Male Southern Elephant Seals Mirounga Research Articles

Cephalopod diet of juvenile male southern elephant seals Mirounga leonina at Marion Island, South Indian Ocean

Cephalopod diet of juvenile male southern elephant seals Mirounga leonina at Marion Island, South Indian Ocean

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

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 sealsMirounga leoninausing 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.

Moulting juvenile male southern elephant seals Mirounga leonina (L.) at Hannah Point, Walker Bay, Livingston Island, South Shetland Islands

Moulting juvenile male southern elephant seals Mirounga leonina (L.) at Hannah Point, Walker Bay, Livingston Island, South Shetland Islands

Moulting juvenile male southern elephant seals Mirounga leonina (L.) at Hannah Point, Walker Bay, Livingston Island, South Shetland Islands

Southern elephant seals Mirounga leonina (L.) were counted at seventeen discrete moulting sites at Hannah Point, Walker Bay on Livingston Island in the South Shetland Islands between 2 January and 16 February 1994. The physical characteristics of each moulting site were assessed. Counts through 46 consecutive days demonstrated that the number of juvenile males in dry moulting sites increased as the muddy wallows were gradually abandoned. This pattern of behaviour is similar to the pattern described for other sex- and age-groups (e.g. adult females) in previous studies.