Motherhood is not a simple state of affairs for cows in the Porcupine Caribou Herd.

These animals live in a demanding environment in which summers are brief, winters are long and cold, and caribou must migrate over huge distances to find enough to eat through the seasons.

Survival can require making hard choices; and for a cow caribou, that can mean weaning her calf at a less than ideal time.

Sometimes cows will put their own well-being first, and wean their calves early. Other times a cow will continue nursing her calf longer than normal, a decision which can take its toll on the mother.

Don Russell with the Canadian Wildlife Service and Bob White, a researcher at the University of Alaska in Fairbanks, studied weaning in the Porcupine Caribou Herd for six years. They used their findings to develop a model showing the different ways in which arctic caribou will wean their young.

The biologists want to establish the weaning patterns that are normal for caribou. Then, if global warming or industrial development bring changes to the North, they can use the model to help predict how the survival of the caribou could be affected.

During a normal year -- when the grazing is good, the mosquitoes are tolerable, and disease is not rampant among the herd -- caribou cows will nurse their calves through the summer, weaning them at the time of the fall rut.

But when times are tough, caribou cows can compensate by weaning their calves either earlier or later than normal, and the main key to the timing seems to be protein and fat.

"Cows are generally selfish in terms of putting on protein," says Russell. He explains that once the caribou have ingested enough calories to satisfy their basic needs, any extra energy goes towards growth and reproduction.

"It seems like the first chunk of excess energy goes to depositing protein for the cow, then to producing milk for the calf and then fat for the cow," says Russell.

During the study, biologists captured and measured caribou and their calves at different times, starting in early summer. They studied both cows with and without calves. In the fall, winter and spring they would recapture as many of the radio-collared animals as possible.

The caribou were caught with net guns shot from helicopters. The net is shot over the front half of the chosen caribou, trapping its antlers and front feet and slowing the animal down so that it can be restrained.

The researchers then weighed the caribou and determined its body fat by feeling how much fat covered the animal's hips, withers and ribs.

They also took milk samples to determine whether the caribou was lactating and how much fat and protein was in the milk.

June is a critical month for both caribou cows and calves, as studies have shown that there is a direct link between the amount of fresh green growth available on the calving grounds and the percentage of calves that survive their first month of life.

If a cow weans her calf during June, the calf will die, but the mother will probably have a better chance of getting pregnant again in the fall.

This scenario also holds true for cows that wean their calves in July because the mothers are not packing on enough protein. In summer, cows normally add 30 grams (dry weight) of protein per day to their body weight, whether they are lactating or not.

The herd is moving constantly by this time and there is usually plenty of food available, so the researchers assume that accidents or disease prevent the cows from gaining enough protein.

If both cow and calf survive into the fall, fat becomes the key variable. Cows that have not fattened up enough by the end of summer will probably wean their calves in early September, reducing the calves' chances of survival.

Normal weaning takes place in early October, but sometimes undernourished calves will continue nursing through the winter. This "extended lactation," while not uncommon in the Porcupine Caribou Herd, lowers the chance that a cow will get pregnant.

"If a calf is particularly small it might nurse more aggressively," says Russell. While these calves usually survive the winter well, their mothers are usually weakened by the demands of the extra nursing.

Caribou have lived in the North for more than a million years, so they have had ample time to fine-tune these different strategies for survival. Russell and White hope that their research will help them predict how changes could affect the herd in the future.

"We are saying that this is the natural system; but if you superimpose industrial development or climate change, then nutritionally the animals will suffer, and the number of caribou that are forced to make the 'hard choices' will increase," says Russell.

For more information contact Canadian Wildlife Service at 393-6700. Additional information on the Porcupine Caribou Herd is also available at in the caribou section of Taiga Net.