In the wild, caribou and reindeer have a very short breeding season. An entire herd may calve in about a ten day period. This is thought to be a strategy to ensure survival of a maximum number of calves: the sudden mass of young animals overwhelms predators that follow the herd and results in a good portion of the calves surviving. However, in captivity, the calving season is not as tightly synchronized and females managed without males may cycle several times during the fall and early winter.
Reindeer are seasonal breeders. The breeding season, or rut, normally begins in early September, lasting 3 to 4 weeks. The gestation period is about 7 months (210 – 220 days). As a result of better nutrition and a change in climate south of the arctic, some of the reindeer on farms start to breed in late August, with calving season beginning in April. An earlier, successful rut requires good summer and fall nutrition to allow fattening of the animals and a tranquil atmosphere.
Bulls tend to separate into smaller herds over the summer to forage. They return to the main herd in preparation for the rut. Their testicles and epidymus increase considerably in size as the rut approaches. Prior to the rut, they loose the velvet from their antlers, their necks thicken, their stomachs draw in and they grow a mane. They become restless, start chasing after cows and testing their strength in short, ferocious fights. During the rut they may fight until one is killed. Yearling bulls may actually keep after a female until they kill her by pawing with the front feet every time she lies down. Pens of bulls with no females do not fight much. Bulls will try to fight with other bulls and the fences can be destroyed unless they are separated by distance from pens of females.
Bulls start breeding at one and a half years old and will remain fertile up to ten years. They do not eat much during the rut and as a result, become thin and may lose up to 1/3 of their body weight. Half a dozen bulls should be efficient to service 100 cows and once bred, bulls no longer pay attention to them. At the onset of the rut, the bulls begin herding smaller groups of cows and will chase small bulls away. The highest sexual activity of the bulls lasts only 10 to 22 days for a total rut period of 25 to 30 days.
Females reach reproductive maturity at one and a half years and also remain fertile for 10 to 15 years. Weak, starved or underdeveloped cows don’t come into heat. The onset of heat depends on body conditions and well nourished, content cows will come into heat earlier. Estrus occurs twice, seldom three times. Reindeer cows in heat are less apparent that other farm animals. Females may mount other reindeer and are more restless than usual. Each heat period last 12 to 24 hours and if the cow doesn’t become fertilized during the first cycle, the heat will return after 11 to 20 days and another breeding can take place. Females never let males approach for repeat mounting if the first one is successful. Some females will conceive as calves, but they usually do poorly in future years and do not serve well as mothers.