The Birthing and Breeding Season
In late November and early December the adult male seals, weighing over two tons, arrive at the rookery and threaten, intimidate or, if necessary, fight for dominance over a portion of the beach. The battles can be both dramatic and bloody but seldom cause any life threatening injury. A dominant or alpha bull emerges from this competition along with a few beta males that are lower in the hierarchy. Throughout the six miles of the rookery there are roughly 100 alphas. As all of the seals, except nursing pups, fast while in the rookery, unnecessary use of energy is avoided.
The pregnant females begin arriving about mid-December forming harems around the alphas, apparently choosing the beach rather than the alpha. Their arrivals peak around mid-January and end in early February. Birthing takes place within a week of arrival and usually takes less than an hour. The new pup is born either head first or tail first and weighs around 70 pounds (32 kilograms). Right after the birth the mother vocalizes to her pup, bonding so that they can keep track of each other on the beach.
Pups nurse for four weeks, gaining weight rapidly on milk that is initially 12% fat and increases to 60% fat at the end of the period. After the four weeks of nursing the pup’s weight has quadrupled and the fasting mother has lost twice as much as the pup has gained. During the fourth week the mother goes into estrus, mates several times then abruptly weans her pup by heading to sea. Family life is over.
With the peak of birthing in mid-January, the peak of breeding comes in mid-February and extends into early March. By that time the number of adults has greatly diminished although most of the dominant males will hang on until the last female has gone. They have been on the beach by then for three months or more and lost almost a ton of their weight.
The beach is a threatening place for the newly weaned pups as long as nursing mothers remain in significant number. Those mothers are very aggressive in defending the space around their pup. When the coast is clear, the “weaners” will make their first entry to the ocean, preferring the sheltered tide pools in the rocky outcrops off shore. There they build muscle and skill while reducing their (buoyant) fat content. There are also physiological changes during the eight to ten weeks between weaning and departure including a considerable expansion of the oxygen storage capacity of both their blood and muscle. One at a time, as they feel ready, they go to sea and begin a foraging trip that will bring them back to the rookery in October or November.
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