Demographic rates of animals (i.e., survival and breeding success) generally increase with experience early in life, but temporal variation in the effect of experience on demography is not well understood. The demographic difference between inexperienced versus experienced breeders can be accentuated during poor environmental conditions when food is scarce, or the difference can remain constant regardless of environmental fluctuation. I tested the hypothesis that environmental variation accentuates the difference between inexperienced and experienced breeders using 20 years of capture–mark–recapture data for northern elephant seals (Mirounga angustirostris) from the Farallon Islands, California. I estimated year-specific survival probabilities, breeding propensity (the probability that an individual will breed in year x), and breeding success of primiparous (inexperienced) and multiparous (experienced) females. Primiparous breeders did not suffer more than experienced breeders during years of environmental stress. The selection hypothesis (a decrease in heterogeneity in individual quality within cohorts over time, based on selection pressure) could explain the experience-related improvement in survival and breeding success. Lower variances in survival of multiparous breeders suggest that primiparous adults constitute a more heterogeneous portion of the population, and the 1st breeding event might act as a selective process leading to a more homogeneous pool of multiparous breeders.
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Vol. 92 • No. 3