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Study: deep-diving marine mammals suffer heart arrhythmias

"In terms of the dive response and heart rate, it's not a perfect system," said Terrie Williams.

By Brooks Hays
A seal diving off the coast of Cape Town, South Africa. Photo by Tim Sheerman-Chase/Flickr.
A seal diving off the coast of Cape Town, South Africa. Photo by Tim Sheerman-Chase/Flickr.

SANTA CRUZ, Calif., Jan. 20 (UPI) -- Scientists and animal-loving laypeople alike have long been impressed by the ability of marine mammals such as seals, sea lions and dolphins to dive deep below the surface and hold their breath for long periods of time. But new research suggests the feat doesn't come without physiological costs.

In a recent study, scientists at the University of California, Santa Cruz found that many seals and dolphins suffer irregular heartbeats during deep dives.

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The study's results shed light on inevitable physiological conflict of exercise on a limited oxygen supply. Biologists have previously shown that deep-diving mammals suppress their heart rate (bradycardia) in order to function underwater for lengthy periods of time and with only a single gasp worth of air.

But basic biological principles require a heightened heart rate for strenuous activities -- like chasing down a meal at fifty feet under the ocean surface.

And therein lies the contradiction -- one that scientists now knows plays in out in the form of arrhythmias.

"This study changes our understanding of bradycardia in marine mammals," Terrie Williams, an ecologist and evolutionary biologist teaching at UC Santa Cruz, explained in a recent press 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."

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In studying the biorhythms of deep dives, Williams and her colleagues found that the expression of arrhythmias varies depending on depth and level of exercise. But scientists found that 70 percent of all dives featured periods of rapid alternation between tachycardia and bradycardia.

"We tend to think of marine mammals as completely adapted to life in the water. However, in terms of the dive response and heart rate, it's not a perfect system," Williams said. "Even 50 million years of evolution hasn't been able to make that basic mammalian response impervious to problems."

Williams thinks the findings may have relevance to humans. The majority (90 percent) of all deaths suffered during triathlons happen during the swimming portion -- when cold water triggers the body to slow down the heart rate, while the need to swim faster requires a quicker pace.

"It may be that the same conflicting signals we saw in dolphins and seals are causing arrhythmias in some triathletes," added Williams, who is now working with triathletes to minimize the risk of arrhythmias on race day.

The new study was published this month in the journal Nature Communications.

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