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The population of southern elephant seals, Mirounga leonina, has been on a constant decline. A research that was initiated in an effort to pinpoint the contributing factors to this decline reveals pressures of extreme polygamy may be driving southern elephant seals to early death. 

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A study published in the journal Royal Society Open Science comprised of these 14,000 southern elephant seals on Macquarie Island in the south-western Pacific. One of the key findings of the study is that male and female survival rates are roughly comparable when they are juvenile. However, after age eight, male survival quickly declined, falling to about 50%, while female survival remained constant at 80%.

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The research reveals that the largest and fattest male seals, known as beachmasters, have an edge when it came to reproduction as they are able to compete for female access better. 

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Sophia Volzke, a PhD student at the University of Tasmania and the study's primary author said, “They can only get food from the ocean." “When they come on land [to breed] they’re competing with other males for access to females.

“They need to have fat resources stored to be able to fight other males and survive on land without eating anything for weeks or months at a time.”

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Volzke said the species demonstrates "extreme polygyny". A small percent of beachmasters control harems of females. One enormous beachmaster might have a harem with up to 100 females, she said. In cases when the harems are so large, they could allow a younger man to serve as an assistant beachmaster.

According on the topography of the beach and the size of the harem, a truly long beach is more likely to have a number of small harems. Just 4% of men work as beachmasters. Males that are still maturing are under pressure to acquire weight fast due to competition. According to the researchers, this leads to reduced survival rates since the males go in search of food at sea in locations where they may be more vulnerable to predators. 

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“Adult males concentrate their foraging effort in shallower waters. These highly productive locations are frequented by other marine predators, such as orcas … and sleeper sharks,” the researchers wrote.

“We might see one male that isn’t a successful beachmaster … come ashore in August and try to challenge a beachmaster. If they lose a fight they will just go back to sea,” Volzke said. “Beachmasters have a really loud roar which deters other males, which means some might not even come to shore at that time.”

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