Sleeping with one eye open – the Delphinidae and curious sleep phenomena

The family Delphinidae, known commonly as the oceanic dolphins, are a clade of toothed whales (Odontoceti) within the order Cetacea. Most people have a decent understanding of what dolphins are, being smaller than most whales, and entirely carnivorous. Like all Cetacea, the dolphins are marine mammals with terrestrial ancestors. DNA sequence data and fossil evidence indicate that millions of years ago, these ancestors (Artiodactyl – even-toed ungulates) made the transition from a land-dwelling to aquatic lifestyle. The extreme change of environment resulted in selection pressures forming a diverse range of adaptations that allow the Delphinidae and close relatives to survive in the oceans. Some of these adaptations are quite well understood, while others are currently the subject of intense research and debate among scientists.

I’d like to give a quick introduction to two fairly obvious adaptations (body-shape and skin-shedding) to give you a taste of the problems faced by marine mammals, and the adaptations that have evolved to solve them. Then, I’d like to take a look at something I find most interesting: the evolution of some truly weird and wonderful sleep phenomena.

We’ll start with the most obvious. A common adaptation in marine mammals is a fusiform (sleek, streamlined) body shape. Even the polar bear has a relatively more fusiform shape than other species of bears. The marine environment makes quick movement difficult due to the drag and resistance caused by the water. A fusiform body shape aids swimming and this is essential for members of the Delphinidae as they chase and catch fast-moving prey such as schools of fish. Both the fusiform shape of dolphins and their reduced limb size decreases the drag of water resistance. A common theme in Delphinidae research is that an adaptation often helps solve more than one environmental problem. Selection pressures may have also brought about the fusiform shape due to thermoregulation. A lot of body heat is lost to the ocean, so marine mammals have evolved a range of adaptations in order to conserve heat efficiently. The fusiform shape decreases the organism’s surface area exposed to the environment, and this reduces heat loss. In fact, we see evidence for this adaptation when we observe the species in deeper, cooler waters that tend to have larger bodies and smaller flippers than coastal species.

The benefits of a fusiform body shape have been relatively well understood for many years. Other phenotypic traits of the Delphinidae have only recently been intensively studied and new explanations for adaptations are emerging. One curious trait is the speed that dolphins shed their skin. Most animals such as insects and reptiles shed a whole layer of skin in one rapid growth known as moulting. In contrast, us mammals continuously shed individual dead skin cells. This process can take a couple of days to completely replace all the dead skin on the mammalian body in most terrestrial species. Dolphins replace every dead skin cell on their body within 2 hours. This striking difference in the rate of sloughing has interested many biologists, but also physicists. This is a fine example of researchers studying naturally evolved solutions to problems and developing new technologies that improve our vehicles and other applications. For example, scientists have looked at how birds and insects fly when trying to develop efficient technologies for air travel. Similarly, some physicists are interested in knowing what adaptations allow dolphins to maximize their speed in water in the hopes that future technologies could mimic the adaptations. As mentioned previously, dolphins have adaptations that allow them to maximize swimming speed in order to catch prey and avoid predators. Hypothesizing that the dramatic rate of sloughing aided swimming speed, scientists in Japan used sophisticated computer simulations modeling every single skin cell and exactly how each is shed from the body of a dolphin. These techniques revealed that the soft, “waviness” of dolphin skin reduces drag and shedding often maintains this condition. More importantly for their own research, they also discovered that drag was reduced significantly because of the shed skin reducing turbulence. The tiny flakes of skin that are lost end up reducing the number-density of hairpin vortices that occur in the flow around the surface of the skin. With less hairpin vortices forming, the drag is once again reduced and the dolphin’s swimming speed is increased. This knowledge might someday be useful in the future design of boats or submarines and underwater equipment.

So, body-shape and the speed of skin-shedding have evolved to help survive in a marine environment. As you can imagine, these characteristics wouldn’t have been found in the terrestrial ancestors. Switching to life in the oceans has required some dramatic changes in body-structure. But we can see these adaptations before our very eyes. We’re talking about fairly obvious physical structures. To discuss the adaptations that I find most curious, we need to consider the less obvious, and think about what’s happening in the brain.

A hot topic in Delphinidae research is the neurological activity and behaviour associated with sleep. Dolphins are known to sleep for 33% of the day. All mammals sleep, but many marine mammals, including all the Cetacea and therefore Delphinidae, demonstrate unusual sleep phenomena not seen among terrestrial mammals. Viewing the various sleep phenomena of Cetaceans as adaptations to a marine lifestyle, biologists have also recently been attempting to identify the original selection pressures that may explain their evolution. Unfortunately, mental processes and behaviour aren’t the most obvious things to discover from the fossil record.

Sleeping behaviour has been observed in whales and dolphins for almost a century. In the early 20th century it was already understood that dolphins sometimes slept with one eye open, and sometimes appeared to sleep while still swimming. Early attempts to explain these behaviours resulted in many novel ideas (especially during the 60s) including the hypothesis that dolphins were capable of unihemispheric sleep, and that breathing was a voluntary process. Dolphins have a brain with two hemispheres, like all mammals. In 1964, J.C. Lilly believed that dolphins were able to put one hemisphere of their brain to sleep while the other stayed awake, explaining why one eye would remain open during these times. His explanation was that breathing was entirely voluntary in dolphins, and they therefore needed to stay partially awake at all times in order to swim to the surface to take a breath when required. Since the 1960s, Lilly’s novel ideas regarding unihemispheric sleep have been confirmed by strong evidence, but his ideas about voluntary breathing have been rejected. Even so, this research still isn’t well understood by many non-scientists and it is still a very common misunderstanding that dolphins only breath voluntarily. It simply isn’t true.

Complex anesthetizing experiments performed by McCormick in 1969 contradicted Lilly’s earlier prediction, and demonstrated that the respiration of dolphins can be cortically controlled or autonomic, just like other mammals. McCormick also demonstrated that while sleeping with one hemisphere at a time, dolphins are partially aware of their surroundings, able to react to other organisms, and swim continuously. It turns out that all Cetacea are capable of unihemispheric sleep (and lack REM). Biologists obviously felt this dramatic characteristic might be explained by the extreme environment the dolphins have evolved to survive, but the adaptation first had to be understood before selection pressures could be considered.

Unihemispheric slow wave sleep (USWS) in dolphins (seen in the image above) is characterized by the brain producing slow delta waves in one hemisphere, while the other half shows reduce voltage activity. Put simply: dolphins sleep with one side of their brain at a time, rather than putting both sides to sleep like we do. In the bottlenose dolphin, Tursiops truncatus, each half of the brain sleeps for approximately 4 hours a day. On the theoretical basis that this interhemispheric EEG asymmetry was an adaptation to an aquatic environment, studies began to focus on other aquatic mammals (non-Cetaceans). Since the 1980s, sleep has been studied extensively in pinnipeds, manatees, the walrus, the sea otter, and even the hippopotamus, and it turns out USWS isn’t exclusive to the Cetacea. All pinnipeds belonging to the family Otariidae were shown to sleep unihemispherically. USWS was also observed in the the walrus and the manatees. Entirely different clades of aquatic mammals with unique terrestrial origins demonstrated USWS, which could be interpreted as evidence that it is a convergently evolved adaptation to the aquatic environment. Further evidence is that the fur seal, Callorhinus ursinus, shows great variability in its sleeping behaviour. USWS is observed in this species while it sleeps in water, but ordinary bihemispheric sleep (like ours) occurs when sleeping on land. Several proposals have been made for the selection pressures that may have driven these sleep adaptations for an aquatic environment.

The first possibility is that USWS evolved simply due to the necessity to breathe. For most mammals, sleeping results in a relatively stationary state. Just picture yourself sleeping in bed. If dolphins fell fully asleep underwater and didn’t move, they would drown, unable to reach the surface. But during USWS, dolphins are able to move freely and swim to the surface to breathe when required. Diazepam can induce bilateral slow wave sleep (like ours) in dolphins, and they are unable to breathe in this state, despite clearly trying to. This means dolphins have evolved to the point where USWS is now required for them to breathe. USWS may have evolved in order to assist breathing, but a much simpler solution seen in other aquatic mammals is to dramatically improve how long breath can be held, and sleep through that period. Perhaps necessity to breathe played a role in the evolution of USWS, but it seems likely that there were other pressures involved.

A second possibility is that USWS evolved to aid sentinel behaviour even while asleep. The ancestors of dolphins and other modern Cetacea made the transition gradually from terrestrial to fully aquatic. Some ancestors would have spent some time on land and in water. Eventually, some of the early aquatic ancestors would have spent all their time in the water, but would not yet have evolved the adaptations required to dive for long periods of time. These ancestors would have to sleep at the surface of the water, where aquatic animals are most at risk of predation. By keeping one eye open and remaining partially awake, the ancestor could watch for danger while getting some sleep at the same time. This may help explain the evolution of USWS together with loss of REM.

Marine mammals can be difficult to study because they live in such an extreme environment. New discoveries are being made every year, but clearly there is still much to be learned. Some adaptations are well understood, but these are often examples of adapted physical anatomy. The evolution of behaviour and neurology are harder to study because behaviour and sleep do not fossilize. Biologists have taken steps to understanding the early evolution of these adaptations we see in modern Delphinidae and close relatives, but there is still a lot of debate over which selection pressures were more powerful in shaping the dolphins we see today. Judging from the burst of research seen recently, perhaps we’ll be more confident of the answers in a few short years. Or maybe not. I’ll keep an eye on it.


Lilly, J. C. (1964). Animals in aquatic environments: adaptations of mammals to the ocean. In: Handbook of Physiology (ed. Dill, D. B.), pp 741-747. Environment, American Physiology Society, Washington, DC.

Lyamin, O. I. and Chetyrbok, I. S. (1992). Unilateral EEG activation during sleep in the cape fur seal, Arctocephalus pusillus. Neurosci. Lett., 143, pp. 263–266.

Lyamin, O. I., Manger, P. R., Mukhametov, L. M., Siegel, J. M. and Shpak, O. V. (2000). Rest and activity states in a grey whale. J. Sleep Res, 9, pp. 261–267.

Lyamin, O. I., Manger, P. R., Ridgway, S. H., Mukametov, L. M. and Siegel, J. M. (2008). Cetacean sleep: An unusual form of mammalian sleep. Neuroscience and Biobehavioral Reviews, Volume 32, Issue 8, pp1451-1484.

Lyamin, O. I., Mukhametov, L.M., Chetyrbok, I. S. and Vassiliev, A. V. (2002). Sleep and wakefulness in the southern sea lion. Behav. Brain Res, 128, pp. 129–138.

Lyamin, O. I., Mukhametov, L. M. and Siegel, J. M. (2004). Relationship between sleep and eye state in Cetaceans and Pinnipeds. Arch. Ital. Biol., 142, pp. 557–568.

Lyamin, O. I., Pryaslova, J. P., Kosenko, O., Lapierre, J. L., Mukhametov, L. M. and Siegel, J. M. (2006). Sleep and rest states in the walrus. Abstract Book of the 34th Annual Symposium of European Association for Aquatic Mammals, pp. 14.

Lyamin, O. I. and Siegel, J. M. (2005). Rest and activity states in the hippopotamuses. Abstract Book of the 33rd Annual Symposium of European Association for Aquatic Mammals, pp. 15.

McCormick, J. G. (1969). Relationship of sleep, respiration, and anesthesia in the porpoise: a preliminary report. Proc. Natl. Acad. Sci. U.S.A., 62, pp. 697–703.

McCormick, J. G. (2007). Behavioral Observations of Sleep and Anasthesia in the Dolphin: Implications for Bispectral Index Monitoring of Unihemispheric Effects in Dolphins. Anasthesia and Analgesia, Volume 104, No.1, 239-241.

Mukhametov, L. M., Supin, A. Y. and Polyakova, I. G. (1984). Sleep in Caspian seals (Phoca caspica). J. High Nerve Activity, 34, pp. 259–264.

Nagamine, H. (2004). Turbulence modification by compliant skin and strata-corneas desquamation of a swimming dolphin. Institute of Physics, Journal of Turbulence, volume 5, no.18.

Ridgway, S. H. (1990). The Central Nervous System of the Bottlenose Dolphin. In: The Bottlenose Dolphin 1990 (ed. Leatherwood, S. and Reeves, R. R.), pp. 69-97. San Diego: Academic Press, Inc.

Ridgway, S. H. (1972). Mammals of the Sea. Biology and Medicine. Springfield, Illinois, Charles C. Thomas.

Ridgway, S. H., Houser, D., Finneran, J., Carder, D., Keogh, M., van Bonn, W., Smith, C., Scadeng, M., Dubowitz, D., Mattery, R. and Hoh, C. (2006). Functional imaging of dolphin brain metabolism and blood flow. J. Exp. Biol., 209, pp. 2902–2910.

Sokolov, V. E. and Mukhametov, L. M. (1982). Electrophysiological study of the sleep on the manatee, Trichechus manarts. J. Evol. Biochem. Physiol., 18, pp. 191–193.

About Peter Harrison

Evo lab rat, student (again), stand-up comedian, magic consultant, writer, skeptic, gamer. Reality is my religion. Follow @Harrison_Peter for English. Follow @kaigishin for 日本語.

Posted on April 6, 2012, in Science and tagged , , , , , . Bookmark the permalink. 33 Comments.

  1. body shapes differ depending on the genetics of a person. i would really love to have a slim body.’

    Our online site
    <a href="

  2. It’s very effortless to find out any matter on web as compared to textbooks, as I found this piece of writing at this web site.

  3. I have read some good stuff here. Certainly worth bookmarking
    for revisiting. I wonder how much effort you put to create
    such a great informative site.

  4. Thank you for any other informative blog. The place else could
    I get that type of info written in such a perfect means?
    I have a mission that I am just now running on, and I’ve been
    on the look out for such info.

  5. You bend at the knees to lift barbell up, and then slowly lower the barbell to the point where you feel the pull on the back of your legs and your glutes.
    In addition to your ideas, the photographer may have
    his or her own idea of how the session should go. s great in that it works the majority of the muscles in your body without placing excessive
    pressure on your body. Just remember to keep your breathing
    steady and your posture correct when doing your double chin exercises.
    In the last 3 years since going to the gym, treadmilling 3-4 miles a day and light
    weight workout, I have increased my Tcells from 300 to 815.

  6. The Fire’s software is based on Google’s Android software.
    Dora Talking Kitchen reviews and store locator available.
    Please read the many book reviews of Stan’s popular book by going to Stan’s website or by doing an internet search of Stan’s many helpful book reviews.

    What you need to do to sell your e-book in the Amazon Kindle store :.
    Most people who submit reviews will be very honest and up front,
    and it is for that reason that you will be able to make the informed purchase choice when shopping over the internet.

  7. Kick back and read about what makes this program so great and
    how you can learn from it. Exercise helps your brain release endorphins and other “feel-good” chemicals which helps with
    both depression and anxiety. Afterward, tilt your head back and repeat the
    process for a second time, for a third time, for a fourth time, etc.
    Each casting or client may want something different.
    I advise you see your doctor first to decide which diet
    program is ideal for you.

  8. It’s hard to come by knowledgeable people for this subject,
    but you sound like you know what you’re talking about!

  9. I love your blog.. very nice colors & theme.

    Did you design this website yourself or did you hire someone to do it
    for you? Plz respond as I’m looking to create my own blog and would like to know where u
    got this from. cheers

  10. What a material of un-ambiguity and preserveness of precious
    knowledge on the topic of unexpected emotions.

  11. Unquestionably believe that which you stated. Your favorite
    justification seemed to be on the net the simplest thing
    to be aware of. I say to you, I definitely get irked while people consider worries that they plainly
    don’t know about. You managed to hit the nail upon the top as well as defined out the whole thing without having side effect , people
    can take a signal. Will probably be back to get more.

  12. It’s a shame you don’t have a donate button!
    I’d certainly donate to this brilliant blog! I guess for
    now i’ll settle for book-marking and adding your RSS feed to my Google account.
    I look forward to brand new updates and will talk aboutt this site with my Facwbook group.

    Chat soon!

  13. You’ve made some decent points there. I looked on the internet to learn more about the issue and
    found most individuals will go along with your views on this site.

  14. What’s up to all, how is all, I think every
    one is getting more from this website, and your views are fastidious
    in favor of new viewers.

  15. Some people աill tell you that thе symptoms only last for a few seconds
    while others will tell you that they last for hours at a time.

    These are sure to wοrk and give you your life back.
    The risk is out there in both the real world and online.

  16. As more familiarity is gained, it will become second nature.
    Article marketing is one of the most effective ways to bring targeted visitors to your website.
    When an article is submitted to A1 Articles,
    it is placed within a locked queue until it reaches our editors.

  17. My partner and I stumbled ovber here coming from a different webb address and thought I should check things out.
    I like wnat I see so i am just following you. Look forward to going over your web page for a second time.

  18. I simply couldn’t depart your website prior to suggesting
    that I actually loved the usual information a person provide on
    your visitors? Is going to be again ceaselessly in order to check up on new posts

  19. Hello, I would like to subscribe for this weblog to obtain newest updates, thus where can i do
    it please help out.

  20. Sooner or later, Google will find all new spam methods.
    These pre-computed numbers, hold on in a very giant information bank
    for millions or URLs on the net. * Page SEO: Your page is
    optimized by various means which include, choosing the right keywords, placing them right,
    adding anchor text linking, adding call to action in various places on the page.

  21. Factors to be considered while submitting in online business directory:.
    The online auction marketplace trend continues to grow
    as we, as an internet community, thrive on the thrill which comes from easy transactions and a wealth of goods
    to browse through. The reason – if you use news-style advertising you will get
    up to 500% better response.

  22. It’s a shame you don’t have a donate button! I’d certainly donate to this brilliant
    blog! I guess for now i’ll settle for book-marking and adding your
    RSS feed to my Google account. I look forward to brand new updates and will talk
    about this blog with my Facebook group. Chat soon!

  23. Hi my loved one! I want to say that this article
    is awesome, nice written and include approximately all
    significant infos. I would like to see more posts like this

  24. I blog frequently and I seriously appreciate your information. This article
    has really peaked my interest. I am going to bookmark your blog and keep checking
    for new information about once a week. I subscribed to your Feed as well.

  25. Everything is very open with a clear explanation of the issues.

    It was really informative. Your website is extremely helpful.
    Many thanks for sharing!

  26. With havin so much content and articles do you
    ever run into any issues of plagorism or copyright violation? My site has a
    lot of unique content I’ve either written myself or outsourced but it seems a
    lot of it is popping it up all over the internet without my agreement.
    Do you know any ways to help stop content from being stolen? I’d really appreciate it.

  27. Hi there! I know this is kinda off topic however , I’d figured I’d ask.
    Would you be interested in exchanging links or maybe
    guuest authoring a blog post or vice-versa? My website addresses a lot of the same topics as yours and I believe we could greatly benefit from each other.
    If you might be interested feel free to send me an e-mail.
    I look forward to hearing from you! Awesome blog by the way!

  28. This paragraph is truly a fastidious one it assists new net viewers, who are wishing in favor of blogging.

  29. It’s an remarkable piece of writing designed for all the internet viewers; they will take advantage from it I am sure.

    orlando bloom orlando bloom افضل شركة كشف تسربات المياه بالرياض افضل شركة تنظيف موكيت بالرياض orlando bloom orlando bloom
    orlando bloom افضل شركة عزل خزانات بالرياض
    افضل شركة نظافة بالرياض افضل شركة كشف تسربات المياه بالرياض orlando bloom افضل شركة مكافحة
    حشرات بجدة orlando bloom orlando bloom orlando bloom افضل
    شركة رش دفان بالرياض
    شركات تنظيف افضل شركة كشف تسربات المياه
    بالرياض افضل شركة تنظيف بجدة orlando
    bloom orlando bloom orlando bloom orlando bloom افضل شركة رش
    مبيدات بالرياض
    orlando bloom orlando bloom orlando
    bloom orlando bloom orlando bloom افضل شركة
    عزل خزانات بالرياض orlando bloom orlando bloom
    orlando bloom orlando bloom رش مبيدات قبل صبة النظافة orlando bloom
    افضل شركة تنظيف بالرياض orlando bloom افضل شركة تنظيف بيارات بالرياض orlando bloom
    orlando bloom orlando bloom افضل شركة تنظيف
    بيوت بالرياض orlando bloom orlando bloom افضل
    شركة تخزين اثاث بالرياض orlando bloom افضل شركة مكافحة حشرات بجدة
    orlando bloom افضل شركة رش مبيدات بالرياض orlando bloom orlando bloom افضل شركة تنظيف خزانات بالرياض orlando bloom orlando bloom orlando bloom
    افضل شركة رش مبيدات بالرياض افضل رش مبيدات قبل صبة النظافة orlando bloom orlando bloom orlando bloom افضل شركة تنظيف فلل بالرياض {افضل شركة تنظيف
    بالرياض نظافة سجاد

  30. We’re a bunch of volunteers and starting a new scheme in our
    community. Your site provided us with valuable information to work on. You’ve performed an impressive job
    and our whole neighborhood can be thankful to you.

  31. Wow! Finally I got a webpage from where I can really obtain valuable facts regarding my study and knowledge.

  1. Pingback: Sleeping with one eye open – the Delphinidae and curious sleep phenomena « Kirsten Writes

  2. Pingback: Sleeping with one eye open – the Delphinidae and curious sleep phenomena « savedolphinsph

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 62 other followers

%d bloggers like this: