Sheep reproduce sexually and, without human intervention, these animals only breed during certain times of the year. For instance, the mating season for bighorn sheep usually starts from November and ends in December, with gestation periods lasting between 5 and 6 months.
Ewes generally reach sexual maturity at the age of 5 to 12 months, although factors such as breed, birth season and diet may influence the age of puberty. During this time, female sheep exhibit estrus, or heat, which makes them receptive for mating. Around 15 to 30 ewes can be mated with one matured ram and 25 to 50 ewes can breed with a 1-year old to 5-year old rams.
Fertility in ewes typically peaks in September, October and November. Ewes that successfully mate within this period are more likely to give birth to several offspring. The bighorn sheep commonly bear a single lamb per pregnancy.