Elephant Seals and Climate Change
Warmer Waters Mean More Males
Male elephant seal at Ano Nuevo State Reserve.
Photo by Marc Smith, used under Creative Commons.
Sex ratios of northern elephant seals may become dangerously skewed due to global climate change, according to a recent study published in the Journal of Mammalogy.
In their study, scientists Derek Lee (of PRBO Conservation Science) and William J. Sydeman (of the Farallon Institute) found that ocean temperature directly impacts the ratio of male to female seal pups born in a given year. With oceans warming globally, scientists worry about the long-term effects of this phenomenon on seal populations.
Normally, this peculiar response to water temperature is healthy for female elephant seals.
This seal species spends 90 percent of its life underwater, most often searching for prey during 25 to 30 minute dives with only a few minutes for breathing in between. After their winter of birthing and mating on the coasts and rocky islands of California, the different sexes of elephant seals migrate to completely different feeding grounds. Male seals swim directly north and feed off coastal prey near the Aleutian Islands of Alaska. Meanwhile, females swim farther east and hunt faster-moving, deep-water prey.
Warmer ocean temperatures mean that females must migrate farther for their prey, which also tends to be more dispersed in these conditions. By giving birth to more males than females in warmer years, adult females reduce competition for food from their daughters.
With natural weather patterns, this change in sex ratio presents no real problems to northern elephant seals. Water temperatures rise and fall with the flux between El Nino and La Nina, and so does the ratio between male and female seal pups. Even a particularly pronounced El Nino, as was seen between 1997 and 1998, has no long-term effect on seal populations. However, the steady rise of water temperatures predicted to continue in the seal’s habitats might create a permanently imbalanced sex ratio.
One possible outcome of this imbalance could be steeper competition for mates between males. Previous studies led by Burney Le Boeuf and Dan Costa at UC Santa Cruz revealed that male elephant seals attempt to woo and cloister several females apiece, so competition is already at play. Weighing in at 1,500 pounds on average and stretching 15 feet long, males employ physical heft to establish dominance among themselves. The effects of even greater numbers of males on these mating practices have yet to be documented. However, researcher Lee refers to the situation as a potential “evolutionary ‘trap’ that results in population decline.”
In light of the new research, it also appears that female survival and reproduction could become much more difficult in higher ocean temperatures. According to Le Boeuf and Costa, female elephant seals spent more time searching for pockets of prey than actively feeding during the El Nino of 1997 to 1998. As a result, their weight was lower than usual when they reached their breeding and mating grounds. Since adults do not eat during this time, blubber reserves are extremely important for successful reproduction. If warm water trends continue, females could be too underfed to birth sustainable levels of pups, and possibly to survive the process themselves.
In the Bay Area, northern elephant seals come ashore at Ano Nuevo state park during winter to molt, birth, and mate before parting ways with their young and the opposite sex for another year. You can visit the park with a special permit starting April 1st. Call (650) 879-2025 or visit www.parks.ca.gov for more information.