Dolphin Mystery: What's Killing Firstborn Calves?

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.

Polluted Milk

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 pollutants—and 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 quantify—and it's difficult to link exposure with symptoms or death.

Accumulating Chemicals

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 milk—and 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.

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