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

Abstract

Developmental differences in vital rates are especially profound in polygamous mating systems. Southern elephant seals ( Mirounga leonina ) are highly dimorphic and extremely polygynous marine mammals. A demographic model, supported by long-term capture–mark–recapture records, investigated the influence of sex and age on survival in this species. The study revealed clear differences between female and male age-dependent survival rates. Overall juvenile survival estimates were stable around 80–85% for both sexes. However, male survival estimates were 5–10% lower than females in the same age classes until 8 years of age. At this point, male survival decreased rapidly to 50% ± 10% while female estimates remained constant at 80% ± 5%. Different energetic requirements could underpin intersex differences in adult survival. However, the species' strong sexual dimorphism diverges during early juvenile development when sex-specific survival rates were less distinct. Maximizing growth is especially advantageous for males, with size being a major determinant of breeding probability. Maturing males may employ a high-risk high-reward foraging strategy to compensate for extensive sexual selection pressures and sex-specific energetic needs. Our findings suggest sex-specific adult survival is a result of in situ ecological interactions and evolutionary specialization associated with being a highly polygynous marine predator.

Modeling climate influences on male survival in the highly polygynous southern elephant seal (Mirounga leonina)

“…There was no evidence of a relationship between SAM1 and the survival of mature breeding adults. By definition, the sample size of individuals within this state is low due to high rates of adult mortality for maturing males (Volzke, Cleeland, et al, 2023). However, the summarized comparison of SAM1 anomalies and breeding adult survival returned reasonable variance estimates and thus instils confidence in this result despite the smaller sample size.…”

“…This identifies a critical developmental phase, because subordinate males have a further need for growth, as their body size is a key determining factor for gaining access to females in this highly polygynous mating system. Subordinate males may employ a high-risk, high-reward foraging strategy that results in fewer highly successful bulls surviving to adulthood (Kienle et al, 2022;Volzke, Cleeland, et al, 2023).…”

“…Southern elephant seals are one of the most sexually dimorphic of all mammals, with large male bulls outgrowing adult females four-fold (Beltran et al, 2022). Only a small portion of males survive old enough to be considered socially competitive for breeding (Lloyd et al, 2019(Lloyd et al, , 2020Volzke, Cleeland, et al, 2023). The male seals that do attain dominant breeding status are extremely large "beachmasters" that will mate with up to as many as 100 females in a harem (Le Boeuf & Laws, 1994).…”

“…Male pups are born significantly heavier on average and require more energy to wean than females (Arnbom et al, 1994). However, male juvenile survival at Macquarie Island was lower than female survival during a period where the population continued to decline (Volzke, Cleeland, et al, 2023). This suggests that there are additional pressures acting on individual survival probabilities of males.…”

Pervasive relaxed selection on spermatogenesis genes coincident with the evolution of polygyny in gorillas

“…Polygyny and reduced male post-copulatory sexual selection are found among many species. Elephant seals ( Mirounga angustirostris and Mirounga leonina ), for example, are one of the most sexually dimorphic and polygynous mammals (Volzke et al, 2023). In one well-studied elephant seal population on Sea Lion Island, in the Falkland Islands, only ∼28% of males sired offspring, while males with harems sired almost 90% of the offspring (Fabiani et al, 2004).…”