Superhuman Tricks: Mammalian Diving Reflex

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A superpower we all have! Some of the quirks of evolution shine through in weird ways. The examples you hear about all the time are enough to make evolution denying believers in an “intelligent” design cringe, but they’re typically along the lines of nipples on men, the appendix, and vestigial tail(bone). Having experienced many a chafed nipple, a broken tailbone, and appendicitis (and subsequent surgery), I tend to think all of those things kind of suck. And rightly so as evolution tends to weed out things (and people!) that aren’t efficient and beneficial. So… Here’es a cool one…

The Mammalian Diving Reflex

As far as we know, every mammal has an automated response system for diving in cold water (less than about 21C / 70F). This mammalian diving reflex allows dive times to be extended by maximizing oxygen expenditure efficiency while submerged. The role of Omega 3s from fish sources in human evolution (especially the explosion *cough* in brain size) isn’t completely known, but diving ability certainly opens up a range of possible food sources. The following three automatic responses are the typical characteristics:

  1. Heart rate slows
  2. Blood flow to extremities constricted
  3. Blood and water allowed to pass through organs and circulatory walls to chest cavity.

The first two items begin to happen as soon as the face hits cold water. The slowing heart rate is almost instantaneous, the constricted blood flow happens more gradually. Both responses are more extreme with more extreme temperatures. The slowed heart rate is generally a useful feature as it actively serves to conserve oxygen depletion and increase the available time underwater without dramatically harming performance. Yeah! You’re holding your breath, so obviously there are limits to the effectiveness! The decreased blood flow provides more of a long-term (minutes-hours) survival benefit, but is detrimental to performance. If you were diving for food, you’d quickly find that your limbs turn into numb rocks in cold water.

The third response is a bit more scary. Essentially, the body intentionally allows fluid to fill the lungs and chest cavity to prevent organs from being crushed from extreme pressure. For surface dwelling mammals, this serves a survival function. It only kicks in as depths become extreme.

Watch Bear Grylls of Man Vs. Wild demonstrate as his heart rate instantly drops from 160BPM to 55BPM!

Bonus Application: Drowning – Another benefit of the reflex, and especially that it’s an automatic response (i.e. reflex, not voluntary), is that it increases the odds of survival in drownings or other accidental submersions. It’s the reason that so many people (and dogs) survive falls into icy lakes. The reflex will kick in even if the person is knocked out before entering the water. It essentially allows the body to enter a state of hibernation in which oxygen depletion is less detrimental to the brain. Thus, in some ways, it’s better to fall into cold water than warm water. Another interesting point is that the dive reflex lessens with adulthood. Children are therefore more likely to survive extended submersion in cold water.

Evolutionary Hack: So what if you fall overboard into cold water with nothing but a flotation device? Well… Knowing that the lowered heart rate response and extremity blood constriction are triggered by cold water to the face, it may be advantageous to voluntarily get the involuntary reflex to kick in by keeping your face cold and wet. Even if oxygen depletion isn’t a problem, the lowered heart rate and decreased circulation may help keep your core temperature up for a bit longer… hopefully in time for help to arrive. It will likely be hard to tell if the reflex has kicked in under panic, especially in icy, rolling seas. The one diagnostic tool you’ll have is your heart rate. Normally, your heart will be racing in this situation. Before “hacking” your heart rate, stabilize your situation, orientation, breathing, and flotation as much as possible. In fact, “cold shock” (kind of the opposite of the mammalian dive reflex) may elevate your heart rate kill you within the first minutes, well before hypothermia sets in. After several minutes of chilling-out, keep your wits about you enough to (if possible in a relatively safe way) completely submerge your face for a few seconds and see if your heart rate drops. It will be a matter of monitoring your heart rate relative to face completely out of the water, and face completely submerged.

The Deep Web of Human Evolution

The mammalian dive reflex is particularly strong in seals, otters, and dolphins. As evolutionary theory would predict… mammals that spend a lot of time in the water. That all mammals have the reflex suggests that it’s been around for a long, long time (125,000,000+ years).

What’s also interesting is that diving birds (penguins) have a similar reflex. As far as I know, it’s not known whether the evolutionary roots can be traced back to a single divergence, or that it evolved independently. If I had to make a guess, I’d say it evolved independently, but… totally a guess.