Reproductive performance of female Alaskan caribou

We examined the reproductive performance of female caribou (Rangjfer tarandus granti) in relation to age, physical condition, and reproductive experience for 9 consecutive years (1987-95) at Denali National Park, Alaska, during a period of wide variation in winter snowfall. Caribou in Denali differed from other cervid populations where reproductive performance has been investigated, because they occur at low densities (less than or equal to 0.3/km(2)) and experience high losses of young to predation. Females first gave birth at 2-6 years old; 56% of these females were 3 years old. Average annual natality rates increased from 27% for 2-year-olds to 100% for 7-year-olds, remained high for 7-13-year-olds (98%), and then declined for females greater than or equal to 14 years old. Females greater than or equal to 2 years old that failed to reproduce were primarily sexually immature (76%). Reproductive pauses of sexually mature females occurred predominantly in young (3-6 yr old) and old (greater than or equal to 14 yr old) females. Natality increased with body mass for 10-month-old females weighed 6 months prior to the autumn breeding season (P = 0.007), and for females >1 year old and weighed during autumn (late Sep-early Nov; P = 0.003). Natality for 2-, 3-, 4-, and 6-year-olds declined with increasing late-winter snowfall (Feb-May; P less than or equal to 0.039) during the winter prior to breeding. In most years, a high percentage of sexually mature females reproduced, and lactation status at die time of breeding did not influence productivity the following year. However, following particularly high snowfall during February-September 1992, productivity was reduced in 1993 for cows successfully rearing calves to autumn the precious year. High losses of calves to predators in 1992 may have increased productivity in 1993. Losses of young-of-the year to predation prior to the annual breeding season can be an important influence on subsequent productivity for ungulate populations where productivity varies with lactation status of females at die time of breeding.