Effects of Male Age and Female Nutritional Condition on Elk Reproduction

Spring calf:cow ratios in some Rocky Mountain elk (Cervus elaphus) populations of northeast Oregon have declined by almost 80% over the last 40 years. Studies have identified the age of breeding males and the nutritional condition of females as potentially contributing factors. We conducted a study in 2 trials, from 1989 to 1993 and from 1995 to 1999, to assess the effects of male age on conception dates and pregnancy rates of female elk in northeast Oregon. Results of the first trial, reported previously, showed a significant influence of male age on conception dates but not on pregnancy rates. The second trial, reported here, was intended to validate findings of the first. trial and to evaluate the interaction of male age and female nutritional condition. We managed an elk population within a 78-km2 enclosure to allow a single cohort of males to function as herd sires as they matured from 1.5 to 5.5 years of age. From animals killed in December, we estimated pregnancy rates, age, nutritional condition (kidney fat index, [MFI]), and lactation status of females and the conception dates of their fetuses. Mean conception dates occurred 1 week earlier as male age increased and were related to KFI in females. Nutritional condition of female elk was 67% higher in 1995 when breeding was by yearling males than in years when breeding was by 4-year-old or 5-year-old males. Pregnancy rates did not differ among ages of males. We used analysis of covariance with female nutritional condition as the covariate to evaluate the interactive effects of male age and female nutritional condition on conception dates of females bred by males of different ages across 2 trials. Mean conception dates (adjusted for female nutritional condition) pooled by age of males decreased from 4 October with yearling male sires to 21 September with 5-year-old male sires. Mean KFI of pregnant, lactating female elk was 118 during the validation and 148 during the initial trial. Pregnancy rates did not differ by male age between trials. We stress the importance of understanding the interactions between age of males and nutritional condition of females prior to interpreting the results of management strategies designed to retain older males because of the many factors that affect calf elk survival.