don't get pregnant. That fertilized egg does not develop during the summer. It's implanted in the uterus in the fall, as the mother bear goes into the den for the winter. It's counterintuitive - just as she's ramping down into a dormant state, she has within her an embryo beginning a rapid-growth state.
Some other animals, such as weasels and bats, are capable of delayed implantation. But they use it to avoid the demands of pregnancy during the lean months of winter.
"Most hibernating animals aren't pregnant," said state wildlife biologist John Hechtel. "But an adult female goes into the den pregnant. She's pregnant, gives birth and lactates all in the den."
The cub is born after just a few months, blind, hairless, helpless, and weighing less than a pound. An eight-pound human baby is about seven percent the weight of a 120-pound mother. The newborn cub is just a fraction of one percent the mother bear's weight.
"It's almost an external pregnancy - the cub is born and then migrates to the teats and nurses," Hechtel said. "The size of the cub in the spring when it comes out of den is closer to what you'd expect to see at birth."
Farley said it's almost as if they are born premature. "They don't even look like the cubs you see when they come out of den."
A first-time mother bear usually has just a single cub, and with subsequent pregnancies generally will have twins. Nutrition plays a key role in litter size.
A mother bear has six nipples. The newborn cub (or cubs) crawls to the nipples closest to her pelvis to nurse. Later, as the cubs get older, they nurse from the top four and the mother often "switches off" production in the bottom two.
At birth a black bear cub is about three-quarters of a pound, and when they emerge from the den black bear cubs average about six pounds. Brown bear cubs are about twice that weight at each stage.
While the cubs are in the den those first five months, they consume less than ten percent of their total yearly milk intake. The mother's milk production is four times greater after they emerge from the den.
As might be expected, the cubs also have a growth spurt their first summer. At about six months, cub growth takes off like a rocket, Farley said.
Bear milk is extremely rich, with a fat content around 20 percent. Human milk is comparable to cows' milk, generally ranging between three and five percent fat. An equal volume of brown bear milk provides three times the calories to the cub as human milk does to a baby.
At one point when bear researcher LaVern Beier was helping Farley with his work he took the opportunity to taste bear milk.
"It was sweet, like that Eagle brand canned condensed milk," he said.
Farley emphasized that the nutritional components and values change considerably over the year, depending on what lactation stage the mother is in. Nutrients include sugar, carbohydrates, protein, lipids, water, fat and what's referred to as the ash or mineral content. They all come together to give a caloric value to the cub.
"In the den the carbohydrate composition is relatively high compared to when she comes out of the den, and the lipid and water values change - they're not a constant value throughout lactation cycle," he said.
As the cub grows during its first summer, its needs outpace the mother's ability to produce. She directs them to more external foods such as plants, berries and fish. She provides less and less milk to the cub or cubs, but she's a long way from weaning them.
"She'll nurse them for a couple years," Farley said. "There's a couple schools of thought regarding that - it's a strong behavioral bond between mom and cub, she can keep them close to her, protect, then, and train them. The other possibility is that there may be some critical micronutrients or fatty acids she can provide for them. I suspect the behavioral bond more important."
Riley Woodford is the editor of Alaska Fish and Wildlife News, the online publication of the Alaska Department of Fish and Game, and he produces the "Sounds Wild" radio program.