While the bulls are more aggressive during the rutting season and make the decisions in the groups, the females take charge during gestation. Pregnant females are aggressive and decide what distance the herd travels in a day and where they will bed for the night. The herds move more frequently when cows are lactating, to allow them to get enough food to nurse their offspring. Cows have an eight- to nine-month gestation period, with calving occurring from April to June. Cows do not calve every year. When winters are severe, cows will not go into estrus and thus not calve the next year. When calving, cows stay in the herd for protection. Muskox are precocial, and calves are able to keep up with the herd within just a few hours after birth. The calves are welcomed into the herd and nursed for the first two months. After that, a calf then begins eating vegetation and nurses only occasionally. Cows communicate with their calves through braying. The calf's bond with its mother weakens after two years.