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Night hunt

Scientists track seal predation behavior through the dark of Antarctica

 

Nearly 30 years ago, Randall Davis and three fellow seal researchers hunkered down for a winter at White Island near McMurdo Station. Their goal was to study the diving behavior of a group of Weddell seals during the long night of the Antarctic winter.

The living conditions were fairly primitive — two orange “fish huts” connected by a covered vestibule, with an outhouse on the side. The technology of the day was fairly simple — mechanical time-depth recorders that the scientists placed on the seal’s hind flippers when the animals occasionally hauled themselves onto the ice shelf.

They learned much about the animals’ behavior through observation, though hard data were sparse. Much of their time was spent looking after their own survival against wood-splintering-strength winds and temperatures that plunged to minus 50 degrees centigrade.

Fast-forward to 2010. Davis and co-principal investigators Lee Fuiman and Terrie Williams, and their teams, will embark on their third field season in August. The quest is still the same as it was three decades ago: How do Weddell seals behave and survive when the polar night dominates and when there is little light for hunting under the ice? But their technologies are decidedly more advanced.

“What we’re finding is that the seals will take advantage of what light is available, but hopefully when we analyze all of the data in detail, we’ll find periods that they’re actually hunting during the dark period of the day-night cycle that exists during Winfly,” explained Davis, a professor in the Department of Marine Biology at Texas A&M University in Galveston.

Operationally, Winfly is the time every August when the U.S. Antarctic Program sends in a vanguard of support workers — from carpenters and truck drivers to cooks and dishwashers — to McMurdo to augment the small winter crew. Their job is to prepare the research station for the main field season, which begins in September this year.

Scientifically, Winfly is about as close to winter darkness as Davis and his team can get, without actually wintering over. This year, they will arrive about a week earlier than the normal Winfly season, when there is still about 24 hours of darkness each day.

“The extra week will buy us some additional periods of darkness. The light level is changing so rapidly, even a week is helpful,” Davis said.

The first sunrise is Aug. 19, with sun visible for little more than an hour. By the end of the month, the sun is up for more than six-and-a-half hours. Daylight then gains about 15 minutes a day in September.

Noted Fuiman, “We run out of darkness pretty quickly.”

Years of research

Much of the research on Weddell seal behavior began in the 1960s with a physiologist from Scripps Institution of Oceanography in San Diego named Gerry Kooyman, who was later a mentor to both Davis and Williams.

Kooyman used a simple but effective time-depth recorder made out of a kitchen egg timer and a glass disk coated with fine charcoal dust to chart the seals’ dives. Changes in hydrostatic pressure as the seals changed depth forced a stylus to make etchings on the glass. The timer ticked off the minutes. Kooyman discovered that the seals could remain submerged for more than an hour, diving down 500 meters in depth.

Kooyman continued his work into the 1980s, when researchers like Davis and Williams took up new problems that required new technologies to address.

“Because we couldn’t observe the animal at depth, it left a lot of questions about what it was doing while diving unanswered,” Davis said.

That motivated him to develop an underwater video data recorder that could be mounted on the seals to record their hunting behavior along three-dimensional maps, swimming performance and environment.

A professor with the Marine Science Institute at the University of Texas at Austin and an expert in behavioral ecology, Fuiman joined Davis and Williams in the late 1990s to test the technological innovations.

“Since the seals’ behavior is done while hold their breath, the physiology is a pretty important part of understanding their ecology. Together the three of us make a really strong team in understanding what these seals are doing,” Fuiman said.

It worked. The technology not only offered insights into seal behavior, but also provided valuable data on two of its prey — the Antarctic silverfish and the Antarctic toothfish. They eventually published a paper on their findings in Marine Biology, along with numerous other papers on the seals.

“I don’t think anybody else has data like this because it is so difficult to collect data from the point of view of the predators. I think there is a goldmine there, and I suspect that there will be something publishable about seal prey in the future,” Fuiman said.

By a hair

In spite of their work on other critters, the focus of the group remains the Weddell seal.

The video data recorders glued onto the seals’ fur collect a variety of data that allow the researchers to create three-dimensional dive profiles, which they can then use to interpret the animal’s behavior. The data include depth, compass bearing and number of flipper strokes. The recorders also report on environmental conditions such as temperature, salinity and dissolved oxygen in the water. 

“It’s a suite of variables that allow us to look at diving and swimming performance, as well as the animal’s immediate environment in three-dimensional space,” Davis explained.

From the videos and other data, the seal biologists have learned a great deal about Leptonychotes weddellii behavior during the daylight hours. For example, after dropping away from a breathing hole in the ice, the seals become negatively buoyant in the first 30 to 50 meters, allowing them to dive with little effort as they make a “meandering descent,” according to Davis.

Davis said he believes that the seals rely primarily on vision when light is available, possibly detecting the silhouette of fish from below when they go in for a kill.

“They’re probably backlighting their prey, which means they’re relying on vision,” he said. “They have exquisite low-light vision. We’re pretty convinced of that.”

Fuiman politely disagrees with the “backlighting” assessment by his colleague. However, both men believe other senses are in play in low-light or dark conditions.

“If they’re catching fish in total darkness, then clearly they’re not using vision alone,” Fuiman noted. “It’s not likely that an animal is going to rely on one sense when it has so many available to it.”

Hearing is out. The seals don’t vocalize while hunting under water, and the fish don’t make noise.

Instead, the team believes the seals rely on vibrissae, or whiskers, which are not just hairs but very complicated sense organs with more than 500 nerve endings that attach to the animal’s snout.

“We think they could detect the wake of swimming fish and use that to capture prey,” Davis said.

Advances in technology

The video will provide some of the key evidence for the hypothesis, according to Fuiman. The camera is positioned in such a way that the scientists can observe the seal’s muzzle and the whiskers, allowing them to see if there is any movement prior to a capture.

“It will be interesting to see if their movement is more pronounced when it’s dark versus when it’s light,” Fuiman said.

So far, the team has been busy building the three-dimensional dive profiles, and is now combining them with the video-recorded behavior. The fourth generation of the video-data recorder system currently in use is about one-fourth the size of the instrument deployed back in 1997, according to Davis.  

While the instrument has shrunk in size, its memory capacity has increased tremendously with digital technology. (The original video was on 8mm tape.) That means more data — and more work.

“One of the curses of improving your technology is that we are deploying the animals for much longer periods of time, so now they’re out for many days,” Fuiman noted.

Davis said he hopes to test a fifth generation this season, miniaturized to a degree that the whole package fits on the head of the animal. The system currently includes a backpack. Digital memory is no longer a limiting factor, at this point, Davis noted.

“What defines the minimum size of the instrument now is the size of the batteries. It dictates how long you can record,” he said. The high-density lithium batteries are good for a one time use only at a cost of about $300 to $400.

On the hunt

The fieldwork itself may not be as difficult as surviving an Antarctic winter, but the temperature is still well below zero and unsettled weather quite common.

But the real challenge is finding a suitable film star, Davis said. The subfreezing water is still warmer than the ambient temperature at that time of year, so it takes a little patience to locate a seal that has ventured out of a hole and onto the ice surface.

Then it’s time for a little muscle, capturing and hauling the seal to a covered tent called a Jamesway where Williams’ team takes metabolic measurements. One experiment involves placing a box over the seal and measuring oxygen uptake by the animal.

“If you understand how far you can push the mammalian heart or lung, you start to better understand the human condition,” said Williams, a professor of ecology and evolutionary biology at University of California-Santa Cruz (UCSC).

Williams said previously in a press release from UCSC that she is also particularly interested in how the seals will respond to changes in their environment caused by global warming. “The seals are monitoring marked changes in the polar environment that will ultimately dictate their survival,” she said.

The team then outfits the animal with the video-data recorder instruments before releasing it into a nearby hole. “It’s about a two-day operation,” Davis said.

Each seal has a satellite transmitter and very high frequency (VHF) transmitter. When it eventually hauls out, it sends a signal to a satellite. Within about 90 minutes, the team receives the latitude and longitude location by e-mail, accurate to within 200 meters or so. Within a kilometer, the scientists can pick up the VHF signal and home in on their equipment.

They can track about a half-a-dozen seals in a season. Some may not reappear for several weeks, though the recovery rate is 100 percent, Davis said.

“It’s a waiting game.”

And, based on the results so far, well worth the wait.

NSF-funded research in this story: Randall Davis, Texas A&M University, Award No. 0739390; Lee Fuiman, University of Texas at Austin, Award No. 0739600; and Terrie Williams, University of California-Santa Cruz, Award No. 0739163.

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Curator: Peter Rejcek, Antarctic Support Contract | NSF Official: Winifred Reuning, Division of Polar Programs