Trade‐offs between age‐related breeding improvement and survival senescence in highly polygynous elephant seals: Dominant males always do better

Abstract

Life‐history trade‐off theory predicts that current reproduction can negatively affect survival and future reproduction. Few studies have assessed breeding costs for males of polygynous species compared to females, despite substantial variation in breeding success among individual males (e.g. subordinate cf. dominant breeders). Specifically, differentiating between the cost of attending breeding events, and the additional cost of successfully securing and mating females is lacking. We investigated whether trade‐offs are present in the highly polygynous male southern elephant seal Mirounga leonina using 34 years of individual‐level data. We compare age‐specific survival, recruitment and future breeding success probabilities of pre‐breeders (males yet to recruit) and breeders (subordinate and dominant social ranks) using multievent models. Pre‐breeders and breeders of overlapping ages had similar survival probabilities, suggesting that there was no attendance cost for early recruits. In addition, the probability of recruiting as a dominant breeder never exceeded recruitment probability as a subordinate breeder of the same age. Therefore, older pre‐breeders that delayed attendance costs generally did not improve their breeding success (probability of being dominant) at recruitment more than younger recruits. Rather, recruitment age may be a function of individual quality, with lower quality individuals requiring more time to socially mature. When comparing subordinate and dominant breeders, we found clear evidence for survival senescence, with subordinate breeders having a higher baseline mortality. In contrast, age‐specific future breeding success (probability of being dominant at t + 1) increased with age, with dominant breeders maintaining higher subsequent breeding success than subordinate breeders. The opposite trends in survival and future breeding success for both subordinate and dominant breeders may indicate a lifetime, population‐level trade‐off. However, we found no evidence to suggest that being a dominant breeder consecutively (and having a higher accumulated breeding cost) accelerated the rate of senescence when compared to individuals that were previously subordinate. Thus, males experienced actuarial senescence regardless of social rank, with dominant (and possibly high quality) breeders showing a reduced trade‐off between survival and future breeding success. We make several novel contributions to understanding polygynous male life histories and southern elephant seal demography.

Selective disappearance of frail juveniles: consequences for understanding social dominance in adult male elephant seals

“…And yet, little is known about the processes governing pre-breeder demographic rates and the implications this has for population dynamics. Using finite-mixture models, we detected frailty in male pre-breeder survival and recruitment probabilities that was not observed at the population level (Lloyd et al 2020a). If pre-breeder survival had not incorporated individual heterogeneity, it would have mistakenly been assumed from population-level models that many more young males reach sexual and social maturity.…”

“…All beaches where elephant seals bred were surveyed every seven days during the breeding season and every ten days outside of the breeding season between 1983 and 2016. Further details of the mark-recapture protocol can be found in Lloyd et al (2020a). The resulting dataset consisted of 27 cohorts, 6245 marked pups and 58 177 observations made over 34 years.…”

“…The multievent component allows for uncertainty in state assignment (Gimenez et al 2012). The general model was based on the analysis of Lloyd et al (2020a), which consisted of nine events observed in the field relating in a probabilistic framework to nine states that an individual could occupy during each year (Supplementary material Appendix 2). Each year started at the beginning of the elephant seal breeding season and ended before the start of the next breeding season.…”

“…The probability of being assigned to a group (π ind ) was initially kept constant and depended on the survival and recruitment probabilities of an individual (Fay et al 2016). All individuals entered the population as pre-breeders, nearly all with two tags (π tag ; Lloyd et al 2020a). Transition probabilities between states were modelled in five steps with each step conditioning on preceding transitions: tag loss from two to one tag (τ 21 ), tag loss from one to zero tags (τ 10 ), apparent survival (φ), breeding state transitions (ψ, representing recruitment -the probability to recruit from the pre-breeder state to the subordinate or dominant breeder state -and future breeding state -the probability to transition from the subordinate or dominant breeder state to the dominant breeder state), and temporary migration by Marion-tagged pre-breeders out of (ψ o ) and into (ψ I ) the study population.…”

“…Males compete for dominance of female harems, with only a few dominant males (or beachmasters) monopolising the majority of mating opportunities (Wilkinson and van Aarde , Galimberti et al ). In addition, pre‐breeders that recruit directly as dominant males are more likely to breed again in subsequent years than males that recruit initially as subordinates (Lloyd et al ). Thus, individual heterogeneity is likely present and changing with age in male pre‐breeder demographic rates, and is likely to carry over into the breeding population.…”