Timing and synchrony of parturition in Alaskan caribou

Timing of parturition of caribou (Rangifer tarandus) varies within populations, but the relative influences of nutritional condition of females during the autumn breeding season and during gestation on that variation is not known. We determined timing of parturition of caribou in Denali National Park, Alaska, during 1984 - 1995, which had wide variation in snowfall that influenced nutritional condition and productivity of females. The first young were observed each year between 4 and 15 May. Annual median dates of parturition for radiocollared females during 1987 - 1995 varied from 13 to 21 May. Synchrony of births (i.e., proportion of births during 7 days centered on the median date) did not vary significantly among years (P > 0.05). Females less than or equal to 4 years old were more likely to give birth after the annual median date than older females (P = 0.005). Dates of parturition were significantly correlated with masses of females in the previous autumn (P = 0.034) but not in late winter (P = 0.155). Births tended to be earlier for parous females that were not lactating during the previous breeding season compared to those that were lactating (P = 0.030). Annual onsets of parturition-varied significantly with snowfall in late winter prior to conception (P = 0.012) but not with snowfall during gestation. Timing of parturition in our study appeared to be a function of physical condition of breeding females in autumn.