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

Abstract

Individual heterogeneity is variation in trait expression observed among individuals of a population. Individual heterogeneity in the pre-breeder stage of development is of importance given its eventual contribution to the breeding population's overall reproductive performance. Yet most studies do not consider the role of individual heterogeneity in pre-breeders when investigating population processes. We investigated individual heterogeneity in the survival and recruitment probabilities of pre-breeding male southern elephant seals Mirounga leonina using 34 years of data collected at Marion Island. Elephant seals are highly polygynous capital breeders, with few male offspring surviving to compete in breeding events and even fewer breeding successfully. Specifically, we fitted finite-mixture models with two hidden groups that represented 'robust' and 'frail' individuals and compared the demographic rates of these groups to that of the population. Survival and recruitment as first-time subordinate breeders could be distinguished between groups. Survival of both robust and frail pre-breeders decreased from age 2, whilst differences between groups were maintained (ontogeny processes). Frail pre-breeders had a much lower survival than the population average, suggesting that frail individuals were preferentially removed from the population (selective disappearance). Differences in recruitment probabilities were apparent from age 5 (earliest recorded age) and increased until age 7, with most recruits comprising robust pre-breeders. Male pre-breeders were more likely to be robust at age 2 when born in years with few conspecifics, suggesting that individual heterogeneity was determined, in part, by density-dependent effects. A population projection model revealed that male breeders of every age class never consisted of more than 5% of frail recruits. Rather, the breeding population increasingly consisted of robust recruits that obtained social dominance with age. We demonstrated that individual heterogeneity in male pre-breeder demographic rates was present and persistent throughout much of development and determined the proportion of adult males that obtained social dominance.

Demystifying individual heterogeneity

“…An individual's position in this hierarchy provides information on its own relative fitness, and on how it may influence population dynamics. Individuals with higher vital rates may experience fewer trade‐offs (Hamel et al, 2009a; Moyes et al, 2011; Oosthuizen et al, 2019), enjoy superior social ranks and competitive success (Hamel et al, 2009b; Hart et al, 2016; Lemaître et al, 2018; Lloyd et al, 2020) and contribute more to population growth than lower condition individuals (Coulson et al, 2006b; van de Pol et al, 2006; Zuidema et al, 2009; Jansen et al, 2012). However, these benefits only last as long as an individual's position in the condition hierarchy is maintained.…”

Uncovering ecological state dynamics with hidden Markov models

“…To assess the costs of reproduction, a biologist will be interested in the probability of breeding in year t, given breeding ψ B,B ¼ 1 À ψ B,NB À Á or not ψ NB,B À Á in year t À 1, as well as assessing any differences in survival probability between breeders ϕ B ð Þ and non-breeders ϕ NB ð Þ. By simply re-expressing the δ, Γ and P x t ð Þ components in terms of the specific state and observation processes of interest, such models can be used to infer the dynamics of conjunctivitis in house finches (Conn and Cooch, 2009), senescence in deer (Choquet et al, 2011), reproduction in Florida manatees (Kendall et al, 2012), interspecific competition between ungulates (Gamelon et al, 2020) and life-history trade-offs in elephant seals (Lloyd et al, 2020). Similar HMMs can also be used to investigate relationships between life-history traits and demographic parameters that are important in determining the fitness of phenotypes or genotypes (Stoelting et al, 2015).…”

Extreme polygyny results in intersex differences in age-dependent survival of a highly dimorphic marine mammal

“…By contrast, males have a very low chance of contributing to future generations. A small proportion of high-quality males gain dominant breeding status and maintain high survival and reproductive success, even in future breeding seasons [ 49 ]. The remainder of mature males struggle to survive in this highly competitive breeding system.…”