Variation in the vital rates of an Antarctic marine predator: the role of individual heterogeneity

Abstract

Variation in life‐history traits such as lifespan and lifetime reproductive output is thought to arise, in part, due to among‐individual differences in the underlying probabilities of survival and reproduction. However, the stochastic nature of demographic processes can also generate considerable variation in fitness‐related traits among otherwise‐identical individuals. An improved understanding of life‐history evolution and population dynamics therefore depends on evaluating the relative role of each of these processes. Here, we used a 33‐yr data set with reproductive histories for 1,274 female Weddell seals from Erebus Bay, Antarctica, to assess the strength of evidence for among‐individual heterogeneity in the probabilities of survival and reproduction, while accounting for multiple other sources of variation in vital rates. Our analysis used recent advances in Bayesian model selection techniques and diagnostics to directly compare model fit and predictive power between models that included individual effects on survival and reproduction to those that did not. We found strong evidence for costs of reproduction to both survival and future reproduction, with breeders having rates of survival and subsequent reproduction that were 3% and 6% lower than rates for non‐breeders. We detected age‐related changes in the rates of survival and reproduction, but the patterns differed for the two rates. Survival rates steadily declined from 0.92 at age 7 to 0.56 at the maximal age of 31 yr. In contrast, reproductive rates increased from 0.68 at age 7 to 0.79 at age 16 and then steadily declined to 0.37 for the oldest females. Models that included individual effects explained more variation in observed life histories and had better estimated predictive power than those that did not, indicating their importance in understanding sources of variation among individuals in life‐history traits. We found that among‐individual heterogeneity in survival was small relative to that for reproduction. Our study, which found patterns of variation in vital rates that are consistent with a series of predictions from life‐history theory, is the first to provide a thorough assessment of variation in important vital rates for a long‐lived, high‐latitude marine mammal while taking full advantage of recent developments in model evaluation.

Individual heterogeneity in life‐history trade‐offs with age at first reproduction in capital breeding elephant seals

“…Female elephant seals recruiting at a young age are not a random subgroup of the population, but mostly those that experienced favorable conditions during early development (Oosthuizen et al, ). Our finding that individuals that recruit earlier in life survive and reproduce better than delayed breeders supports the hypothesis that recruitment age is an indicator or proxy of individual quality (Fay et al, ; Paterson et al, ).…”

“…In theory, recruitment at an early age benefits individuals by shortening generation time and increasing the number of possible breeding opportunities over a lifetime (McGraw & Caswell, 1996). In support of this prediction, numerous empirical studies indicate a fitness gain among individuals that recruit at an early age (Fay, Barbraud, Delord, & Weimerskirch, 2016;Paterson, Rotella, Link, & Garrott, 2018;Zhang, Vedder, Becker, & Bouwhuis, 2015). However, the advantages of early breeding are often counter-balanced by elevated reproductive costs among younger first-time breeders (Desprez et al, 2014;Millon, Petty, & Lambin, 2010).…”

Joint estimation of survival and breeding probability in female dolphins and calves with uncertainty in state assignment

“…Adult female bottlenose dolphins invest heavily over several years to raise their offspring (Arso Civil, Cheney, Quick, Thompson, & Hammond, ), like other mammal species such as elephants (Lee, ) and chimpanzees (Van Lawick‐Goodall, ). Providing such intensive parental care may have an impact on the female's subsequent reproduction (cost of reproduction, Paterson, Rotella, Link, & Garrott, ), as well as other demographic traits (trade‐offs) such as survival, which is the predominant trait for long‐lived species. In this study, we did not observe a trade‐off between current reproductive status and survival (see the “Adult survival” section).…”

Variation of annual apparent survival and detection rates with age, year and individual identity in male Weddell seals (Leptonychotes weddellii) from long‐term mark‐recapture data

“…We built models using the R package NIMBLE version 0.6‐10 (de Valpine et al, ) in R version 3.5 (R Core Team, ). As previous studies have suggested that age and year affect φ and p of Weddell seals (Cameron & Siniff, ; Stauffer, Rotella, & Garrott, ), and we had data from a large number of years for which at least some female vital rates have been shown to vary (Cameron & Siniff, ; Hadley et al, ; Paterson et al, ; Stauffer, Rotella, & Garrott, ), we structured our a priori candidate models for males such that temporal variation was allowed for both φ () and p () by including random effects of year for each parameter that were normally distributed with a mean of zero. To evaluate support in the data for alternative predictions of how φ might change with age, we used five different model structures for the log‐odds of φ .…”

“…Survival rates of female Weddell seals are moderate for pups and yearlings and quite high for older animals; adult females endure costs to survival from pup production and rearing (Chambert et al, ; Chambert, Rotella, & Garrott, ; Hadley, Rotella, & Garrott, ; Paterson, Rotella, Link, & Garrott, ). Additionally, actuarial senescence that commences immediately after age at first reproduction, which typically occurs at 8 years of age, was recently detected in female Weddell seals (Paterson et al, ). Unique reproductive costs might cause differences in age‐specific survival rates and senescence patterns between male and female Weddell seals.…”