Wednesday, January 19, 2011

The Farallones Elephant Seal Factory

First thing is first. A new introduction is of primary importance. The winter researcher colony gained another member just before the holidays. Jaime Neill has joined the crew as our third winter intern. She hails from North Carolina and has recently been working with nesting sea turtles in Florida. She has experience with a variety of both terrestrial and aquatic species, but is really enjoying her transition into fieldwork with pinnipeds - which has really started to pick up since our last update.

Jaime preparing to stamp some seals

 The elephant seal pupping season is well underway and is keeping the winter crew quite busy here on the Farallones. The Northern Elephant seal breeds on rocky islands and coastlines such as the Farallon Islands throughout the winter and early spring. The males arrive first, generally in early December, with older sub-adults and full-grown bulls staking out their territories and battling each other for prime real estate. Pregnant cows begin to haul out shortly after the males and within a week of arriving they give birth to a single pup weighing 60-80 lbs. They nurse their pup continuously while fasting for about 27 days during which the pup gains an average of 10 lbs a day. During the breeding season, cows generally huddle in large groups, or harems, protected by a dominant bull from the pestering of younger males.  Bulls fight violently for possession of a territory and access to all the cows in one harem - up to 100 females - with whom they will mate. Sub-adult males may gain access to a few cows at the periphery of an alpha’s territory, but are often chased off. After mating, cows wean their pups and head back into open waters in search of food, returning to land only one other time during the year to molt.

Mirounga Beach harem
Sand Flat harem

We are now within the height of the breeding season on the South East Farallon Island (SEFI) and familiar patterns are beginning to emerge. Rusty, a bull that held alpha status on Sand Flat during the entirety of the breeding season last year, has claimed the same territory and is currently king of a 47 cow harem (which grows larger by the day). He is seen daily completely surrounded by rows of cows with pups on their sides. Bedlam Boy, one of the largest males that we’ve seen on SEFI, was holding down Mirounga Beach with 11 cows until he was usurped by MC Hammer a few days ago. MC Hammer was alpha of Mirounga Beach last year, so it was only a matter of time before he fought to get his territory back. Two other potential players are B-boy and Herzog, who have been hanging around the periphery of Sand Flat and may end up facing off with the alphas later in the season once cows become sexually receptive. Herzog was not tagged prior to arriving on SEFI this year and may be a bull that has not previously bred here at the island. Despite their enormous size, the bulls have been moving around the island quite a bit in their attempts to claim territories. We have not witnessed a major territorial battle just yet, but the larger males have been chasing younger animals that try to sneak into the periphery and occasionally displaying dominance by mounting females in their area. Some younger males have been seen as far away as the cart path near East Landing, undoubtedly having been forced out by more dominant animals. 

Rusty at Sand Flat
Rusty guarding his cows
Rusty performing a dominance display
Bedlam Boy at Mirounga Beach

Subadult male near old telephone pole by East Landing

The birth of the first pup on SEFI was witnessed on Sand Flat on December 23. We presume that the cow that gave birth was a young and inexperienced mother because we never observed her nursing. Cows start giving birth at 3 or 4 years of age but their pup mortality is high until they become more experienced at around 7 or 8 years. Unfortunately, this already doomed pup was unwittingly crushed by MC Hammer during a dominance display, a common fate in crowded elephant seal colonies, and did not survive. Pregnant cows started showing up in larger numbers at the beginning of the year, with at least two new cows and an average of two pups born daily starting on the 2nd of January. Currently, we have an estimated 24 live pups on SEFI and 5 on West End. We have been stamping new cows and new pups with numbers coated in hair dye in order to more easily keep track of both, a task that is becoming increasingly difficult as the colonies increase in density. Today, we counted 47 cows on Sand Flat with more cows (and pups) arriving daily. Tagging pregnant cows has proven to be a challenge because placing tags in the hindflippers of large animals is only successful if the animal is fully asleep, which cows never seem to be. Even prior to giving birth, a cow is constantly vigilant. We’ve witnessed a number of violent displays between protective mothers, some of which involved errant pups being bitten in the head by other cows. 

Stamped pup
Tagged and stamped cow
New tag!

We have been making every attempt to mark new pups with numbers matching their mothers’ stamps as pup switching can happen in crowded elephant seal colonies. One incident in particular has captured our attention. A stillborn pup was found on Sand Flat one morning with no mother in sight. Later the same day, we witnessed a very young-looking cow showing particular interest in a pup as it was being born to Gypsy, another resident Sand Flat cow. As soon as the pup emerged and started to vocalize, the young cow began vocalizing in return to the pup and seems to have imprinted on him/her. Although we have not witnessed her nursing, she seems to respond to Gypsy’s pup vocally and remains near the pup as much as possible. Occasionally, we’ve seen her touring the periphery of the harem and eyeing other pups, usually resulting in being chased off by their mothers. Although she reacted violently to her initial attempts to claim the pup, Gypsy now allows the abductor cow, whom we’ve informally named Abby Ductor (or Baby Stealer), to remain near the pup. We hypothesize that Baby Stealer is another young and inexperienced cow that gave birth to the stillborn pup and is confused about her role as a mother. We have marked the three animals and are keeping an eye on them to see where the pup ends up. Some pups become superweaners - after being weaned from their own mother, they continue sucking from other cows that may have lost or switched their pup later in the season. It is possible that Gypsy’s pup may end up as one of these “double mother sucklers.” Indeed, we've already witnessed some greedy pups nursing from moms which are not their own.

Abductor cow (top) on the prowl for pups
Three pups trying to nurse from one cow

We will continue to monitor the elephant seal population here on the Farallones throughout the winter while attending to some of our other responsibilities – one of which includes surveying the endemic Farallon Island Arboreal Salamander. More detail will be given to the salamanders in our next blog entry.