Lesions in the lungs and shrunken adrenal glands distinguish dolphins that washed up dead in the Gulf of Mexico between June 2010 and December 2012 compared with those found in beachings elsewhere. As a result, researchers have linked the mass deaths to BP's oil spill. "The dolphins have adrenal disease and lung disease consistent with exposure to petroleum products," explains Stephanie Venn-Watson, lead author of the study published in PLoS One and veterinarian at the National Marine Mammal Foundation in San Diego.

The researchers did not start out by blaming oil, first considering the most likely possibilities for many dolphins washing ashore dead. That list of suspects includes diseases like morbillivirus, poisoning by blooms of toxin-producing algae or even unusually cold weather. But BP's blowout in the Gulf of Mexico beginning on April 20, 2010, added oil to this collection. There were three mass strandings of dolphins on the Gulf coast from the Texas–Louisiana border to Florida, more than 1,000 dead dolphins in all, including babies. Based on the timing and the lack of other discernible causes, scientists had hypothesized that there might be a link between the oil spill and the unusual dolphin deaths.

Now the analysis of tissue from 46 dolphins from the region of the spill compared with 106 dead dolphins from elsewhere—North Carolina, South Carolina and unaffected parts of Florida and Texas—shows that the Gulf dolphins had abnormal adrenal glands and weaker lungs. This analysis matches the health problems found in a survey of living Gulf dolphins in Barataria Bay, La., in 2011, which showed lung debility and poorly functioning adrenal systems. This also matches the results from other toxicology experiments, such as feeding mink oil to understand the impacts of exposure. The mink had shrunken adrenal glands, among other lab animals to show such impacts from oil exposure.

The Gulf coast dolphins were also four times more likely to have died from an infectious disease—such as opportunistic bacterial pneumonia—than the dolphins from elsewhere. Without enough of the adrenal hormones like cortisol, which protects against physical stresses, the Gulf dolphins were liable to be more susceptible to infection and cold temperatures.

People who inhale hydrocarbons like those present during the oil spill often contract pneumonia. Dolphins are even more vulnerable to inhaling some of the hydrocarbons that can be present in air during an oil spill, given that a single breath is taken right at the water's surface amidst the oily sheen. A single dolphin inhalation/exhalation also exchanges more than three quarters of the lungs’ air capacity without any kind of filter, like nose hairs. The air that reaches the lungs is held for an extended period of time as well. Kathleen Colegrove, a veterinary pathologist from the University of Illinois at Urbana–Champaign, characterized the lung lesions in the Gulf bottlenoses as the "most severe" she had seen in 13 years of examining dead dolphins' lungs from all U.S. coastal waters.

The lack of information on the health of bottlenose dolphins prior to the 2010 spill—and chronic exposure to petroleum in the industrialized Gulf of Mexico coastal waters—complicate this diagnosis. BP disputes the link, according to BP America spokesman Geoff Morrell. But the absence of any of the other typical causes of mass dolphin deaths suggest that BP's oil spill helped kill these dolphins. As Venn-Watson says: "The extensive evidence leaves no feasible alternative causes to explain the timing, location and nature of these unusual and deadly conditions."