Synchronous Mating in Canadian Barren-Ground Caribou

The dates of the 1966 and 1967 mating seasons for the Kaminuriak barren-ground caribou (Rangifer tarandus groenlandicus) population were estimated by examination of the reproductive organs of 129 females collected from Manitoba and the District of Keewatin, N.W.T., in September, Novem- ber, and December. Ovulation and pregnancy were diagnosed from the size and microscopic appearance of ovarian corpora lutea and follicles. Dates of conception were determined by estimating the age of embryos with a growth curve (crown-rump length) predicted from the growth rates of three other spe- cies of Cervidae. Conceptions were highly synchronous with 80 percent of 64 occurring in the first 11 days (19-29 October) of a 4- to 5-week mating season. The females' ages, body weights, and fat measurements were not significantly (P > 0.05) correlated with their conception dates. The syn- chronous occurrence of conceptions, confirmed by observations of rutting behavior and the timing of births in the study and other caribou populations, appears to be important to calf survival.