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NEWS Sink or Swim ... or GlideVirtual rides reveal energy strategy of deep-diving marine mammals.By A.J.S. Rayl
Whales, dolphins, and seals seem to defy reality with every deep dive into the big blue. They descend tens to hundreds of meters and, despite limited oxygen, can stay submerged for 30 minutes or more. How they achieve these feats has mystified oceanic observers since physiological studies first began some 70 years ago. During the last several years, a team of researchers headed by Terrie M. Williams, professor of biology at the University of California, Santa Cruz, has taken a series of virtual journeys into the ocean worlds of a bottlenose dolphin, a blue whale, an elephant seal, and three Weddell seals. Now they've surfaced with data that reveal these animals not only 'got game,' they 'got strategy.' The researchers recently reported1 that these deep-diving marine mammals conserve energy by gliding rather than swimming to the depths--a finding that will rewrite the books on marine mammals. Even more intriguing, Williams says, each of these distinct species appears to shut down its locomotor systems at depth. Marine biologists have long dreamed about following these graceful creatures. "We had been wanting to do this forever but had to wait for the technology to catch up, specifically for it to get small enough to be unobtrusive to the animals and retrievable by us," Williams offers. Once the miniaturized equipment was available, the most significant challenge was attaching all the tiny devices. "With marine mammals and especially dolphins, nothing sticks," explains Williams. "The cells on their skin turn over really quickly, so it's almost as if you have a thin surface on top of glass."
For the first-phase research, where one trained bottlenose dolphin dived on command to various submerged targets near San Diego, Williams and crew used a prosthetically designed, vacuum-molded plastic piece custom-fit onto the dorsal fin. The prosthetic fin carried a camera synchronized with time-depth recorders and velocity meters, three-dimensional compasses, hydrophones, and heart rate meters. "The camera was equipped with [light-emitting diode] lights that [allowed us] to see the objects that were in front of or behind the animal by about a meter or so. But they were invisible to the animal itself, so it didn't interrupt hunting," explains Williams. The researchers enlisted other preselected mammals, all of which, according to Williams, were "mild mannered and easily accepted the equipment." They recruited a northern elephant seal diving freely in a canyon off Monterey, Calif.; three adult Weddell seals at McMurdo Station in Antarctica, who dived nearly 700 meters down through an isolated hole in the ice; and one adult blue whale, a passerby, roaming Cordell Bank off the northern California coastline. Custom-designed prosthetic tools weren't practical for these animals, so the researchers used a specially designed neoprene belt with suction cups on the bottom--something that "looks a lot like a bathmat," says Williams. "That allowed us to place the cameras, which are about the size of the small Sony Handi-Cams broken down into separate pieces, and all the other instrumentation." They outfitted the blue whale with the National Geographic Society's CRITTERCAM, which was attached via a "low-profile silicon" suction cup held in place with a magnesium plug that dissolves in the salt water. Reviewing the videotapes, the investigators found themselves on "E-ticket" virtual rides into the depths. "For the first time, you actually get a feel for what it's like to be one of these animals," says John Francis of the National Geographic Society's research and exploration committee, who worked with the blue whale. Lee A. Fuiman, behavioral ecologist at the Marine Science Institute at the University of Texas, Austin, who worked with the Weddell seals, adds, "You can only imagine how exciting it is to see things people haven't seen before."
The Long Glide HomeThe first major discovery--that these mammals glide rather than actually swim to the depths--may appear minor to the unstudied in marine biology. But the finding is wiping out previously held notions and raising a host of new questions. It had been previously theorized, for example, that marine mammals swim constantly at cost-effective routine speeds during diving.2,3 "When you look at animals swimming near the water's surface, which is all we've had to view in previous years, they all had to stroke to keep moving," explains Williams. "We had an idea of how big their oxygen 'tanks' were, and an idea of what they were going down with when they were holding their breaths and how long they were able to dive. The problem was they kept beating all the records and not showing any big increase in anaerobic metabolism. That left us with a paradox." Still, to theorize that these mammals somehow glided with virtually no movement "just wasn't intuitive in terms of swimming animals." In hindsight, of course, it makes sense. "It's almost as if the physics demands that kind of response," Williams suggests. "Think about biking and how people draft. It's one of those things where you recognize, 'Hey this is an easy way out,' and [take] advantage of that. We're seeing the same kind of behavior here. These animals are taking the easy way out. Why take a stroke when you don't have to?" Scientists have also previously suggested that the mammals somehow reduce their body temperatures and/or metabolic rates to conserve energy for their deep dives. "Those theories on dropping temperature and metabolisms may enter into this, but on a more subtle level," says Williams. Interestingly, the "prolonged gliding behavior by diving marine mammals appears to be a general phenomenon, irrespective of the method of propulsion and size of the animal," the investigators stated in their study. "We saw the same strategy for conserving energy among four very different marine mammals, and that is interesting," says Fuiman.
The Science of SinkingJust how little the animals moved while gliding, however, is what immediately surprised the researchers. Williams points out, "We'd watch a depth gauge flicking by--50 meters, 100 meters, 150 meters--and the animals weren't moving a muscle. It was just beautiful to watch them glide through these water columns and yet never take a stroke. That was the very first impression for all of the species; nothing seemed to move. We could tell when the animals took a breath, but that was about it." The buoyancy changes that result from lung compression at depth, the researchers found, is what allows them to glide so easily. Escalating pressure as they descend causes their lungs to shrink in volume and at some points completely collapse. This compression reduces the animal's overall volume without altering its mass; therefore, buoyancy decreases while density increases, allowing them to descend fairly rapidly with virtually no movement. "If you look at calculations in Terrie Williams' lab, the bottlenose dolphins are still positively buoyant in the upper 30 to 40 to 50 meters or so, because they haven't expelled all the gas in their lungs," Fuiman expounds. "Once they get below 50 meters, the residual amount of gas compresses enough so they become negatively buoyant, and then they glide without expending any energy. At that point, their lungs are essentially collapsed." At deep destinations, the animals "seem to play these tricks of shutting everything down. It seemed like, as they got to depth, they just turned everything off," says Williams.
The blue whale didn't seem to move at all. It was, at first, confounding. "Watching the tapes over and over again for two hours of a dive, I thought, 'Gee, this is real nice and blue, blue ocean, great big blue back.' But we didn't see anything. I'm thinking, 'There's no science in here,'" she remembers. But what if they sped up the tape? "Suddenly," says Williams, "it was all there, like magic." It was then they began to realize that whales--from a human perspective--live in a whole different time scale. "In the depths, the blue whale moved at a rate about 10 times slower than humans move, so all the movements it was making were so subtle and were happening so slowly that we couldn't detect them by looking at it in real time." The team has just completed a more detailed study of the elephant seal that reports on its three-dimensional movements under water.4 They've found that in a series of eight dives on which the seal covered several kilometers under water, the animal is moving in a remarkably straight path. "How do you do that when there are crosscurrents and no visual signals?" Fuiman asks. "We don't have the answer yet, but now we have the question." Right now, the team has turned its attention to the Weddell seals. "There's a host of things we've been looking at, and one of them is how the animals hunt, how they go about finding prey when it's cold and pitch black," says Williams. The team has also begun to analyze the acquired data to determine whether the Weddell seals do have some mechanism by which they reduce their metabolic rates at depths. If what they've already learned from these marine mammals wasn't enough to justify their grant, the tapes are yielding other data about other marine life. From the Weddell seals, Fuiman, whose background is in fish behavior, is preparing a report on the behavior and distribution of the Antarctic silver fish, the center of the food chain in the ice-covered waters. The team has another year to review and report on the data in hand. Even so, they are already looking to go into the wild. "We want to get a little bolder with the cameras and send them out on animals in open-water environments," says Williams. That, however, requires an even easier, quicker method of attaching and retrieving the equipment. To develop a method, they called on Jack O'Neill, designer of the premier wetsuit that bears his name and is worn by surf riders worldwide. O'Neill has already created a sort of wetsuit-belt backed with the suction cup material. "It will allow us to put the instrumentation on really quickly, with the help of a Velcro strap. Then, by using magnesium bolts or links, the belt connections will dissolve away and the whole package will come off and float until retrieval," Williams explains. Another century-old mystery researchers will explore on future missions is the intelligence of the whales and dolphins and how that intelligence manifests in their environments. In the initial study, intelligence did come into play with regard to the dolphin, because, Williams points out, the dolphin was trained. "We worked with the dolphin as a scientific subject as opposed to just observing," she explains. "It was a cooperative thing in terms of the scientist asking the question and then the subject understanding what we wanted in terms of performance and being able to do it." Intelligence, however, opines Francis, is "a relative" term. "Blue whales travel the world ocean and have to find patches of food that are quite dispersed and unpredictable. How do they navigate the deep dark ocean? How do they know where to go to feed? What do they do once they find a patch? These are questions we might address that bear on intelligence." The most intriguing mystery about the blue whale, however, is its use of deep vocalizations. "These are the loudest calls in all the animal kingdom, and nobody knows what purpose they serve," Francis says. "One of the things we hope to do is to measure the call frequencies in association with the videotapes and find out what these whales are calling about."
The Big PictureFor each of the researchers, the big-picture implications are what the research is all about. Insight into the three-dimensional worlds of these mammals, all agree, may help humans save animals and humans. "Understanding what's going on the 95 percent of the time that we don't see the animals will have, in the long run, implications in terms of what these animals need in order to make it, to survive in the marine environment," Williams says. "This kind of research will give us the data needed to determine how big of a reserve we would need to create for a dolphin or a blue whale. It's a world we humans just don't yet really understand at all. Their territory ranges not only in a horizontal plane but in a vertical plane." Their work has not come without opposition. "We have gotten some backlash from people who want to wish all the animals well and not have us invade their privacy," admits Williams. "I do appreciate that point of view. I wish we could just stand on a cliff and look at happy dolphins and sea otters frolicking. It's not that simple. We have damaged the oceans in ways we haven't even realized yet. We see all these animals come up on beaches and gray whales dying. You just know something is very, very wrong. If we don't do something and find out a little bit more about what these animals are doing and what they need, we're going to lose them." Williams pauses, then adds: "There's a much bigger mission here. Trust me, it's not the money. This is for the long run." S
A.J.S. Rayl (ajsrayl@loop.com) is a freelance writer in Malibu, Calif.
References 1. T.M. Williams et al., "Sink or swim: strategies for cost-efficient diving by marine mammals," Science, 288:133-6, April 7, 2000. 2. D. Thompson et al., "How fast should I swim? Behavioral implications of diving physiology," Zoological Society of London Symposia, 66:349, 1993. 3. T.M. Williams et al., "Balancing power and speed in bottlenose dolphins (Tursiops truncatus)," Zoological Society of London Symposia, 66:383, 1993. 4. In press, Journal of Comparative Biochemistry and Physiology
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