Abstract

This study investigates the influence of inbreeding on California sea lion (Zalophus californianus) pup survival at San Miguel Island, California, between June 2002 and January 2003. Of the 349 dead California sea lion pups examined, full necropsies and histopathologic examinations were conducted only on 187 fresh carcasses. Hookworm (Uncinaria spp.) load was determined by visual examination of the intestinal tract. For molecular analyses we genotyped all pups at 13 highly polymorphic (3–15 alleles) microsatellite loci and calculated internal relatedness (IR) as an estimator of inbreeding. Contrary to our initial hypothesis, relatively inbred pups do not die earlier in the season than less inbred pups. Instead, mean IR was highest in October (0.10; ANOVA, F₆, ₁₈₀=2.08, p=0.05), when 100% of pup mortality was attributed to hookworm infection. A generalized linear model showed both pups’ IR and age as key predictors of Uncinaria load (p<0.0001). Testing each main cause of death (hookworm-associated hemorrhagic enteritis, starvation, and trauma) separately revealed that although all examined pups showed varying numbers of Uncinaria spp., the association between IR and Uncinaria load was only significant for those pups that died due to hookworm enteritis (r²=0.09, p<0.001, n=138) in contrast to pups that died due to starvation (r²=0.04, p>0.1, n=18), or trauma (r²=0.11, p>0.1, n=13). These results suggest that more inbred pups may be less successful at mounting an immune response against hookworms and are thus more likely to die due to related lesions, particularly when the prevalence of Uncinaria is highest in the population.