The population of the southern elephant seal, Mirounga leonina, has been steadily declining. A study initiated to identify the factors contributing to this decline shows that extreme polygamy pressures may be driving southern elephant seals to early deaths.
A study published in the journal Royal Society Open Science included these 14,000 southern elephant seals on Macquarie Island in the southwest Pacific. One of the key findings of the study is that survival rates for males and females are roughly comparable at a young age. After eight years of age, however, the male survival rate declined rapidly, dropping to about 50%, while the female survival rate remained constant at 80%.
Research shows that the largest, fattest male seals, known as beachmasters, have an advantage when breeding because they are better able to compete for female access.
Sophia Volzke, a PhD student at the University of Tasmania and the study’s lead author, said, “They can only feed from the ocean.” “When they come to land [to breed] They compete with other males for access to females.
“They must have stored fat stores in order to fight other males and survive on land for weeks or months without eating.”
Volzke said the species displays “extreme polygyny”. A small percentage of beach masters control women’s harems. A giant beach master could have a harem of up to 100 women, she said. In cases where the harems are this large, they might allow a younger male to serve as an assistant beachmaster.
Depending on the topography of the beach and the size of the harem, a really long beach tends to have many small harems. Only 4% of men work as beach masters. Males who are still sexually mature are under pressure from competition to gain weight quickly. According to the researchers, this leads to reduced survival rates as the males forage in the sea in places where they may be more vulnerable to predation.
“Adult males focus their foraging on shallower waters. These highly productive sites are frequented by other marine predators such as orcas… and sleeper sharks,” the researchers wrote.
“We may see a male who is not a successful beachmaster… come ashore in August and try to challenge a beachmaster. If they lose a fight, they just go back to sea,” Volzke said. “Beachmasters have a really loud roar that scares off other males, meaning some may not even come ashore at this time.”
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