Age‐Specific Weaning Success of Northern Elephant Seals in Relation to Previous Breeding Experience

Abstract

We studied the effect of age and previous breeding experience on the probability of successful reproduction in female northern elephant seals (Mirounga angustirostris). Reproductive data, collected over a 12—yr period, were analyzed by multiple logistic regression to describe the functional relationships among the variables. We also examined the relationships among age, experience, date of breeding, and reproductive success to determine if differences in timing explain age—specific productivity. Finally, we investigated the relationship among age, experience, and maternal behavior to determine if correlates of reproductive effort could be related to age— and experience—specific patterns of reproductive success. Reproductive success increased between the maternal ages of 3 and 7 yr and then levelled off, demonstrating an asymptotic relationship. However, the relationship between reproductive success and age, after statistically controlling for previous experience, showed instead that success increased linearly throughout an animal's lifetime. In contrast, after statistically controlling for age, reproductive success was related to experience in a parabolic fashion; success increased with experience for the first few breeding attempts, but declined later in an animal's reproductive lifetime. The effects of experience depended on age: among young animals experience was apparently beneficial, but among old animals greater experience was deleterious. These results suggest experience—related senescence. Timing of breeding was related to experience, rather than age, in a curvilinear fashion; arrival and parturition dates were later for inexperienced and very experienced females, and were earliest for moderately experienced animals. Young animals that gave birth later in the season experienced reduced reproductive success; we observed no such decline in older animals. Thus, among young females, arriving and giving birth earlier each year was adaptive. Aggressiveness and dominance increased with age and experience, but we were unable to separate the effects of age and experience. We contend that the experience—related drop in reproductive success reflects the cost of reproduction. Breeding at a young age and/or continuously appears to result in lower reproductive success later in life. Furthermore, curvature in age—specific reproductive success can be explained by experience—related senescence. This result has general applicability, suggesting that age—related senescence instead may be due to reproductive "burn—out". These results indicate that previous breeding experience and differences in individual quality do not explain the pattern of increasing reproductive success with age in northern elephant seals, and provide indirect support for the hypothesis that animals increase reproductive effort as they age to offset a concomitant decrease in residual reproductive value.