A recent study suggests that marine mammals such as dolphins and seals that dive deep below the surface and hold their breath for a long time can place a strain on the heart, suppressing their heart rate (bradycardia) to keep them functioning at such depths. These arrhythmias observed in the mammals can provide some insight into how humans function as well, and how the mammals evolved over time. Science Recorder interviewed Dr. Terrie Williams, a professor of ecology and evolutionary biology at the University of California Santa Cruz, about what exactly these findings mean and what further research will be conducted on the issue.
Have you always suspected this to be the case with marine mammals? How did you find this out?
A lot of it had to do with the technology. What happened is, we’ve been using heart rate recorders on marine mammals. I’ve been doing this since the 1980s, and you get an average heart rate and you see how the heart rate changes, but the question in my mind — and we didn’t have the technology — is how much of it was muscle noise and heart rate recorders picking that up as a heartbeat, and how much is variability. So in this study, one of the major developments is an EKG recorder that we put on marine mammals, and it can go 500 meters in depth. The thing that was amazing was, when we got the recorders back, and I was watching this thing, the heart rate was just going back and forth. It was pretty erratic at times.
What problems can this cause? Can a marine mammal get a heart attack or stroke from a deep dive?
It’s not a lethal condition. It starts off with something in the chest as if you were drinking too much coffee or caffeine. The thing that’s concerning to me is in humans when you see this condition. … When you see this kind of conflict with the dive response and the exercise response in the heart, you realize it can set you up with a situation that first is unstable. So I’m not saying marine mammals are having heart attacks when they go for a dive. There are things that cause the heart to become unstable, and so that just raised a red flag for me. Because the way marine mammals get away with it is they use behavioral tricks, so we’re seeing animals gliding when going on a dive, or we see animals going horizontal at periods. Very rarely do we see them go at high speed at really deep depths to the surface. They don’t tend to do that. It would really set the heart up for an unstable situation. But you then look into the literature and see a list of responses some animals have, and it really makes you wonder if you should pay a little more attention to this. What triggers are there for an unstable heart situation? Also, we wondered how does this apply to humans, and are taking a second look at triathletes. The swimming portion, especially in cold water swims, humans experience the same kind of things.
Will you be conducting further research on what all of this means?
We’re actually in the middle of that right now. We took a look at some extreme divers that are out there. We just got back from a trip in Florida where we looked at a really slow kind of diver that doesn’t go very deep, and that is the manatee, and see how their heart responds to exercise and diving. Then this past summer we looked at narwhals, which is a very deep diver — 1000-plus meters — and how swimming affects those animals. We’re looking at different species of marine mammals, and now we’re asking questions: are all animals the same or not? Obviously, they’re not the same, but the hope is we get to a point where we say, “This species is particularly vulnerable to sounds, more so than these other species.”