Since our sample consisted of only three cohorts born over three years, we should be circumspect about generalizing. The number of branded adults was small, and our results on
survival in mature females hinges on the 15 animals observed at age 10 or older. Moreover, declining survival in old females might be attributed to poor conditions that all three cohorts experienced after 2002, rather than senescence. We found, however, that a model based on age outperformed a model based on calendar year, and there is no evidence that feeding conditions were better in the 1990s than after 2000. In fact, the switch in the Pacific Decadal Oscillation around Dactolisib ic50 1998 apparently favored elephant seal foraging (Le Boeuf and Crocker 2005), as females tracked at sea gained more weight in 2004–2005 than in 1995–1997 (Simmons et al. 2010).
In contrast, there is ample precedent for attributing declining survival in mammals to aging (Nussey et al. 2008, Turbill and Ruf 2010). The southern elephant seal offers an illuminating comparison of lifetime survival because many of its populations are declining while the northern elephant seal’s is expanding (McMahon et al. 2005a). Differences in survival rates between the species might thus indicate factors regulating population growth (Le Boeuf et al. 1994; McMahon et al. 2003; Pistorius et NVP-BGJ398 supplier al. 2008, 2011). For example, juvenile survival at Año Nuevo is low relative to
the southern species, suggesting that the Año Nuevo colony is not sustained by internal recruitment but by immigration (Le Boeuf et al. 1994). Contrary to the pattern in juveniles, we found higher adult female survival in the northern species, averaging 86%/yr compared to 81%/yr or lower at both Marion and Macquarie colonies (Hindell 1991, McMahon et al. 2003, Pistorius learn more et al. 2008), two southern elephant seal colonies where populations have declined, and 84% at Peninsula Valdes, the only expanding population of the southern species (Pistorius et al. 2004, Ferrari et al. 2012). Moreover, survival in the branded cohorts of Año Nuevo females remained high until age 16, whereas a life table based on branded animals at Macquarie Island showed steadily declining survival in southern elephant seal females after age 11 (Hindell 1991). High survival through age 15, however, was observed in the southern species at Marion Island (Pistorius and Bester 2002a, Pistorius et al. 2011). Our results to date thus lead us to hypothesize that high survival of adult females has been a key factor in the recovery of northern elephant seals from the population nadir in 1890. It follows that reduction in female survival will be important in curbing population growth. In southern elephant seals, Pistorius et al.