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February 23, 2006

Northern Elephant Seals: Mating

Alpha Male Mating In The Harem
Año Nuevo State Reserve

The first males that haul out in December are often the alpha males from the previous season, followed by the other large males. The males fight each other until they have established a dominance hierarchy. The alpha males, or males that successfully defend their territory and establish harems, will mate with anything from 30 to 70 cows.

Cows begin breeding at around 5 years old, and have one pup a year for their entire lifetimes. Bulls cannot mate until they are 4, but really, they do not mate until they are old enough and large enough to establish themselves as dominant alphas, usually over 8 years old.

Northern Elephant Seals Mating
Año Nuevo State Reserve

Only 5% of all male pups born will eventually become alpha males and mate with cows. And since there were less than 100 seals left less than a century ago, the several hundred thousand elephant seals today all are descendants of this small population. This has led to a genetic bottleneck - all the seals are vulnerable to the same factors. Also, since only the largest males are fathering all the pups, the elephant seals are growing larger as the generations pass.

Beta Males Mate When They Can
Año Nuevo State Reserve

Alpha males will often allow Beta males - the mature males that lose to the Alphas but are still high up in the dominance hierarchy - to hang out on the edges of his harem. When any cow arrives, very pregnant and looking to join a stable harem, she must get past the Beta males. Any arriving male that wants to challenge the Alpha for his harem will have to run a gauntlet of beta males first. The bulls fast through the mating season, losing weight and fighting advantage as the season progresses. At the beginning of the season, you're likely to see one large bull surrounded by lots of cows and little pups, but by the end of the season, when the alpha has fasted for almost three months, you'll see many large bulls fighting to get a little mating in.

Posted by sorsha at February 23, 2006 1:41 PM


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