SEASONAL SURVIVAL AND THE RELATIVE COST OF FIRST REPRODUCTION IN ADULT FEMALE SOUTHERN ELEPHANT SEALS
´ PIERRE A. PISTORIUS,* MARTHAN N. BESTER, GREG J. G. HOFMEYR, STEPHEN P. KIRKMAN, AND
FRANCES E. TAYLOR
Mammal Research Institute, Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa
The annual cycle ofadult female southern elephant seals (Mirounga leonina) can be divided into 2 pelagic phases, separated by relatively short terrestrial phases: breeding and molting. We used resighting data collected from tagged female southern elephant seals at Marion Island during 1986–1999 to investigate seasonal survival during the 2 pelagic phases in relation to reproductive experience. Mean postbreeding(pelagic phase between breeding and molting, about 62 days) survival of primiparous females was 0.830 compared to 0.912 for moreexperienced females. Postmolting (pelagic phase between molting and breeding, about 255 days) survival was 0.847 (0.960 when rescaled to 62 days for comparative purposes) and was not dependent on reproductive experience. Postbreeding survival of experienced females was higherthan postmolting survival, but per unit time the opposite applied. A 2-stage survival model, in which survival was constrained to be constant before 1994 (when the population was declining) and from 1994 onward (during the stable phase), had overwhelming support from the data. Postbreeding survival of primiparous females increased from 0.799 before 1994 to 0.880 from 1994 onward. Postmoltingsurvival of all females also increased from 0.817 to 0.872 over the same period. Postbreeding and postmolting mortality risk varied independently over time, demonstrating the importance of an intra-annual approach in population studies of southern elephant seals.
Key words: survival environmental change, Marion Island, mark–recapture models, Mirounga leonina, population ecology, seasonalThroughout the course of their annual cycle, animals may be subjected to different mortality risks as a result of activities such as breeding and long-distance movement, which will shape the evolution of basic life-history traits (Beauplet et al. 2006; Gauthier et al. 2001; Hadley et al. 2007; Sendor and Simon 2003; Sillett and Holmes 2002; Stearns 1992). This is especially relevant in long-lived speciessuch as southern elephant seals (Mirounga leonina), in which population changes are primarily regulated through survival rather than fecundity (Gaillard et al. 1998). Although a growing number of studies have quantiﬁed annual survival in wild populations in a mark– recapture context, relatively few such studies have considered how annual survival varies on a seasonal basis (e.g., Gauthier et al.2001; Madsen et al. 2002; Pistorius et al. 2006; Sillett and Holmes 2002). These studies have mostly been based on wildfowl and to our knowledge no previous attempt at such a study has been made on a population of marine mammals. In the case of southern elephant seals in particular, this lack of knowledge
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on the seasonal components of annual survival and the interaction between seasonal survival and reproductive experience, expenditure, or both limits our understanding of population regulation in this species. The last half of the 20th century saw a decline in numbers of southern elephant seals at most of their breeding sites in the southern Indian and Paciﬁc Oceans and someof them have stabilized in recent years (reviewed in McMahon et al. 2005a). Despite the lack of empirical support, the decline in numbers was probably related to food limitation (Bester and Wilkinson 1994; Hindell et al. 1994; McMahon et al. 2005a; Pistorius et al. 1999a, 2001). Although breeding sites of southern elephant seals are limited to the antarctic and subantarctic regions, their vast...