Environmental correlates of reproduction, patterns of maternal allocation, and variation in adult female vital rates in the Weddell seal

Abstract

The dynamics of populations are the integrated result of multiple processes affecting variation in vital rates. Using a long-term mark-recapture dataset from a population of Weddell seals (Leptonychotes weddellii) in Erebus Bay, Antarctica, I investigated three processes related to population dynamics: environmental correlates of reproduction, sources of variation in maternal allocation to offspring, and sources of variation in the vital rates of adult females. First, I assessed the strength of the association between primary production in the McMurdo Sound and Ross Sea polynyas and the number of pups born in Erebus Bay. I demonstrated both a strong coupling between trophic levels and a surprising timing in the relationship. Pup numbers were most strongly associated with primary production in the months after birth, consistent with a response by mothers to take advantage of the environment of relative abundance. Second, I showed that the patterns of maternal allocation to offspring differ in the prenatal and post-parturition periods. Maternal and pup masses at parturition increased with maternal age (maximum near age 16) prior to declining for older animals, consistent with both restraint and senescence. In contrast, maternal allocation to offspring continued to increase with maternal age during the post-parturition period. Together, these patterns are strong evidence for terminal allocation. Furthermore, I found extensive among-individual heterogeneity, such that some mothers consistently produce heavier pups and allocate more resources during lactation. Finally, I assessed the sources of variation in the vital rates of adult females, using a multistate model to jointly estimate the probabilities of survival and reproduction. Survival rates steadily declined with age, consistent with the onset of senescence at the age of first reproduction, whereas reproductive rates increased for young animals to a maximum 8 years after the age of first reproduction before exhibiting a senescent decline. I found extensive among-individual and yearly variation in reproductive rates, coupled to minimal variation in survival rates. This dissertation adds novel information to the understanding of the complex demography of Weddell seals by revealing an association between trophic levels and patterns of variation in both the allocation of resources to offspring as well as vital rates of adult females. Together, these results shed further light on the flexible life-history patterns of a long-lived marine mammal.