"Harems" of elephant seals cause early death in males
Extreme polygamy may lead to early death of thousands of male elephant seals, new research suggests, reports “The Guardian".
The study surveyed 14.000 elephant seals on Macquarie Island in the south-west Pacific and found that although survival rates for males and females were roughly comparable in the early years, male survival declined rapidly after eight years of age, falling to a 50% survival rate, while female survival remains constant at 80%.
This species of seal varies considerably in size: adult males can weigh up to almost five times as much as adult females. Differences in size usually begin to appear between the ages of three and six, when animals undergo maturation.
Sofia Volzke, first author of the study and a PhD candidate at the University of Tasmania, said the biggest and fattest male seals have a reproductive advantage.
"They can only get food from the ocean," she said. "When they come to land [to breed] they compete with other males for access to the females. "They need to have stored fat resources so they can fight other males and survive on land without eating anything for weeks or months," she says in the study.
The species exhibits "extreme polygamy", in which a small number of the largest and most dominant males – known as "beach masters" – control "harems" of breeding females.
"A huge beach master can have a harem of 100 females," says Volzke. "Once harems get that big, it can lead to the early death of the male." It all depends on how big the harem is and the geography of the beach – if you have a really long beach, it's more likely to have a bunch of little harems,” she added.
Only about 4% of men become "beach masters".
Researchers believe that competitive pressures on mature males encourage them to gain weight as quickly as possible, resulting in lower survival rates because males forage in waters in areas that may put them at greater risk.
"Adult males concentrate their foraging effort in shallower waters. These highly productive sites are visited by other marine predators, such as orcas and sharks," the study said.
Although male elephant seals become biologically capable of reproducing at around six years of age, they are rarely socially competitive enough to breed with females aged nine to 12, Volzke said.
Elephant seals spend much of the year on beaches. The males come ashore each August on Macquarie Island to try to establish dominance over the females, Volzke said. "The females arrive in September and gather themselves into groups, the males come and try to defend those groups.
"We could see a male who is not a successful beach master come ashore in August and try to challenge a beach master." "If he loses the fight, he'll just go back to the sea," Volzke said.