Diving Mammals Endanger Their Hearts For Food

Human divers have long known that they risk heart trouble every time they make for deeper depths. Holding your breath places strain on your heart, and the longer you do it for during physical exertion, the more you risk. Now new research has found that even mammals meant for deep-diving, like seals and dolphins, run these risks, proving that there are some things the heart simply cannot overcome.

That's at least according to a study recently published in the journal Nature Communications, which details how seals and bottlenose dolphins suffer from heart arrhythmias at a surprisingly high rate during their deepest dives.

Experts have long known that to achieve their remarkable dive times, well-adapted aquatic mammals can slow their heart rate (a process called bradycardia) to put less stress on the organ. Other physiological changes have also allowed these species to conserve limited oxygen levels during a dive.

However, exertion has always been tied to a heightened heart rate in mammals, as the heart is normally required to deliver more oxygen as muscles burn through the supply. How marine mammal hearts cope with this exertion when hunting prey during a dive has long remained a mystery.

After all, if a seal's body is telling the heart to slow, but its muscles are asking it to speed up, wouldn't there be trouble?

And yet that, the new study implies, is exactly what happens.

"This study changes our understanding of bradycardia in marine mammals," study lead Terrie Williams, from the University of California, Santa Cruz, said in a statement. "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."

This was all determined after wild Weddell seals and trained bottlenose dolphins were outfitted with monitoring devices that recorded their heart rate, swimming stroke frequency, depth, and time, during long dives.

The data showed that when chasing prey, the Weddell seals in particular appeared to alternate between easy glides and short chases to ensure their hearts remained calm. However, in rare instances where increased activity was too much, arrhythmias were noticed.

"This study is not saying that these deep-diving animals will die if they exercise hard at depth," Williams said.

However, she adds that this may explain why many triathletes and other disciplined swimmers face elevated arrhythmia risk. She is currently working with triathlon groups to help mitigate such problems during races.

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