Caribou in Denali

By Layne Adams (last updated April, 2016)
a caribou walks across a dirt road

NPS Photo / Lian Law

Population dynamics of the Denali Caribou Herd have been investigated continuously at Denali National Park and Preserve since 1984.  The long-term goal of this research is to document population trends, primary vital rates (calf production, calf recruitment, adult female survival), and other important population characteristics (female age structure, adult sex ratios) that determine the status of the Denali Caribou Herd. In addition, we are investigating the growth and survival of adult males and the  movements and habitat selection of both sexes. This progress report summarizes work conducted during October 2014-September 2015.

graph that shows the caribou populations have increased from the late 1990's to present day
Figure 1. Late September population estimates for the Denali Caribou Herd, 1986-2015 (2015 estimate is preliminary).

Herd Size
The preliminary herd size estimate for September 2015 is 2,780. That number will be adjusted and finalized based on results of the 2016 census. The Denali Herd appears to have increased by about 10% per year since fall of 2013 (Figure 1). During these 2 years, winter snowfalls were light and adult female survival over winter was high (> 98%).

Adult Sex Ratio
In September 2015, it was determined that an adult sex ratio of 47 bulls to 100 cows. Adult sex ratios declined from an average of 56 to 100 during 1984-1989 to a low of 29:100 during 1997-98. This was a result of increased mortality of males during severe winters in the late 1980s and early 1990s along with low recruitment of calves. Bull to cow ratios have increased slowly since 1998 but are still below levels at the beginning of the study.

Calf Production and Survival
Productivity of cows that were one year or older was estimated at 76% in mid-May 2015. This is based on 72 radio-collared females in the age-structured sample. The calf production has varied from 59% in 1990 to 92% in 1994. It is primarily influenced by the number of yearlings in the herd, the variable productivity of two-year-olds, and the proportion of females 13 years or older in the herd. During the mid-June 2015 post calving surveys, we noted 39 calves to 100 cows. By the late-September composition count that ratio had declined to 24 to 100. Based on these measures of calf production and survival, and accounting for adult female survival between mid-May and late September, survival to fall was estimated at 30% for 2015 calves. Fall calf survival has averaged 28% over the last 12 years. This is compared to 42% during 1987-1990 and 15% during 1991-2003.

Female Survival and Age Structure
During October 2013-September 2014, we estimated an annual mortality rate of only 4% for adult females. This percentage is lower than the long-term study average of 12% (range of annual values = 2-23%). Females 13 years or older made up 11% of the population. With continued moderate recruitment, the proportion consisting of these older females can be expected to decline over the next few years. This is because the herd is made up of 15-17 year-olds that are nearing the end of their lives and there are few females 12-14 years old to come in behind them. Based on data collected from radio-collared caribou since September 1986, age-specific survival rates of females are high during two-seven years of age, averaging 0.94. The then decline slowly during 8-13 years of age before declining markedly as individuals become senescent. The 2 oldest caribou females we have monitored died in May as they turned an estimated 20 years old.

Bull Growth Patterns
During September 2007-2015, we weighed 195 bulls that were one year or older at their initial captures and recaptures to replace their radio collars. In mid-September, bull caribou should be at their maximum body for the year in preparation for the rut and coming winter. Overall, body weights of males ranged from 205 to 612 lbs. Body weights increased with age from 1 to 6 years, gaining an average of 54 lbs each year, and plateaued at 509 lbs on average for bulls ≥ 6 years of age. Antler size was strongly correlated with body weights, thus, mean antler length showed a similar pattern to body weight, increasing by 5.6 in/year for bulls 1 – 6 years of age, and averaging 48.6 in for bulls ≥ 6 years of age.

Adult Bull Survival
During our studies of bull survival since September 2007, we have noted that age-specific survival rates are high for males 1-4 years-old, averaging 88%. As bulls approach full adult size at 5 years of age and become active in the rut, their survival declines to 78% and that level of survival continue through 6 and 7 years of age. After 7 years, survival drops off markedly each year with very few caribou bulls surviving to 10 years. Interestingly, bulls ≥ 5 years old die predominantly during July – November (83% of annual mortality) with nearly half this mortality (39% of annual mortality) occurring prior to the onset of the rut in mid-September.

Movements and Habitat Selection
During September 2010-2013, we maintained a sample of 20 adult females instrumented with GPS collars that acquired and stored locations every 2 hrs. Those collars were retrieved in September 2013 and we are currently analyzing the mass of data collected. We deployed GPS radio collars on 20 bulls in September 2014 and 19 bulls in September 2015. The bull collars are designed to automatically release after 3 years of data collection in September 2017 or 2018. The resulting data sets for females and males will be used to compare and contrast seasonal movements and fine-scale habitat selection patterns of the 2 sexes.

Last updated: April 20, 2016