Caribou cows are selfish. That's one of the findings of a study of summer weight gain in two northern caribou herds.

Don Russell, a biologist with Environment Canada in Whitehorse, is involved in the study. He says he assumed at the beginning that cows are altruistic, that they would sacrifice their own well-being in order to provide the best possible start for their calves.

"What we find is that the cow is selfish," he says. The study shows that cows put on weight through the summer at roughly the same rate, whether or not they are producing milk to feed calves.

"That means that, come hell or high water, they're going to put on the protein," says Russell.

Protein is the important factor in summer weight gain. In order to make protein, the vital ingredient in both milk and muscle, the animal must eat food containing nitrogen. In winter, caribou eat lichen, which has virtually no nitrogen, so in summer they have to make up their nitrogen/protein deficit by consuming green plants.

An altruistic cow would turn much of that protein into milk for her calf, and turn the remainder into muscle mass on her own body. However, the study showed that both lactating cows (the cows producing milk) and non-lactating cows in the Porcupine Caribou Herd increased their muscle mass at roughly the same rate, about 30 grams of protein a day.

The conclusion is, says Russell, that summer nutrients go first to producing protein, or muscle, in the cow herself and then to producing milk. If there is nutrient energy left over after those two demands, it goes to producing fat on the cow.

A study of the Central Arctic Herd in Alaska showed similar results. Although body protein wasn't measured in animals from the Central Arctic Herd, body weight measurements showed that the caribou cows tend to build up their weight to about the same level by the end of the summer, whether or not they are feeding calves.

One of the reasons behind the study, says Russell, was to determine what impact disturbance in their summer range might have on caribou. The Central Arctic Herd spends its summers in an area with considerable oil development activity. In contrast, the Porcupine Caribou Herd's summer range is largely undisturbed.

The fact that the cows are selfish, that summer nutrients go for their use first and the calves' use second, means that if feeding is disrupted the calves are likely to suffer, Russell says. He adds that the so-called "selfish cow" is a strategy that benefits the herd as a whole since strong adult animals are more important than calves in the herd's long-term survival.

The study turned up some differences between the Central Arctic Herd and the Porcupine Caribou Herd. By the end of the summer, the cows in the Porcupine Herd are heavier than the cows of the Central Arctic Herd. The non-lactating cows in the Porcupine herd gained roughly the same amount of weight as their counterparts in the Central Arctic Herd, but lactating Porcupine cows gained about twice as much weight as lactating cows in the other herd.

It's too early to tell, says Russell, whether Central Arctic cows with calves gain less weight because of human disturbance. There are several possible reasons for the discrepancy: the range of the Porcupine herd might provide better nutrition; climate change might be affecting the amount and quality of green feed available; or human disturbance might have pushed the Central Arctic animals into less plentiful pastures.

An Alaskan researcher is currently using satellite images to assess the range quality in the core calving areas of both herds, Russell says, and studies on the herds' use of their fall range are also planned. Researchers hope eventually to determine what accounts for the differences between the two herds and how much impact human activity has on caribou.

For more information about caribou and caribou research, contact Environment Canada in Whitehorse.