REPRODUCTIVE BIOLOGY OF NORTH AMERICAN MOOSE

Abstract

An understanding of the reproductive biology of moose (Alces alces) facilitates wise management. Moose are polyestrous cervids with relatively high ovulation rates in adult females. Puberty varies among populations, but no calves are sexually mature. In populations on good range or below carrying capacity, yearling ovulation and pregnancy occurs. The estrous cycle averages 24 days and ranges from 22-28 days. If not bred, moose have up to 6 recurrent estrus cycles. The period of heat when a female will accept the male is short, lasting from numeral 1-36 hours. Gestation length ranges from 216-240 days with a mean of 231 days. Gestation length is not different for single vs. twin litters, or primiparous and pluriparous females. Pregnancy rates in adult moose are remarkably constant averaging about 84%. Twinning rates vary with range quality and may be a good indicator of carrying capacity. Bull moose reach puberty as yearlings. Antler growth is initiated in response to increasing day-length and other endogenous rhythms. High levels of testosterone activate Leydig cells which begin spermatogenesis. By fall, bull moose are ready for the breeding season with hardened antlers and fully developed sperm. Breeding season is relatively short, with >85% of all pregnancies occurring in <10 days. Peak rut occurs in late-September and early-October. Rutting season is relatively constant across North America. Out of season births are rare, but have been reported as late as August. Declining levels of testosterone following the rut are responsible for antler drop in bulls, which occurs from early-December through March. Large bulls tend to shed their antlers earlier than young bulls.