EFFECTS OF AGE, SEX, DISEASE, AND DENSITY ON SURVIVAL OF BIGHORN SHEEP

Longitudinal studies of survival are valuable because age-specific sur vival affects population dynamics and the evolution of several life hi story traits. We used capture-mark-recapture models to assess the rela tionship between survival and sex, age, population, year of study, dis ease, winter weather, and population density in two populations of big horn sheep (Ovis canadensis) in Alberta, Canada. The Ram Mountain popu lation, monitored for 20 yr, more than doubled in density; the Sheep R iver population, monitored for 13 yr, experienced a pneumonia epizooti c. Yearling survival varied among years and was lower than that of old er sheep of the same sex, except for yearling males at Ram Mountain. Y earling females at Ram Mountain were the only sex-age class exhibiting density dependence in survival. Senescence was evident for both sexes in both populations. Female survival from age 2 to age 7 was very hig h in both populations, but males aged 2 and 3 yr enjoyed better surviv al than males aged 4-6 yr. Our data support the suggestion that where hunters remove many males older than 5 yr of age, the natural mortalit y of males increases at 3-5 yr, possibly because young males suffer a mortality cost of participating in rutting activity. The decline in su rvival for sheep older than 7 yr was greater for males than for female s. Survival was lower for males than for females, both among prime-age d sheep (0.896 vs. 0.939 at Sheep River; 0.837 vs. 0.945 at Ram Mounta in) and among older sheep (0.777 vs. 0.859 at Sheep River; 0.624 vs. 0 .850 at Ram Mountain), but not among yearlings. Survival of sheep aged 2-7 yr was not significantly different between the two populations. W inter weather did not affect survival. Survival of sheep 2 yr of age a nd older did not vary significantly between years, except at Sheep Riv er where survival of prime-aged sheep of both sexes was lower in the y ear of the pneumonia epizootic. Studies of survival of mountain sheep based upon skull collections may have overestimated survival of young rams. Our results underline the need for accurate information on age-s pecific survival.