Extreme polygamy is taking a toll on male southern elephant seals, leading to early deaths, according to new research. The species exhibits extreme polygyny, with the largest males controlling harems of up to 100 females. The largest and fattest males are at an advantage, as they need to have fat resources stored to survive on land without eating anything for weeks or months at a time. This pressure to maintain dominance, however, can be driving males to early death. The study discovered that only 4% of male southern elephant seals become “beachmasters,” while the remaining males have to compete for access to females. Southern elephant seals differ significantly in size, with adult males weighing up to nearly five times that of adult females. The study found that male survival rates rapidly decreased after eight years of age, dropping to around a 50% survival rate, while female survival remained constant at 80%.
Extreme Polygamy May Be Shortening the Lifespan of Southern Elephant Seals
New research suggests that the pressures of extreme polygamy may be driving male southern elephant seals to early death. Southern elephant seals exhibit “extreme polygyny,” in which the largest males, known as “beachmasters,” control harems of breeding females. The males can command a harem of up to 100 females, and this study of 14,000 southern elephant seals at Macquarie Island in the south-western Pacific found that males, who can gain weight quickly by foraging in areas full of predators, become driven to gain weight as quickly as possible.
The species differs significantly in size, with adult males weighing up to nearly five times that of adult females. Maturation begins between three and six years of age when the size differences typically start to emerge. The study found that while survival rates for males and females are roughly comparable for juveniles, male survival rapidly decreased after eight years of age, dropping to around a 50% survival rate, while female survival remained constant at 80%.
According to Sophia Volzke, the study’s first author and a PhD candidate at the University of Tasmania, the largest and fattest male seals had a reproductive advantage because they have stored fat resources that allow them to fight other males and survive on land without eating anything for weeks or months at a time. When they come on land to breed, they are competing with other males for access to females. The study found that only around 4% of males become beachmasters, while the remaining males have to compete for access to females.
The species exhibits “extreme polygyny,” in which a small proportion of the largest and most dominant males known as beachmasters control harems of breeding females. “One huge beachmaster can have a harem of up to 100 females,” said Volzke. “Once the harems get that big they might allow for a younger male to be an assistant beachmaster. It depends on how big the harem is and the geography of the beach – if you have a really long beach, you’re more likely to have a bunch of small harems.”
In summary, southern elephant seals are under pressure to gain weight and become the largest and fattest to have a reproductive advantage. However, this pressure to maintain dominance could be driving male southern elephant seals to early death due to extreme polygyny. The research discovered that only 4% of male southern elephant seals become beachmasters while the remaining males have to compete for access to females.
Competitive Pressures on Southern Elephant Seals Result in Lower Survival Rates
A new study suggests that competitive pressures on maturing male southern elephant seals result in lower survival rates. Although the males become biologically capable of reproducing at around six years of age, they are rarely socially competitive enough to breed with females until nine to 12 years old. The researchers believe that the competitive pressures drive them to gain weight as quickly as possible, resulting in lower survival rates because the males forage at sea in areas that may put them at higher risk of predation. The study found that adult males concentrate their foraging effort in shallower waters, which are highly productive but also frequented by other marine predators, such as orcas and sleeper sharks.
Southern elephant seals spend much of the year at sea, and the males come ashore each August on Macquarie Island to try to establish dominance over beach areas. The females arrive in September and aggregate in groups, while the males come and try to defend those groups. Only around 4% of male southern elephant seals become “beachmasters,” the largest and most dominant males who control harems of breeding females. Beachmasters have a really loud roar that deters other males, which means some might not even come to shore at that time. The study was published in the journal Royal Society Open Science.
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