Wednesday, December 07, 2005

Elephant Seal Biology 101

Elephant seals are the biggest seals on the planet. The northern hemisphere species breeds from Mexico to northern California, but their feeding range reaches far offshore and north to Alaska. Males are significantly larger than females and also have a large, dangling proboscis (nose) that gives them their name. Adult males can weigh up to 2 tons at a lenth of 12-15 feet, and females can range between 800 to 1500 pounds at a length of about 9 feet. Elephant seals spend most of their lives in the water and are the deepest diving seals.
Here's a picture where you can see the size differences between a cow and her pup and a bull.
During the breeding season these animals haul out on land and fast the entire time. So all the energy they need to sustain themselves and give milk to their pups, plus all neccessary water for a month is provided entirely by their body fat. Cows give birth to a single pup a few days after hauling out on a rookery and nurse it an average of 28 days. In that time they manage to increase the weight of the pup from 80 pounds at birth to 200 pounds at weaning.

Mother and newborn pup

Then the cows return to sea, leaving their now weaned pups behind. Male elephant seals will stay on land not just a month, but the entire breeding season of about 100 days, fasting all this time. It is no wonder that this physiological stress, combined with their ferocious fighting for mating privileges, restricts a male life span to only 10 - 12 years. Females on the other hand, can live to over 20 years of age, generally producing a single pup each year, starting to breed between age 3-6. Elephant seals are physiological marvels, fasting twice a year during the breeding and molting haulouts, migrating vast distances twice each year between these haul outs, and diving to depths where steel is crushed.


A healthy weanling

The weanlings left behind molt their long black lanugo fur to a short silver coat soon after their mothers depart. They remain for a month or so, learning to swim in tidepools and shallow coves at night. When their hunger finally triggers their departure, they swim along the bottom. These behaviors are thought to be adaptations to shark predation pressure. White sharks are daytime ambush predators that strike prey near the surface suddenly from below. The weanlings will return to the Farallones in the fall to molt again, and that is when you will learn about the sharks and their seal prey in realtime.

Posted by: Danny

1 comment:

Richard said...

Danny-You say weanlings, and I say weaner...??