By Dr. Becker
It's long been known that marine mammals have a slower heart rate on deep dives, but it was long thought that this was an "all-or-nothing" reflex. However, research conducted in the 1970s in pinnipeds, including sea lions, revealed that the response is not only autonomic but is also controlled cognitively by the animal. Decades later, researchers decided to find out if cetaceans, such as porpoises, can also consciously control their heart rates.1
Lead study author Siri Elmegaard of Aarhus University in Denmark told New Scientist, "Until now, we knew that the heart rates of porpoises and cetaceans in general correlate with different dive factors, such as dive duration, depth and exercise. Now we can conclude that harbor porpoises have cognitive control of their heart rate."2
New Study Reveals Porpoises Consciously Control Their Heart Rate
To find out if porpoises maintain cognitive control over their heart rate, researchers fitted two captive porpoises with special monitors, then trained them to dive either a 20-second or 80-second dive. The animals slowed their heart rates much more at the beginning of the longer dive than they did at the start of the shorter dive. Their heart rates measured between 15 percent and 26 percent slower on the longer dives than they did on the shorter dives, which is likely done in order to optimize oxygen usage.
The porpoises not only lowered their heart rates but also constricted blood vessels. "Combined, these decrease heart output and perfusion of oxygen into organs, maintain blood pressure and conserve blood oxygen, essentially redistributing oxygen to the brain and heart, which are the organs most sensitive to lack of oxygen," New Scientist reported.3
While the ability to consciously manage their heart rate, and therefore their ability to stay under water longer, is certainly beneficial to the species (and likely exists in all cetacean species, including dolphins), it could be affected by underwater noise.
If the animals are startled by an unexpected noise from shipping traffic or sonar, for instance, the researchers suggested they could panic and lose control of their heart rate, causing them to surface too quickly or lose control of oxygen and nitrogen levels in their bodies. As in humans, cetaceans can suffer from decompression sickness — a toxic buildup of nitrogen in tissues that leads to pain and potentially paralysis and death.
Paul Jepson, Ph.D., at the Institute of Zoology in London told New Scientist that this could even be implicated in some unexplained strandings. "If abnormal cognitive physiological control of dive profile occurred, for example due to naval sonar exposure, this could result in radically altered nitrogen gas kinetics and — potentially — increased risk of decompression sickness — as has been suggested for some mass strandings of beaked whales linked to human-made mid-frequency sonars," he says.4
Are Marine Mammals at Risk of Heart Irregularities on Deep Dives?
Marine mammals are uniquely suited for their aquatic environments, but that doesn't mean that holding their breath while diving and chasing prey comes easy. In fact, research suggests it presents a physical challenge that may even increase the incidence of heart problems on deep dives. In research published in the journal Nature Communications, researchers monitored bottlenose dolphins and Weddell seals diving from the surface to depths of more than 200 meters.5
Both the exercise and the depth altered the animals' heart rate, and the researchers unexpectedly found that cardiac arrhythmias, or irregular heartbeat, occurred in more than 73 percent of deep, aerobic dives.
"Such marked cardiac variability alters the common view of a stereotypic 'dive reflex' in diving mammals," the researchers noted, adding that their findings also suggest marine mammals may still have cardiac function traits that are similar to those in found in their land-based ancestors, and may explain why some deep-diving marine mammals are so sensitive to environmental pollution and other stressors.
The cause of the cardiac arrhythmias was suggested as due to "autonomic conflict" similar to what may occur in humans who are exercising heavily then exposed to cold water (such as during a triathlon). That being said, a 2017 study in The Journal of Experimental Biology suggested that benign heartbeat fluctuations are common in marine mammals and the generation of deadly cardiac arrhythmias in marine mammals due to exercise or deep dives is "unlikely."6
The Many Ways Marine Mammals Are Adapted to Life in the Water
Controlling their own heart rate is but one way marine mammals are suited for life at sea. Their ability to dive underwater to catch prey is the result of "multi-level adaptations, from cellular biochemistry to behavior," and includes increases in blood volume, blood hemoglobin and muscle myoglobin to help store oxygen.7
The myoglobin is particularly elevated in muscle areas that produce greater force and consume more oxygen during aerobic swimming, according to research published in the Journal of Comparative Physiology. Further, the researchers noted, "Marine mammals minimize the energetic cost of swimming and diving through body streamlining, efficient, lift-based propulsive appendages, and cost-efficient modes of locomotion that reduce drag and take advantage of changes in buoyancy with depth."8
Other marine mammals, such as beluga whales, have other specialized adaptations like being able to move their heads up, down and side-to-side, which likely helps them find and catch prey in ice-covered areas. Beluga whales, by the way, can dive to depths of nearly 3,000 feet, although most of the time they stay between depths of 650 feet and 1,000 feet.
There's a reason for staying within their "aerobic comfort zone," which most marine mammals do, researchers said — it "maximizes time underwater and minimizes recovery time at the surface."9 It really is amazing how marine mammals thrive in the water. To gain a greater sense of appreciation, the next time you're going for a swim think about the coordination it would take to consciously control your heart rate while you go in for a dive — it's a truly impressive feat.