Cohort variation in male survival and lifetime breeding success in red deer.

We investigated cohort differences in the lifetime breeding success and survival of male red deer Cervus elaphus L. in an increasing population on the Isle of Rum, Scotland. There were significant differences in survival through different stages of the life span between 15 cohorts of males, ranging between: 0.26-1.00, calf survival through first winter; 0.56-1.00, yearling survival; 0.44-0.94, adult survival. This variation in survival was related to environmental conditions in the cohorts' year of birth, whilst controlling for annual effects. For 10 cohorts of males with complete lifetime data, mean breeding success also varied significantly, between 0.83 and 3.86. This variation, too, was associated with environmental conditions in the cohorts' year of birth. Since in many sexually dimorphic mammals, the growth and survival of males is more strongly affected by adverse environmental conditions than that of females, we expected that cohort variation would be more extreme amongst males than females. This was true, for both cohort survival and reproduction. We expected to observe selection on cohort adult body size, related to survival or reproduction. However, there was no evidence of variation in adult body size between cohorts, nor for relationships between differences in body size between cohorts and measures of survival or breeding success. Cohorts which underwent high intial mortality subsequently experienced higher adult survival than cohorts not subjected to high density-related selection early in life.