Marine mammals, such as dolphins, are thought to be the descendants of land animals that returned to the sea. For about 50 million years dolphins, seals and other aquatic mammals have been diving for food. However, a new study says that despite their adaptations, bottlenose dolphins and Weddell seals have a high frequency of heart arrhythmias during deep dives.

It has long been understood that marine mammals show a sharp reduction in heart rate bradycardia and undergo other physiological changes to help them conserve their limited oxygen while underwater. It is also understood, however, that the normal response to exercise is an increase in heart rate (called tachycardia). How the diving animals balance the two responses has not been clearly understood.

The study, published January 15 in Nature Communications, show that the conflict between these responses can lead to a high rate of cardiac arrhythmias.

“This study changes our understanding of bradycardia in marine mammals. 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,” said Terrie Williams, a professor of ecology and evolutionary biology at UC Santa Cruz, in a statement.

The study was done using a newly developed device to measure heart rate, depth, swimming stroke frequency and time for swimming mammals. Measurements were taken on bottlenose dolphins in pools and open water as well as free-ranging Weddell seals in McMurdo sound Antarctica.

The researchers found that the heart rates varied with the intensity of exercise and depth. The data also showed that the animals could alternate quickly between periods of bradycardia and tachycardia and that Cardiac arrhythmias occurred during more than 70 percent of deep dives.

“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,” said Williams.

Different neural circuits have been found to be involved in exercise-induced tachycardia and dive-induced bradycardia. The sympathetic nervous system is triggered to increased the heart rate during exercise and the parasympathetic system is triggered by the dive response. The researchers also noted that similar cardiac responses have been noted in beaked and blue whales during the flight response triggered by mid-frequency sonars.

The team believes that these findings could help to explain stranding events which occur when deep diving animals strand themselves on land.

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

The research may also have implications for human who have a similar dive reflex which is most frequently triggered by contact with cold water. A 2010 study found that swimming in cold water accounts for 90 percent of race day deaths among triathletes.

“It may be that the same conflicting signals we saw in dolphins and seals are causing arrhythmias in some triathletes,” said Williams, who is currently working with triathlon groups to improve safety.