Estrous Cycles

Reindeer are seasonally polyestrous, short-day breeders, with an estrous cycle length of approximately 24 ± 3.4 days (range: 18–29 days).² The endocrine changes during the estrous cycle have been described in detail and are very similar to other deer species. Primiparous Norwegian reindeer exhibited considerable variation in estrous cycle length (19.4 ± 5.7 days) at the onset of the breeding season.³ This contrasts to our work with reindeer at a similar latitude in Fairbanks, Alaska, where estrous cycle length did not vary significantly from the beginning to the end of the breeding season or among individuals. Seasonal ovarian activity among females penned without a bull is initiated in late August.^2,4 Like other ruminant species, a small transient progesterone rise, lasting between 4 and 9 days, precedes the first full-length estrous cycle of the breeding season, and reindeer may experience two or more short cycles before the onset of full-length cycles.² Even though an estradiol and luteinizing hormone (LH) peak may precede short cycles, these hormonal events have not been clearly linked to estrous behavior. However, the possibility that males may detect these initial endocrine changes and attempt to mount cows at this time could explain the numerous earlier reports of a 10- to 12-day cycle length.

Females left open will continue to cycle well into spring (April), having 6 to 8 estrous cycles over the winter.² The transition to anestrus in five adult females was characterized by either an abrupt cessation of luteal activity in April (n = 2) or the formation of a persistent corpus luteum (CL) (n = 3) beginning in late February or March. Sampling did not continue long enough to evaluate the lifespan of the prolonged CL in two cases, but from previous work, it is known that persistence into the next breeding season can occur.⁴ Further investigation is needed to assess the potential effect of a persistent CL on breeding synchrony and timing of mating.

Antlers and the Onset of Estrus

Reindeer and caribou are the only members of the deer family in which females grow antlers. Although males depend on rising testosterone to harden and clean the antlers, increasing estradiol at the onset of the breeding season is responsible for female antler cleaning.⁵ Anecdotal information suggests that antlers are cleaned 2 to 3 weeks before the onset of estrus, but there is no clear documentation on the interval between antler cleaning and first ovulation. Although preliminary work cannot predictably link antler cleaning with ovulation, polished antlers are a useful criterion for timing harem formation.⁶

Puberty

Recommendations generally suggest that harem cows be at least 1.5 years old or a have a body weight of at least 60 kg. Yearling and adult females initiate the first full-length cycle at approximately the same time,^2,4 suggesting that once pubertal weight is attained, age differences in body weight have little effect on the onset of breeding readiness. Both males and females can reach sexual maturity at 6 to 8 months of age, and there are numerous reports of females producing their first offspring as yearlings. However, calf pregnancies have resulted in reduced postpartum growth rate of the young females, low birth weight calves, and high offspring mortality rates. Although male reindeer can attain physiologic puberty in their first year, their subordinate position in a herd generally precludes them from breeding. If isolated with receptive females though, larger male calves can take over a dominant role and successfully breed.

Bull Introduction

In the absence of a bull, adult cows in good body condition will come into estrus, the timing of which may vary with latitude.² However, introduction of a bull to a group of females before the initiation of estrous cycles significantly hastened the onset of ovarian activity by approximately 2 weeks and resulted in synchronized calving the following spring.⁴

Estrous Behavior and Detection

Behavioral estrus can be subtle, and published descriptions are inconsistent. Various authors report dilation of the preorbital glands and upright position of the tail, especially in response to touching of the perineal region. Redness and swelling of the vulva, accompanied with a mucous discharge, is not always apparent.³ Anecdotal descriptions of cows appearing restless, searching for a bull, and mounting other cows can be found in the husbandry literature. The behavior of the male toward the female is probably the most frequently used indicator of estrus. However, there are problems with consistency and precision here, too. Once-a-day observations or observations collected only during daylight hours may not be frequent enough to detect all instances of mating.

There are few reports on the use of artificial aids for the detection of estrus. Heat mount strips and vasectomized, marked bulls have been occasionally used with variable success. We recently investigated the use of a radiotelemetric estrous detection system in reindeer. In two separate studies, successful detection of estrus and breeding was 70% (n = 10) and 42% (n = 19), respectively. The former study was the only one in which we encountered two false positive mounts, at 18 and 21 days after conception.² Collective radiotelemetric data indicate that individual mounts are swift, lasting 3 to 9 seconds and estrous females are mounted one to three times over a 24-hour period. The brief mount duration coupled with the low frequency of mounting most likely contributed to the poor success rate of radiotelemetric or visual estrus detection. Studies to improve mount detection with radiotelemetry are ongoing.

Estrous Synchronization and Superovulation

Interest in truncating the breeding season and artificial insemination has focused attention on synchronizing estrus. Among captive reindeer in Fairbanks, two 15-mg IM injections of PGF_2α (Lutalyse, Pharmacia and Upjohn Company, Kalamazoo, MI 49001, USA) administered 10 days apart, caused luteolysis in cycling reindeer, while a single 15-mg injection at 6 weeks after conception terminated pregnancies.^6,7 Polished antlers among a group of cows are an indication that most of the females are cycling and, hence, a reasonable cue for initiating a synchronizing protocol dependent on a normal CL. However, it does not preclude the fact that some cows may be experiencing sequential short cycles and fail to respond appropriately to prostaglandin.⁷ In addition, care must be taken to ensure that the full dose is delivered into the muscle and not into the substantial subcutaneous fat. Conception following successful synchronization with PGF_2α was 88% with mounting occurring 44 to 56 hours after the end of treatment.⁷

The use of goat controlled intravaginal drug release (CIDRg: 0.33 g progesterone) is common in Europe, although very little has been published. In Finland, 8 of 10 females conceived following a 14-day treatment regimen with CIDRg devices. Mating occurred 43 hours after CIDR removal. Concurrent superovulation with follicle-stimulating hormone (FSH) resulted in an embryo recovery of only 20%.⁸

Gestation Length

Published estimates of gestation length for reindeer and caribou range from 198 to 240 days. This variability in reported gestation length may be partially explained by the limited reliability of behavioral observations of estrus and breeding, thus producing inaccurate estimates of conception. Our accumulated data, using only individuals with conception dates verified by endocrine data, provide a gestation length of 216.9 ± 6.8 days (n = 39). In one study, gestation length among eight captive reindeer, housed and managed identically throughout gestation, differed by 23 days and conception date was negatively correlated with gestation length (r² = −0.98, P < 0.0001). A Norwegian study has also reported a negative correlation between gestation length and conception date.⁹ More information on the variability in gestation length and its association with calf viability and growth is clearly warranted and our current studies are addressing this phenomenon.