Deep-Diving Marine Mammals Experience Arrhythmias During Challenging Dives

It turns out that even though dolphins and seals are made for deep dives, exercising while holding their breath still remains a physiological challenge for marine mammals. Scientists have actually detected a surprisingly high frequency of heart arrhythmias in bottlenose dolphins and Weddell seals during their deepest dives.

The normal dive response in mammals involves a marked reduction in heart rate, called bradycardia, and other physiological changes in order to conserve limited oxygen reserves. How mammals cope with the exertion needed to pursue prey deep beneath the waves, though, has long been unclear. That's why scientists decided to take a closer look.

The researchers developed a monitoring device to record heart rate, swimming stroke frequency, depth and time throughout the dives of trained bottlenose dolphins diving in pools or open water, as well as free-ranging Weddell seals swimming beneath the ice in Antarctica. The animals used low-intensity swimming modes during dives; when hunting beneath the ice, the seals alternated between easy glides and short chases in pursuit of prey.

"This study changes our understanding of bradycardia in marine mammals," said Terrie Williams, one of the researchers, in a news release. "The heart is receiving conflicting signals when the animals exercise intensely at depth, which often happens when they are starting their ascent. We're not seeing lethal arrhythmias, but it is putting the heart in an unsteady state that could make it vulnerable to problems."

The findings reveal how these marine mammals react to the pressures and exertions of deep water. In addition, the findings may be relevant to humans; the dive response also occurs in humans, and is triggered when the fact contacts cold water.

The findings are published in the journal Nature Communications.

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