Peak of the Elephant Seal Breeding Season

The last days of January are the peak of the northern elephant seal breeding season on the South Farallon Islands. Most cows have already arrived and given birth on the rookeries, which echo with the sounds of cows calling to their screaming pups and males bellowing to intruders. We can hear the cacophony all the way up at the PRBO house! SEFI's Sand Flat (pictured on the left), where Rusty is the reigning alpha bull, currently has 73 cows, 52 live pups (9 dead), and 2 weaned pups, including the cow Daniela's weanling. The Marine Terrace colony has 15 cows, 11 live pups (1 dead), and 2 weaned pups, including the cow Shaye's weanling, with Flaccido Domingo the alpha defending this harem. Mirounga Beach, where McHammer is alpha male, has only 4 cows and 2 pups, after last week's massive storms washed out 3 pups and forced 16 cows up to Sand Flat, including the cow Aprile and her pup who are doing fine! On West End, 8 cows and 5 live pups are at Pastel Cave Highlands, defended by Felipe, and 10 cows, 4 live pups (3 dead), and 1 weaned pup are on Shell Beach with Bedlam Boy as alpha bull.

Flaccido Domingo claims the cow Carmen, in a display to a male challenger.

Northern elephant seals are fascinating in their ability to live off their blubber for long periods of time, a critical part of their life-history strategy. Called 'capital breeders' because they feed their pups entirely from their stored blubber reserves, adult females fast the entire time they are nursing their pups, about 30-35 days. 'Income breeders' like California sea lions must leave their pup on the beach occasionally to forage for more energy to maintain themselves and their growing pups. By not having to forage while nursing, the elephant seal cow transfers a great deal of milk energy to her pup in a short time. This allows her to wean the pup quickly and return to sea to prepare for future offspring by foraging on her own without encumbrance. The largest breeding males fast for more than 100 days, allowing them to be vigilant in fighting and defending their harem of cows from other male challengers, and then mating with every receptive female as their reward. Weaned pups live off their blubber for several months while they learn to swim and dive before first heading off to sea to forage.

Rusty bellows a warning to a male attempting to intrude on "his" cows.

Rusty's reward for defending the harem is to mate with >70 receptive females!

Given that elephant seals must live entirely off their blubber while on land, it is no surprise they spend as much time as possible sleeping. Elephant seals undergo sleep apnea (holding their breath) for up to 25 minutes, enabling them to reduce water loss from respiration and to slow lipid metabolism - the only source of water for a fasting seal.

Cows, pups, and a bull sleep long hours to save water and energy.

Cows do not eat for 35 days or more while transfering milk energy to their rapidly growing pups.

Weaned pups live off a thick layer of blubber while learning to swim and dive.