The Weddell's new coat
Scientists study how biology and environment influence seal molting, breeding
Posted January 10, 2014
The Weddell seal lifts her head high from her cold recline on the sea ice as the researchers approach – an impressive display of strength given her blubbery bulk and lack of dexterity out of the water.
The plastic tag on the hind flipper identifies her as an 11-year-old female that has had four previous pups but none this year. Her sheer immensity – scientist Linnea Pearson estimates the animal weighs at least 400 kilograms – implies she skipped this past year’s breeding cycle.
In contrast, the mothers still nursing their pups around the small island called Turtle Rock in McMurdo Sound appear downright skinny and wane. They’ve literally been drained from the energy demands of their hungry pups, which can be heard whining in whistles and chirps that sound like special effects sounds from a science fiction movie. The researchers pause for a moment to remark on the beautifully big, dark, watery eyes of one wee pinniped.
But it’s the so-called “skip breeder,” the behemoth of a Weddell seal that didn’t give birth this summer, that principal investigator Jennifer Burns and her team are interested in on this overcast day. They had hoped to find at least a couple such animals near Turtle Rock, located about 15 kilometers from McMurdo Station, based on earlier reconnaissance.
It took all of five minutes to locate the 11-year-old female from a list of about seven possible candidates, though a widening crack in the ice shooting away from Turtle Rock will make it difficult to search the rest of this section of the Erebus Bay seal colony.
But that can wait for later. The researchers have much work ahead of them as they prepare to sedate the seal and take a variety of biological samples. The study is part of a National Science Foundation -funded project to learn more about what drives the timing of a seal’s critical life history events – such as breeding and molting – and how disruptions in that natural cycle by changes in climate and environment might affect the world’s southernmost mammal.
The idea for the study actually emerged from a previous project on the same seal population, according to Burns. That research involved tracking Weddell seal behavior over the winter, as scientists know very little about what the animals do and where they go during that time of the year. Instruments attached to the seals also collected valuable physical oceanography data about the Ross Sea, such as temperature and salinity. [See previous article — Motion capture: Scientists track seals through winter to learn about ecology and oceanography.]
Photo Credit: Jenn Burns
Instruments glued to a Weddell seal for a previous project to study over-winter behavior.
The satellite tags and instrumentation had to be glued to the back of the animal’s head, meaning they could only be attached after the seals had molted; otherwise, the device would be lost once the Weddell had shed its fur. Burns noticed a curious trend among the seals outfitted with tracking instrumentation.
“Over the course of the study we had never put a satellite tag on a female that was known to pup [that season]. The chances of that happening were incredibly low,” explains Burns, a professor at the University of Alaska in Anchorage . About 80 percent of Weddell seal females in the colony give birth in a given year, she says.
In fact, thanks to a long-term database on the Weddell seal population maintained by researchers from Montana State University , Burns and her team determined that only about seven percent of all the females observed in January and February, when they were deploying satellite tags for winter observations, had pupped earlier that year.
“That was really unusual. It suggested there was a clear link between the probability of pupping and the timing of molting,” she says.
As is typical in science, the pursuit of one question led to a host of new problems to solve: What are the costs of a new fur coat?