Age-specific reproductive success and cost in female Alpine ibex

In female mammals, reproduction requires high energy expenditure because of gestation and lactation, possibly leading to a fitness cost. Several studies, however, failed to find the expected negative correlation between current and future reproductive success, likely because of individual heterogeneity in reproductive potential. We compared reproductive performance and costs of reproduction for 40 female Alpine ibex in one established population with 29 females translocated from the same population to a new colony. We investigate factors affecting pregnancy, fecundity and overwinter survival of juveniles, after accounting for individual heterogeneity. In both populations, prime-aged females experienced a strong reproductive cost. Senescent females, however, showed no evidence of reproductive costs. The colonizing population showed lower reproductive cost and better age-specific reproductive performance than the established population. We found a general pattern of low age-specific fecundity and reproductive success that was affected by environmental constraints. Age-specific reproductive success was unrelated to longevity. Although about 84% of adult females appeared to conceive, independently of environmental constraints, energy was allocated to reproduction in a highly conservative manner, leading to low age-specific fecundity (only 36 and 21% of prime-aged and senescent females were seen with a kid) but high kid survival (100% to weaning and 92% to 1 year). Our results suggest that females embarked on lactation only if they had a very high probability of raising their offspring. Our study highlights how reproductive performance and costs in this species vary with age and environment, and are the result of a highly conservative reproductive tactic.