Population and individual life history consequences of polygyny in male southern elephant seals

Abstract

Our understanding of polygynous life history is biased towards females. Few studies investigate the processes governing male life history because of the difficulty in measuring male reproductive effort, and because population growth is thought to be limited only by females. Therefore, clarity is needed on the drivers and predictors of polygynous male life history and the long-term consequences this has for male fitness. My thesis achieves this overall aim by answering specific questions about the life history of male southern elephant seals at Marion Island using a 34-year mark-recapture dataset. The male elephant seal served as a model organism to investigate the most important life stages of polygynous males in general. For pre-breeders that survived their first year of life, individual heterogeneity in survival probabilities was expressed from age 2. Males born in years with relatively few pups were more likely to be robust in quality than males born in years with many pups. Survival probabilities of robust individuals and the population average became more similar as pre-breeders aged, suggesting that frail individuals were preferentially removed from the population during development. Therefore, the majority of pre-breeders that survived to breed successfully were robust in quality. Pre-breeder recruitment probabilities increased with age. In addition, pre-breeders of the same age often recruited as first-time subordinate breeders than as first-time dominant breeders. First-time subordinate breeders started recruiting from a younger age than first-time dominant breeders. Males likely need time to socially mature or require some breeding experience to outcompete older breeders. Pre-breeders expressed individual heterogeneity in recruitment probabilities, with robust individuals (i.e. higher survival) being more likely to recruit than frail individuals (i.e. lower survival). This supports the individual quality theory, which predicts that life-history traits are positively, rather than negatively, correlated. Pre-breeders born during low pup production years generally recruited at younger ages than pre-breeder born during high pup production years. Males attending breeding events did not suffer lower survival probabilities than same aged males that were still pre-breeders, suggesting that there was no reproductive cost for attending breeding events for early recruits. However, all males attending breeding events experienced actuarial senescence from recruitment age, with subordinate males suffering higher baseline mortalities. Given that this coincided with breeding improvement (the probability of becoming dominant), males appear to pay a reproductive cost for attempting to obtain dominance. When comparing survival probabilities between individual dominant males, there was also a reproductive cost for breeding successfully that accumulated with age. Males that were dominant at beaches with above average harem sizes accumulated higher reproductive costs than males that were dominant at beaches with below average harem sizes. Nevertheless, dominant males still maintained higher survival probabilities than subordinate males of the same age. Therefore, individual quality (inferred from breeding state) may play an important role in modifying resource allocation trade-offs between reproduction and survival. Future breeding success (measured as social status) increased with age for both subordinate and dominant males, but dominant males were more likely to remain dominant than subordinate males were in obtaining dominance. This apparent improvement in breeding success with age was predicted by birth cohort size, again suggesting that males born into cohorts with few conspecifics performed better as adults. In conclusion, I provide support for several population theories and show how they shaped male elephant seal demographics from birth to death. My findings provide valuable insight into the drivers and predictors of male life history in a highly polygynous breeding system.