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The aim of this study was to extend 40 yr of prior demographic work on northern elephant seals (Mirounga angustirostris) at Año Nuevo, California, by including the oldest animals. We used a Bayesian mark-recapture analysis to estimate lifelong survival and lifespan of a cohort of 372 weaned pups branded in 1985–1987 and resighted until 2008. Annual survival probability of females averaged 86.3%/yr at ages 5–16, then declined until age 21, the age of the oldest female. Male survival was lower, averaging 67.7%/yr from age 1 to age 15, the age of the oldest male. Northern elephant seal females in the expanding population at Año Nuevo live longer than southern elephant seal females (M. leonina) at colonies whose populations are declining. This comparison suggests that high survival of females is a key factor in population growth.
The population of northern elephant seals (Mirounga angustirostris) has been increasing in number and expanding in range since near extinction over a century ago (Townsend 1885, Bartholomew and Hubbs 1960, Stewart et al. 1994, Lowry 2002). The demographics of this growth phase have been documented at the Año Nuevo colony in central California over the last four decades, addressing variation in male survival and mating success, primiparity in females, pup mortality, and juvenile survivorship (Le Boeuf 1974; Reiter et al. 1978, 1981; Le Boeuf and Reiter 1988; Reiter and Le Boeuf 1991; Clinton and Le Boeuf 1993; Le Boeuf et al. 1994). Most of this research focused on young animals and prime-age adults. The aim of this paper is to extend earlier work by documenting survival rates of the oldest animals, testing for mortality-related senescence, and comparing the lifespan of males and females. This yields a full life table for adult northern elephant seals of both sexes, necessary for understanding population growth of this long-lived mammal (Pistorius et al. 1999, Eberhardt 2002).
Our previous demographic studies were based on numbered plastic tags affixed to the interdigital webbing of the hind flippers. These worked well for studies of juveniles and young adults. With time, however, tags wore smooth or broke, necessitating retagging (Le Boeuf and Reiter 1988, Clinton and Le Boeuf 1993). Thus, survival estimates in older animals may be unreliable, even when tag loss is modeled (Pistorius et al. 2000, McMahon and White 2009). Branding offers a more permanent alternative for marking, and in southern elephant seals (Mirounga leonina) permitted identification of individuals throughout life without deleterious effects (Hindell 1991, McMahon et al. 2006, Schwarz et al. 2012). We thus undertook a branding study of northern elephant seals at Año Nuevo in 1985 aimed at studying survival rates of seals throughout their lifespan.
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Annual survival of adult females was high from age 5 to 16, averaging 86%/yr, but then declined abruptly. This is a higher rate and a longer duration of prime survival than we expected and the first evidence for senescence in survival rates of northern elephant seals. Our earlier work did not detect the decline in female survival because there were no data on females older than 15 yr (Le Boeuf and Reiter 1988, Reiter and Le Boeuf 1991). Schwarz et al. (2012) found limited power in estimating survival beyond age 15 due to the small number of animals retaining tags.
Average male survival was <72%/yr at all ages and lower than female survival after age 3, as reported in earlier studies (Clinton and Le Boeuf 1993). Neither our current analysis nor the earlier work detected senescence in male survival, but high mortality throughout life meant few males were still living at age 12 when senescence would be most likely. On the other hand, our earlier study did detect declining competitive ability in males past age 12 (Clinton and Le Boeuf 1993).
Juvenile survivorship in the current study was 31% from weaning to age 3 and similar in the two sexes, a rate close to the average reported across several previous cohorts (Le Boeuf and Reiter 1988, Le Boeuf et al. 1994). This average masked variation, however, and low survival in 1986–1987 may have been due to poor foraging conditions associated with an El Niño event (Trenberth and Stepaniak 2001, Crocker et al. 2006). Our earlier study of juvenile survival also described substantial year-to-year fluctuations (Le Boeuf et al. 1994). These rates of survivorship, though, began at weaning and omit pup mortality, and 10% of pups in the Año Nuevo mainland colony died before weaning in 1985–1987 (Le Boeuf et al. 2011). In population modeling, the relevant rate of juvenile survivorship (from birth) was thus 28%, not 31%.
Dispersal of branded animals to nearby colonies—“prospecting” for alternative breeding sites—also confirms earlier observations (Le Boeuf et al. 1974, 2011). Seven of the 37 females observed to breed did indeed choose an alternative, but two of those subsequently returned home.
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 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 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 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. (2004) attributed differences in population trends at different colonies to variation in adult survival, but juvenile survival, fecundity, and age at primiparity have also been implicated as key density-dependent factors (Pistorius et al. 2001; Pistorius and Bester 2002b; McMahon et al. 2003, 2005a, b; de Little et al. 2007). In the northern elephant seal, we have likewise found substantial variation in juvenile survivorship and annual fecundity (Huber et al. 1991, Reiter and Le Boeuf 1991, Le Boeuf et al. 1994, Crocker et al. 2006), suggesting ample opportunity for either to affect population growth. Except for pup mortality, however, density-dependent variation in survival and fecundity has not been demonstrated (Le Boeuf et al. 2011).
On the other hand, when compared to other large mammals, elephant seals are short-lived. Adult females of most large herbivores have survival rates >90%/yr (Gaillard et al. 1998), as do many pinnipeds (Cameron and Siniff 2004, Hastings et al. 2011). In gray seals (Halichoerus grypus), 95% of adult females survive annually (Harrison et al. 2006) and a 42 yr old has been observed (Bowen et al. 2006). Elephant seals must have higher fecundity than gray seals in order to sustain population growth with their relatively short lifespan. At least one pinniped, though, is similar to elephant seals: in monk seals (Monachus schauinslandi), survival declined starting at age 17 (Baker and Thompson 2007).
Our next steps are to study fluctuations in vital rates over time by studying other cohorts, then to build models of the Año Nuevo colony and the entire population of northern elephant seals based on complete life tables. Given our current estimate of a 21 yr life span and 86% annual survival of adult females, we will explore variation in juvenile survival to find a rate that would support the rapid worldwide recovery in the 20th century. We can also use the observed life table at Año Nuevo to quantify the immigration rate needed to account for local population growth. Other pinniped species offer excellent precedents for this sort of modeling (Cameron and Siniff 2004, Harrison et al. 2006). With the northern elephant seal, we will soon have the bonus of observing the cessation of population growth, allowing us to document vital rates across the transition to stability and test hypotheses about environmental and demographic factors important in regulating the population.