Effects of pup age, maternal age, and birth date on pre-weaning survival rates of Weddell seals in Erebus Bay, Antarctica

Abstract

Identifying factors affecting juvenile survival is important to understanding the dynamics of populations and may also provide insights into life history theory and the selective forces shaping evolution. Quantifying the relative influence of the various potential selective forces for the post-birth, maternal dependency, and independent periods is difficult and often limits investigators to estimating a single juvenile survival rate for the first year of life, or from birth to recruitment. We examined survival of individually marked Weddell seal Leptonychotes weddellii pups during the maternal dependency period in Erebus Bay, Antarctica from 2005–2007. We used mark-recapture models to evaluate competing a priori hypotheses regarding variation in daily pre-weaning survival rates (φ) during the first 42 days of age. The a priori model receiving the most support from the data supported several of our predictions: increased with pup age and was higher for pups born later in the season and to older mothers. Increases in with increasing pup age may have been due to improved resilience to the environment, phenotypic selection against the frailest pups, or both. Maternal age was more important to than was maternal experience or age of primiparity, potentially indicating that age-related increases in body mass allow greater offspring provisioning, or age-related improvements in competitive abilities benefit offspring during the period of maternal care. Depending on the timing of birth and the age of the mother, 42 days ranged from 0.79 (SE = 0.05) to 0.98 (SE = 0.01). These exceptionally high pre-weaning survival rates contrast with estimates from other large terrestrial and marine mammal species where neonate survival is considerably lower and suggest that in species with similar life histories, pre-weaning survival probability may be buffered from both predators and environmental fluctuations during the period of maternal nutritional dependency. Climatic changes affecting stability of ice used for pupping substrate or extent of fast-ice buffering pupping colonies from predators have the potential to reduce pre-weaning survival and may have important implications for population growth rates.