The Diving Reflex Explained

October 21, 2024 Neuroscience Evolution

Think about how you would feel if you dip your head underwater for about 10 to 20 seconds.

Specifically, what do you expect to happen to your heart rate during this time? Try to really think about it.

Most people might assume that the sudden submersion would trigger stress responses. Something along the lines of, “We’re underwater, we can’t breathe, help!” Thus, this sudden submersion should cause our heart rate to spike.

Perhaps surprisingly, this is not the case at all.

In fact, your heart rate slows down. This is part of a natural response known as the diving reflex—a fascinating set of physiological responses our bodies undergo during submersion.

Let’s break it down further.

The diving reflex, also known as the mammalian dive response, kicks in when the face, particularly the nose and mouth, is exposed to cold water while holding one’s breath. While most pronounced in marine mammals such as cetaceans (e.g., whales), and pinnipeds (e.g., seals), it is also present in terrestrial mammals (Hochacka & Mottishaw, 1935). This includes humans!

Irving et al. (1935) first detailed this reflex, which comprises three key components: apnea (cessation of breathing), vasoconstriction (narrowing of blood vessels), and bradycardia (slowing of the heart rate). These adaptations allow mammals to preserve oxygen and extend the time they can remain underwater.

This evolutionary mechanism is particularly evident in marine mammals. Nonetheless, let’s briefly explore a possible theory that explains why the diving reflex is present in humans.

The diving reflex in humans is an ancient adaptation that likely traces back to ancestors who lived in aquatic or semi-aquatic environments. This theory, known as littoral evolution, suggests that early humans developed adaptations such as the diving reflex to survive in water. This theory was proposed by Hardy, and also explains why we look so physically different to our immediate ancestors (1960). Hardy proposes that the primitive apes have primarily been living in trees, thus evolving to have physical features that support this. Due to competition, we were forced from life in trees to feed on the seashores and to hunt for food in the shallow waters off the coast. This transition would explain our superior diving reflex compared to our ancestors, and our physical differences (e.g., that we do not have fur, and have a streamlined body that suits swimming well).

What is the takeaway from this all?

When you compete—which, of course, happens often—in a “let’s see who can hold their breath the longest”, quickly wetten (or submerge) your face. You will have activated your diving reflex, allowing you to hold your breath for a longer time. (If you win because of this, you’re welcome).