Temporary emigration by Weddell seals reveals behavioral responses to environment
Posted October 10, 2014
The prize for toughest mammal on Earth is probably debatable, but the Weddell seal would have to make anyone’s list of contenders based just on its choice of habitat.
Leptonychotes weddellii lives farther south than any other mammal, hauling out of the water during the brief Antarctic summer to breed on the consolidated sea ice that surrounds the continent. A colony of seals found in an area known as Erebus Bay lives at the extreme southern limit, which also happens to be a short snowmobile ride away from the U.S. Antarctic Program’s largest research facility at McMurdo Station.
For nearly a half-century researchers funded by the National Science Foundation have studied the Erebus Bay population, amassing one of the longest population datasets on record for a mammal species in the wild. The latest paper in a series of recent journal articles suggests that Weddell seal behavior can remain flexible even in the face of changing environmental conditions.
The paper in Ecology, in press but available online, proposes that some Weddell seals will skip returning to the Erebus Bay colony in McMurdo Sound in years when conditions prove too tough even for them, such as when the sea ice under which they must navigate to their breeding site is more extensive than normal.
“The more ice there is locally, the harder it is for the seals to come in, so it’s more likely for them to choose not to come in,” said Thierry Chambert, who led the field team from Montana State University for three austral summers in Antarctica. Lead author on the Ecology paper, Chambert is now a postdoctoral researcher at Penn State.
Other factors also play into whether or not a Weddell female returns to the breeding colony, such as its age, experience and physical condition, according to Chambert.
In fact, on average, about 86 percent of non-breeding females still return to the colony in a given year. Work by Chambert and his colleagues, including co-principal investigators Jay Rotella, Donald Siniff and Robert Garrott at Montana State University, has suggested there are a number of advantages to returning to the colony each summer.
For example, the colony offers protection against predation, since it’s generally located far enough from the edge of the sea ice where killer whales prowl for prey. There is also likely a social aspect to gathering at the colony, which begins coming together in October: The synchronization of estrus for the females, helping set the biological clock for fertility and reproduction.
Follow the expedition
Chambert’s predecessor, Glenn Stauffer, was the lead author of another paper in Ecology earlier this year that also suggested prebreeders – females that haven’t had a pup yet – might return to the colony to learn breeding behaviors and to synchronize estrus.
Rotella said that the drive for most non-breeding Weddell seal female seals to return to their breeding grounds is fairly unique for a colonial animal. “A lot of [terrestrial] animals don’t do that. If you’re not breeding, you don’t want to be there,” he explained.
That’s because colonies are crowded and competitive, inviting aggression for scarce resources, whether for food or a crack in the ice for hauling out. However, unlike habitats where temperate, terrestrial animals gather to breed, the Weddell seal colony is generally located away from predators.
Those animals that choose to take a sabbatical from the colony in a given year fall into some broad categories, though there is some individual heterogeneity involved. Many are either young or old animals, unable to make the journey under the sea ice. Others may just be in poor physical condition, requiring them to remain closer to the ice edge where food resources – and predators – are more plentiful.
What Chambert refers to as the “quality” of a seal is certainly a factor. A handful repeatedly avoids the colony.
“That’s a minority of individuals,” he said, adding that surveys of other Weddell seal colonies farther north has rarely found these “temporary emigrants” breeding elsewhere.
“It’s extremely rare that a seal born in Erebus Bay is having a pup outside of Erebus Bay,” he said.
Photo Credit: Jay Rotella
Field team members weigh a Weddell seal mother a few days after she gave birth to learn how body mass might vary among individuals and between years to see if such variables are a factor on whether or not the individuals will return to the colony the following year.
The Montana State University-led study will enter its 47th year during the 2014-15 field season. Researchers have tagged more than 22,500 Weddell seals during that time. Since 1973, all the pups that have been born into the Erebus Bay population have also been tagged, meaning about 65 to 70 percent of the colony is of known age, providing the team a wealth of information for its studies.
“Knowing how often they do leave for a year – skip being there for a year – helps put our survey counts in perspective,” said Rotella, a day before the current MSU field team was preparing to leave for Antarctica to begin its annual census work.
Rotella explained that, in part, the impetus for the studies on “temporary emigrants” goes back about a decade to the austral summer of 2004-05. Giant icebergs had formed something of a blockade into McMurdo Sound following a major calving event from the Ross Ice Shelf in 2000. Thick, multi-year sea ice was trapped, extending the distance the seals needed to travel to reach their colony at Erebus Bay.
“When we first showed up in that colony, we had very low numbers of pups, very low numbers of moms, very low numbers of seals in general,” Rotella recalled.
In fact, 2004 turned out to be a record low with only 169 pups, followed by 249 pups in 2005. No other year had ever fallen below 345, according to Rotella. The colony generally averages about 425 newborns, though only about 20 percent will survive to adulthood.
Within two years the colony returned to relative normalcy.
“They clearly survived and did something different,” Rotella said. “That’s one of the things that got us into this temporary emigration issue.
“They’ve been doing pretty well ever since,” he added.
The population is doing so well, in fact, that five of the last seven years have witnessed record numbers of pups born at Erebus Bay. The record was set in 2010 with 603 pups, the first and only time the number has exceeded 600.
Why that is remains a mystery. One theory is that there may simply be more food available for the seals in the region. Data are scarce on the amount of fish biomass in the Ross Sea, so the MSU team, in collaboration with Kevin Arrigo at Stanford University, is attempting to determine the amount of resources indirectly using satellites.
Remote-sensing techniques can spy large phytoplankton blooms from orbit. The microscopic plants are a key part of the polar food web, and major blooms during the summer may lead to more fish and prey for larger predators like the Weddell seal.
A commercial fishery that operates in the Ross Sea may also be playing an indirect role by removing Antarctic toothfish. Weddell seals prey on Dissostichus mawsoni, but both species also favor Antarctic silverfish. Some researchers suspect the Ross Sea fishery may be depleting a major competitor for silverfish.
Research into these and other puzzles will continue into 2014-15. Rotella said it’s impossible to say how the upcoming field season will unfold, as each year presents its own challenges and surprises, from the behaviors of the animals themselves to the dynamic nature of the sea ice on which the team lives and works for up to three months.
“It’s always fun to see how the year is going to stack up against the previous years,” Rotella said.
[See related article — Sealed and delivered: Long-term population study of Weddell seals makes some 'super' discoveries.]