Dolphin Mystery: What's Killing Firstborn Calves?
National Geographic Channel
|January 8, 2004|
Off Florida's Gulf Coast, firstborn bottlenose dolphins are dying in
disproportionate numbers, and biologists can't pinpoint exactly why.
Scientists working with the Sarasota Dolphin Research Program in Florida have added this question to their research agenda. Now when they collect dolphin data, they run lab tests to determine the level of man-made toxins they've accumulated in their bodies.
In search of information on dolphin behavior and health, a team of researchers, veterinarians, and dolphin handlers periodically takes to the sea to study them in the wild.
On a recent expedition, a flotilla of nine boats with 60 scientists aboard converged on a group of bottlenose dolphins frolicking near the shoreline in Sarasota Bay.
The lead boat encircled the dolphins in a net "pen," and a few researchers jumped in with them. Working quickly and gently, they wrapped each dolphin in a bear hug, conducted a quick exam, and took a blood sample from a vein in their tail. If the animal wasn't pregnant, a veterinarian gently placed it aboard a second boat on a stretcher for some quick measurements and additional body fluid samples, carefully monitoring for any signs of distress.
Although dolphin numbers in these waters have risen slightly over the last few decades, the scientists are trying to understand why so many firstborn calves die: Less than a quarter live out their first year, compared with 70 to 80 percent of subsequent births.
One reason may be man-made pollutantsand the transfer of chemical contaminants to calves through milk, according to Randy Wells, a conservation biologist with the Chicago Zoological Society and director of the Mote Marine Laboratory for Marine Mammal and Sea Turtle Research in Sarasota. Since 1970, Wells has led the world's longest-running dolphin study.
"There's thousands of contaminants that man has put in the water," he said. "Finding a smoking gun is very challenging, but the weight of evidence shows that environmental contamination is one factor in calf mortality." However, exposure is difficult to quantifyand it's difficult to link exposure with symptoms or death.
Sea mammals sit high on the food chain and are long-lived. Through the fish they eat, they accumulate high concentrations of an array of pesticides, polychlorinated biphenyls (PCBs), and other chemicals in their blubber. "Some of the highest levels we find anywhere in the animal kingdom are found in dolphins," said John Kucklick, an environmental chemist with the National Institute of Standards and Technology in Charleston, South Carolina.
When a female dolphin is producing milk, she's using up blubber, says Wells, and the chemicals stored there pass into the milkand to her calf. Mothers give birth at about eight years old, so their firstborn calves receive an extra-large dose compared to later-born calves that come every two to three years, when the toxic load is lighter.
Northern fur seals in the Bering Sea and killer whales off the Alaskan coast also lose firstborns, said Todd Ohara, a wildlife veterinary toxicologist at the University of Alaska Fairbanks. "We're very worried about neonatal exposure. But trying to link specific contaminants [with the deaths] is difficult," he said.
Since the Second World War, more than 75,000 new chemicals have been developed and put into use according to the Washington D.C.-based Environmental Working Group. Many of these chemicals are dumped into oceans or waterways. United States law does not require chemical companies to conduct health and safety tests on their products, so no data exists for the vast majority of chemicals.
Florida's Gulf Coast provides an excellent natural laboratory to study dolphins. Mote researchers have followed families of dolphins here for over three decades and still track about one-third of their original 47 subjectsas well as their calves, grand-calves, and great-grand calves.
"We currently have about 140 dolphins regularly using Sarasota Bay, with another 2,400 individuals roaming up and down the central west coast from Tampa Bay to Charlotte Harbor," said Wells. Studying the environmental effects of contaminants is just one of 20 studies currently underway under the Sarasota Dolphin Research Program.
The Gulf carries higher concentrations of contaminants than the open ocean, says Kucklickand firstborn dolphin deaths are higher there, too. In a study of Atlantic spotted dolphins in pristine Bahamas waters, just 25 percent of firstborns die, says Denise Herzine, research director of the Wild Dolphin Project in Jupiter, Florida.
An Array of Toxins
Pesticides are chemicals that were designed to act on the central nervous system, explains Olson; they impair immunity and disrupt hormonal activity in the thyroid and other glands. PCBs have been linked to premature births, sterility, and other reproductive failure. "The levels are at those where we can expect to see some of these effects," said Kucklick.
PCBs are a family of 209 chemical compounds originally used as flame retardants and electrical insulators that were eventually used in products from cereal boxes and varnishes to bread wrappers until they were banned in the U.S. in 1979.
But the effects of these chemicals can be subtle. Studies on humans have shown that PCBs affect learning and hearing developmentboth of which are particularly important to dolphins, famously social creatures who communicate via sound.
Lowered immunity may mean that a calf doesn't recover from a wound or minor infection. When combined with other factors, like the less-than-optimal attention of inexperienced, first-time mothers, firstborn dolphins are at a clear disadvantage. "We know much more about the effects of these chemicals in people than we do in wildlife," said Kucklick. Little is known about chemical interactions in mammals.
Last year, a study in the journal Environmental Toxicology and Chemistry pointed out that chronic exposure to PCBs hampered the reproductive success of dolphin populations in Sarasota Bay; Beaufort, North Carolina; and Matagordu Bay, Texas.
The mystery of the firstborn dolphin die-offs reflects the complexity of the species. Dolphins are highly intelligent creatures, and "nothing about them is simple," said Wells. "I wouldn't be surprised if a number of factors might contribute to the deaths."
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