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Dolphins appear to be true masters of swimming—leaping, frolicking, and spinning. But as new research shows, dolphins aren’t quite so adept at a related skill: diving. Some dolphins can be stressed to the brink during deep dives, and in some cases, the consequences can be fatal.

Ever since the 1940s, and the early experiments of Per Scholander and Laurence Irving, scientists have assumed that marine mammals had perfected the deep dive. “We thought: oxygen is stored, and they don’t get the bends, and it’s fine because they’re so well adapted,” says Terrie Williams, a comparative ecophysiologist at the University of California, Santa Cruz. But in a new study, in which Williams and her colleagues measured dive responses in bottlenose dolphins and Weddell seals, they found that all is not fine.

Typically, when dolphins dive, they save energy by adjusting their swimming technique, and regulate their airflow by deliberately collapsing their lungs. It was thought that they would also slow their heartbeats. But when a dolphin’s deep dive is coupled with high energy output, which forces them to use precious oxygen, the effects on their physiology can be startling.

In her experiments, Williams showed that diving dolphins often experience irregular heartbeats—the deeper the dive, the higher likelihood of cardiac arrhythmia. The animals aren’t having heart attacks, but it isn’t great for them, says Williams. “The implications show it’s not a completely perfect system,” she says.

Even Weddell seals, deep dive experts that remain underwater for long periods as they hunt in the frigid waters off Antarctica, had irregular heart rates in more than two-thirds of the measured dives.

This surprising shortcoming could possibly be traced back to these animals’ land-dwelling ancestors, says Williams. After all, seals are related to bears, and dolphins to hippopotamuses. The question of whether these animals’ ancient terrestrial physiologies have adapted enough for their current aquatic existence is a point of debate among marine biologists. Williams says her thoughts on the matter have changed. “In the end, I really did a turnaround when I realized that these animals are taking tissues and organs packaged for land” and using them in the water.

The finding that dolphins may be stressed by diving led Williams to another question: how does this affect an animal that is fleeing from danger?

For a separate study, the scientists measured how much energy bottlenose dolphins use during their flight response. They measured the animals’ oxygen use and swimming kinematics both at rest and during a dive, and used the results to model how much energy would be spent during a flight response.

What they found seems obvious—diving and swimming quickly take a lot of energy. But the level of energy needed is significant: the scientists measured a spike in the dolphins’ metabolic rates of more than 30 percent in some instances. Such a rapid pace of energy expenditure just isn’t sustainable, says Williams.

Williams started thinking about dolphins and other marine mammals that die in mass strandings. If something triggers a dolphin’s flight response (say, excessive human noise), then a dolphin will expend a massive amount of energy to escape, depleting its oxygen stores. If the dolphin is one of the majority that experience heart irregularities during dives, the exertion could be fatal, Williams says.

“It starts to get to the heart of mass strandings,” Williams says.

The demands put on an animal’s cardiovascular system in high-pressure environments and situations are “a big challenge for these animals to undertake,” Williams says. “Why wouldn’t we expect that it sometimes goes wrong?”