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. 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.
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. 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. 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.