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Bottlenose DolphinA bottlenose dolphin breaks the oily surface of Chandeleur Sound, Louisiana (see map), on May 6, 2010, two weeks after an explosion at the Deepwater Horizon oil rig sent crude gushing into the Gulf of Mexico.Oil, gas, and chemical dispersants contaminated thousands of square miles of marine and coastal habitat. Many animals were killed or sickened outright, but on the one-year anniversary of the Gulf oil spill, scientists still don't know the extent of the spill's effects on most species.(See: "Gulf Oil Spill Pictures: Ten Animals at Risk [May 2010.]")Bottlenose dolphins have been dying in unusually high numbers in northern Gulf waters since February 2010, two months before the oil spill began, and the trend continues today. Since January, 68 premature, stillborn, or newborn calves have washed ashore.The Gulf oil spill is certainly on the list of suspects in the recent dolphin deaths, but it's too early to say for sure, Blair Mase, coordinator of the Southeast Marine Mammal Stranding Network of the National Atmospheric and Oceanic Administration, told National Geographic News in March.Only a handful of obviously oiled dolphins have been recovered. But a recent study from the University of British Columbia estimated that the actual number of dolphins and whales killed by the spill could be 50 times higher than official tallies suggest, putting the death toll in the thousands. —Rebecca Kessler

Photograph by Alex Brandon, AP