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Crittercams Provide Insights into Nurse Shark Behavior |
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Brian Handwerk and Mark R. Holmes for National Geographic News |
| July 4 , 2002 |
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Shark Photo Gallery: Go >> Soon, the shark specialists and their crew will fold their tents, dismantle the observation platform, and wrap-up another successful research season. Though they may relish their return to the world of fresh food and hot showers, leaving brings up mixed emotions. Jeff Carrier, a biologist at Albion College in Michigan, and Harold "Wes" Pratt, a biologist with the National Marine Fisheries Service, have come to this remote cove in the Florida Keys for the last 12 years to study nurse shark breeding and behavior. Carrier is reflective as another successful expedition comes to a close. "You know, it's all an adventure," he says. "It's all exciting. We're sitting in a pristine area of the tropics, surrounded by wildlife, and we're looking at what is probably how Florida used to be." Breeding Ground for Sharks, Science Carrier and Pratt have been able to fill in some of the blanks about the growth, maturation, and life cycles of nurse sharks because they are able to capture the same animals year after year. They've discovered that the nurse sharks in the Keys breeding grounds grow 3 to 5 inches (8 to 13 centimeters) a year, and don't reach sexual maturity until they are 18 to 24 years old. Such a slow reproductive cycle can become a problem if species populations are depleted by natural or human occurrences. Other findings shed some light on the still-mysterious processes of shark breeding and mating behavior. Wes Pratt calls nurse sharks the "couch potatoes" of sharks because they don't migrate like many other species. "They're born here in a tropical paradise, so why would they leave? But this sedentary nature creates a potential genetic problem," he said. Carrier agreed. "When you consider the fact that they don't migrate, how in the world can these animals maintain genetic diversity without seeing the negative effects of inbreeding?" Using DNA analysis, the scientists have determined that a female's litter of 25 to 35 pups can contain the offspring of four or five different male sharks. This may be a natural adaptation that helps to maintain the population's genetic diversity. "It's a strategy that helps remove the inbreeding problem," said Carrier. The team's observations suggest that only about 8 percent of the mating events result in copulation. Years of regular observation enabled Carrier and Pratt to identify a complex set of behaviors surrounding the mating process. Before copulation occurs, for instance, the scientists often observe several males competing for a grip on the female. Only one will succeed, but some of the others will stay in the area and may play some role in preventing the female from escaping. The group technique helps to ensure reproductive success, said Carrier. While it doesn't help the males who lose the mating battle, it's a survival strategy that aids the reproduction of the species as a whole. Crittercam Takes Shark Study to New Depths In his 30-plus years of shark research, Carrier has seen a lot of technology come and go. When asked what innovation has been the most significant technological advance for shark research, his answer is quick and somewhat surprising: "It's the video camera," he said. "It gives us a degree of quantification that just couldn't be attained without it. In my experience, behavioral sciences frustrated me before the video camera because it didn't appear that individual behaviors could be quantified. "Reviewing video provides the opportunity to reexamine behaviors in a way that single observations cannot hope to attain." These days, researchers employ Crittercam technology to take the research utility of the video camera to a new level. Mike Heithaus, a biologist at National Geographic's Remote Imaging Laboratory, and his team attach the small cameras to the shark's dorsal fin. This lets the scientists see the world from a shark's point of view, and also gives them the ability to observe what happens far away from human contact—scenes that could never have been witnessed before Crittercam. The mating sharks don't seem to be bothered by wearing the device; the males resume chasing after females once the Crittercam has been attached. "They are soon right back in business," said Heithaus. Heithaus is able to find his cameras once they release from the animals at a predetermined time by using radio transmitter signals. He hopes that the film they contain can help researchers solve two key puzzles. The first is the role of the shallow water nursery in the "big picture" of shark mating. Pratt and Carrier have observed the animals breeding in the shallow lagoon for years and have often asked themselves whether this is the only place they mate or whether they breed in deep water also, and if so, how that breeding differs from what happens in the lagoon. Crittercam may provide answers to these questions. The camera could also provide information on how the scientists themselves do, or do not, affect shark breeding while observing it in close quarters, Heithaus said. "They've been observing for years by being right in the sharks' faces while they are mating," he said. "They might be a foot from a shark's face while it's mating, so they want to see if they are affecting the process, although it seems that once those sharks get locked on they are ignoring pretty much everyone else." The first hints from this year's Crittercam footage suggest that mating in the lagoon is the same whether the scientists are present or not, but much more study is needed to know for sure. Early video returns also hint at some surprising events that occur when the sharks are away from the lagoon, such as displays of aggression between male sharks. Such findings are part of Crittercam's value. "The thing is that whenever we put these cameras on animals we find things that we never expected," says Heithaus. "And those things bring up more and more questions. Hopefully, we can answer a few along the way as well." Unexpected new questions are part of the appeal that drives these scientists back to the site each year. As much as they have learned about sharks, they still say that the "hows" and "whys" of what the animals do are a "big black box" inviting exploration. "Sharks are exhilarating, and awe inspiring and majestic," Carrier said. "It took nature 400 million years to perfect these animals." Now, dedicated scientists are finally making steps toward understanding them. Follow the progress of this National Geographic expedition to the Florida Keys: Expedition Report One: Scientists Study Nurse Shark Mating Habits Expedition Report Two: Researchers Tag Sharks to Study Breeding Habits National Geographic Shark Resources News Stories: Jaws Author Peter Benchley Talks Sharks Do Hammerheads Follow Magnetic Highways in Migration? Shark Nursery Yields Secrets of Breeding South Africa Rethinks Use of Shark Nets Sharks Falling Prey to Humans' Appetites Satellites Clear Up White Shark Mysteries Are People Eating Sharks Out of Existence? Shark Sites on Nationalgeographic.com: Creature Feature: Great White Sharks Ten Cool Things That You Didn't Know About Great White Sharks Print 'N' Go Coloring Book: Great White Sharks Shark Surfari: Online Quiz Related Lesson Plans: Use this National Geographic News article in your classroom with these Xpeditions lesson plans: Lesson Plan: A Trip to the Beach Lesson Plan: Are Sharks As Dangerous As We Think They Are? Lesson Plan: Does the Hammer Help? Lesson Plan: Sharks—Setting the Record Straight Lesson Plan: Sharks—Should They Be Afraid of Us? Lesson Plan: What's the Hammer For? Join the National Geographic Society Join the world's largest nonprofit scientific and educational organization, and help further our mission to increase and diffuse knowledge of the world and all that is in it. Membership dues are used to fund exploration and educational projects and members also receive 12 annual issues of the Society's official journal, National Geographic. Click here for details of our latest subscription offer: Go>> |
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