Females ovulate at 6-7 years. Between 1950-1975, females ovulated at 4-8 years, but by the late 1980s the age of first ovulation had increased by about 2 years, presumably due to changes in the food supply (Fay 1982). Pacific females first give birth generally at about 10 years; males are sexually mature at 8-10 years but generally do not successfully mate until about 15 years old. Gestation lasts 15-16 months, including 4-5 months before implantation of fertilized egg. One calf (rarely 2) is born April to mid-June (mainly May). Calf is weaned by 2-2.5 years. Females in their prime give birth in alternate years. "Mobile lek" polygynous breeding system (Fay 1982). May live up to at least 40 years.