Scientists have detected high frequency of heart arrhythmias or irregular heart beats in bottlenose dolphins and Weddell seals during their deepest dives.
This shows that despite their remarkable adaptations to aquatic life, exercising while holding their breath remains a physiological challenge for marine mammals.
"This study changes our understanding of bradycardia (a marked reduction in heart rate) in marine mammals," said lead author Terrie Williams, professor of ecology and evolutionary biology at University of California-Santa Cruz.
"We are not seeing lethal arrhythmias but it is putting the heart in an unsteady state that could make it vulnerable to problems," Williams added.
Instead of a single level of reduced heart rate during dives, the researchers found that heart rates of diving animals varied with both depth and exercise intensity, sometimes alternating rapidly between periods of bradycardia and tachycardia.
Cardiac arrhythmias occurred in more than 70 percent of deep dives.
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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 is not a perfect system. Even 50 million years of evolution has not been able to make that basic mammalian response impervious to problems,' Williams noted.
The new findings have implications for efforts to understand stranding events involving deep-diving marine mammals such as beaked whales.
The findings may also be relevant in humans.
The mammalian dive response or dive reflex also occurs in humans and other terrestrial animals and is triggered when the face contacts cold water.
The study was published in the journal Nature Communications.