Despite their remarkable adaptations to aquatic life, exercising while holding their breath remains a physiological challenge for marine mammals, finds an international study of dolphins and seals. The study, published today in Nature Communications, found a surprisingly high frequency of cardiac arrhythmias in bottlenose dolphins and Weddell seals during the deepest dives.
The normal diving response in marine mammals has long been understood to involve a marked reduction in heart rate and other physiological changes to conserve limited oxygen reserves while the air-breathing animals are underwater. Since the normal physiological response to exercise is an increase in heart rate, how marine mammals cope with the exertion needed to pursue prey at depth has previously been unclear.
This new study, led by Dr Terrie Williams at the University of California Santa Cruz, shows that these conflicting signals to the heart can lead to alternating periods of faster and slower heart rates and cardiac arrhythmias.
Professor Michael Shattock from the Cardiovascular Division at King’s College London and co-author of the study, said: ‘We believe that these arrhythmias may be caused by something we have termed ‘autonomic conflict’ – where the nervous inputs to the heart (the ‘accelerator’ and the ‘brake’) are applied simultaneously.’
US researchers developed a monitoring device to record electrocardiograms (ECGs), heart rate, swimming stroke frequency, depth, and time throughout the dives of 10 trained bottlenose dolphins diving in pools or open water, as well as three free-ranging Weddell seals swimming beneath the ice in McMurdo Sound, Antarctica.
ECG analysis revealed that heart rates of diving animals varied with both depth and exercise intensity and cardiac arrhythmias occurred in more than 70 percent of deep dives.
Dr Terrie Williams, UC Santa Cruz and lead author of the study, said: ‘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. Even 50 million years of evolution hasn’t been able to make that basic mammalian response impervious to problems.’
Professor Michael Shattock, King’s College London, said: ‘While swimming and diving may trigger these arrhythmias, these are by no means the only environmental triggers that may initiate autonomic conflict. We think that autonomic conflict may occur in response to a range of triggers: stress, anxiety, or even the alarm clock going off and getting out of bed in the morning! Studying these events in expert divers such as dolphins and seals gives us a clearer insight into what may also occur in humans under stress.’
Source: King’s College London