For reasons that scientists still do not 100% understand, animals need to sleep. Humans, for example, typically need about eight hours of sleep a day. This means we spend one-third of our lives looking like this:
This apparently is King Olav V of Norway. I have no idea what the story is behind this image. I found it on Wikimedia Commons.
…except maybe not dressed in a tuxedo or while posing for a portrait.
If you have ever snuck your iPad into your bed without your parents knowing, and then stayed up until 3:00 in the morning because you were binge-watching Bob’s Burgers on a school night, you may have noticed the next day at school that you did not function very well. Sleep is necessary for the maintenance of our brains, but scientists are still gathering evidence to figure out why we need sleep and how it is able to help us.
For many animals, sleep is necessary, but also kind of dangerous. A sleeping animal is a lot less likely to notice that another animal is about to eat it than an animal whose eyes are open. Animals that are low on the food chain, like mice and antelope, tend to be very light sleepers. Predators that are not anyone else’s prey are usually deep sleepers, with at least one exception. Dolphins always sleep with one eye open.
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This dolphin isn’t sleeping, but it is cute
As cute as bottlenose dolphins may seem to us, they are not cute to fish like mullets. Small fish see dolphins as giant monsters that want to eat them. Being a large predator means dolphins rarely get eaten themselves. When a dolphin sleeps with one eye open, it is not worrying about becoming someone else’s prey. It is not even worrying about a gang of mullets beating it up to get revenge. A bottlenose dolphin’s main worry during sleep is breathing.
Dolphins spend the majority of their life underwater, but they cannot breathe underwater. Like us, they are mammals that must breathe oxygen from the atmosphere with their lungs. If a dolphin tried to breathe underwater, it could drown.
Dolphins do not breathe the exact same way we do, though. For one thing, they don’t breathe through their nose. They breathe through a hole on top of their head, called a blowhole.
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Another difference is humans are involuntary breathers (meaning we breathe all the time without thinking about it) and dolphins are not. You may have already noticed we breathe automatically if, like many 10-year olds, you have decided that you should learn how to meditate. You may remember the first thing your meditation teacher told you to do was to pay attention to your breathing. As you did this, you may have thought to yourself, “Wow! My breathing just keeps happening.” Then you probably thought to yourself, “It makes sense for us to keep taking breath after breath without thinking about it, because our noses are almost always in the atmosphere where we can safely take in oxygen… Also, who wants to go around telling yourself to breathe every 4 seconds?” And then as you kept paying attention to the breaths you took you probably thought about how “Every Breath You Take” by the band The Police is your mom’s all-time favorite song. That then made you think about how you want to be a police officer for Halloween this year, and that you better get more candy than last year when you were a giant banana. And then your meditation teacher became aware that you weren’t actually focusing on your breathing, and then you failed your meditation class.
Dolphins are voluntary breathers, meaning they have to tell their body when to breathe and also that dolphins can’t really begin meditating by focusing on their breathing because they already do that all the time. If a dolphin’s breathing was involuntary, like ours, and its brain automatically told its blowhole to breathe every few seconds, there is a good chance it would end up breathing underwater until it drowned. Dolphins are voluntary breathers to make sure they only breathe when they rise to the surface and their brain knows their blowhole is up in the air.
When dolphins come out of the water like this, they are exhaling and then inhaling.
Because dolphins need to tell themselves when to breathe, they can never completely fall asleep. Instead, dolphins only let one half of the brain sleep at a time. Dolphins let the right side of their brain go to sleep while the left side stays awake to control the breathing. Then a little while later the right side wakes up and the left side falls asleep. Dolphins need about eight hours of sleep a day, but, unlike us, their brain sleeps in shifts. The right half gets four hours of sleep and the left half also gets four hours of sleep, just at different times.
The awake half of of a dolphin's brain not only helps keeps the dolphin breathing. It also keeps one eyeball working so the dolphin can continue to see what it going on around it while the other eyeball shuts down. The awake half of the brain can also keep the dolphin swimming. If you ever see a dolphin doing what the dolphin is doing in the GIF above, there is a chance that half of dolphin is sleeping while it swimming through the water.
And, yes, this whole sleeping thing I just described is an adaptation that allows dolphins to survive in ocean and estuary environments.
To learn more about dolphins, read my books Sharks and Dolphins: A Compare and Contrast Book and A Day in the Salt Marsh.
Online References and Resources:
LiveScience. "How Do Dolphins Sleep?"
http://www.livescience.com/44822-how-do-dolphins-sleep.html
National Geographic. "Dolphins sleep with half their brain awake."
http://voices.nationalgeographic.com/2009/05/06/dolphins_sleep_with_half_their_brains/
PHYS.ORG. "What animals can tell us about sleeping,"
http://phys.org/news/2016-06-animals.html
PLOS ONE. "Dolphins Can Maintain Vigilant Behavior through Echolocation for 15 Days without Interruption or Cognitive Impairment"
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0047478
RN. "How dolphins sleep and why we should care." (Audio interview with scientist Annie Aulsebrook).
http://www.abc.net.au/radionational/programs/rnafternoons/how-dolphins-sleep-and-why-
we-should-care/7526868
Scientific American. "How do whales and dolphins sleep?"
https://www.scientificamerican.com/article/how-do-whales-and-dolphin/
Photos and Images:
Click the photos, GIFs, and images used above to find their sources.