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Researchers from the University of California, Santa Cruz have found that marine mammals like seals and dolphins can suffer from irregular heartbeats when they dive deep in the search for food. According to researchers, who believed they have evidence of cardiac rhythm complications in both Weddell seals and bottlenose dolphins, said the problem takes place as they hold their breath during deep dives.

When marine mammals dive, a dive response is activated that drops the heart rate sharply (called bradycardia) to conserve oxygen.  However, when they do locate prey, their heart rate must then increase so they can match the moves of their target and catch their prey.

The new study, published in the journal Nature Communications, found that these conflicting signals to the heart can lead to cardiac arrhythmias, according to lead author Terrie Williams, a professor of ecology and evolutionary biology and the University of California, Santa Cruz.

"This study changes our understanding of bradycardia in marine mammals," Williams says. "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."

Researchers found that the heart rates of diving animals varied with both depth and exercise intensity, and sometimes alternated rapidly between periods of bradycardia and tachycardia.  They also found that during seventy percent of deep dives, the marine mammals experienced a cardiac arrhythmia.

"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 says. "Even 50 million years of evolution hasn't been able to make that basic mammalian response impervious to problems."

The findings will help scientists understand the stranding events involving deep-diving mammals such as beaked whales.  These cardiac responses are similar to that of the whales' flight response when exposed to shipping noise and mid-frequency sonar. 

"This study is not saying that these deep-diving animals will die if they exercise hard at depth," Williams says. "Rather, it raises questions about what happens physiologically when extreme divers are disturbed during a dive, and it needs further investigation."

The study may also provide relevant incites to humans, as well, as these responses are also found in humans and other animals when the face contacts cold water.  A 2010 study of triathlons found that the cold water swim accounts for over ninety percent of race day deaths.  "It may be that the same conflicting signals we saw in dolphins and seals are causing arrhythmias in some triathletes," Williams says.

Williams is currently working with triathlon groups to help reduce these such problems during races.