Why only do it with half a brain?

Bottlenose dolphin (Tursiops truncatus). (from wikimedia commons)

There are many reasons why you may want to punch a dolphin in the mouth, but their sleep definitely isn’t one.

The brain does not sleep equally in all regions. Local differences in brain activity during sleep are partly caused by anatomical differences between the regions and partly by the activity of that brain region during wakefulness.
All animals that have been studied so far have these local differences in sleep, but not many have them in such a spectacular way as the dolphin.

Examples of EEG recordings showing unihemispheric sleep in a dolphin. A: Location of the electrodes on the dolphins brain (1,2,3=left, 4,5,6=right); B: The dolphin is asleep on the left, and awake on the right; C: sleeping on the right and awake on the left side. (Mukhametov et al., 1977)

Dolphins are able to sleep with only one half of their brain at a time, alternately napping with their left and right hemisphere. Early EEG research in dolphins seemed to show that dolphins are only able sleep in this unihemispheric way. Moreover, when researchers kept one hemisphere awake, the dolphin only showed recovery sleep in that hemisphere. This was some of the earliest direct evidence that sleep is regulated on a local level in the brain.

Sleeping dolphin hovering at the tank surface (McCormick, 2006)

Some more recent research shows that dolphins are in fact able to sleep with both hemispheres at the same time. They then hover near the water surface of their tank with their blowhole above the surface to breathe, displaying a reflexive swimming motion, and don’t respond to external stimuli. So bihemispheric sleep in the dolphin is very similar to ‘normal’ sleep in land-dwelling mammals. But, even though they are capable of sleeping with their entire brain at the same time, dolphins don’t do it very often. Bihemispheric sleep, or at least sleep-like surface hanging, makes up about 15% of their total sleep/resting time. Surface hanging sleep has been observed in sperm whales (see video), but I don’t know of any observations of this behavior by dolphins in the wild.

It is a bit of a puzzle why an animal would sleep with only half of their brain at a time. After all, bilateral sleep allows you to get the same amount of sleeping done in half the time. What kind of selection pressures could lead to unihemispheric sleep?

One obvious benefit of unihemispheric sleep is that it allows an animal to nap and keep some control of their environment at the same time. There is little risk of sinking and not being able to come up for air in time, or being carried off by a current and finding yourself in a completely different part of the ocean after dozing off for a while. Sinking may not be a major problem for a dolphin, they are good swimmers after all, but drifting off into unknown parts of the ocean is.

Another reason to sleep with one hemisphere is to be able to sleep with one eye open. Dolphins usually live and sleep in social groups. When sleeping, keep on swimming without bumping into each other much (or into the aquarium walls for that matter). And while doing that, they keep one eye open towards their groupmates. If it is time for the other hemisphere to go to sleep, they simply change their position in the group accordingly. So it seems that keeping track of your group is more important than keeping an eye out for potential predators. This could be because dolphins don’t face many predators, or perhaps that there is little hope of escape when one shows up while they are taking a nap. It is also possible that they simply watch each other for cues as to when to turn and where to swim. The published research on this is done within the safe walls of an aquarium though; it would be interesting to see how the sleep behavior changes in the presence of predators.

For example: ducks, who are also able to sleep with half of their brain at a time, spend more time keeping an eye out to the world outside of their flock than towards it while taking a nap when a picture of a bird of prey was projected in the lab.

Eye closure is a nice way to determine which hemisphere is asleep (it is usually the one opposite the closed eye). Sadly, the relation between eye closure and sleep is not 1:1, which makes the EEG still the most reliable way to determine sleep. It will obviously be tricky to watch wild dolphins sleep without disturbing them. Doing that while sticking some electrodes on them is even harder. In the below picture you can see a researcher (I’ve been told it’s Lev Mukhametov, but I’m not sure) measuring the EEG of a dolphin that’s swimming around in an aquarium. There are wireless and more portable EEG recording devices available nowadays, and it would be great to see in how far the aquarium observations match what dolphins do in the wild.

Recording EEGs in freely moving dolphins can be tricky. Here it is done with waterproof cables and a patient researcher. (Dont know source)

If watching out for predators is not the main reason for unihemispheric sleep, what else could it be? Pillay and Manger (2004) propose that dolphins sleep with one eye open to keep warm. Heat loss is a major issue for warm-blooded animals that live in the ocean, more so the smaller they are. The heat-loss hypothesis would also explain another peculiarity of dolphin sleep: the very low amount of REM sleep compared to animals with a similar-sized brain. For dolphins, and other animals, the main way to generate body heat is to move their muscles. During unihemispheric non-REM sleep, they may moves less than during wakefulness, but they are still using their muscles and consequently generate heat. During REM sleep, however, they may not be able to do so: postural muscles are then typically completely relaxed, and body temperature is no longer regulated to a normal, stable level. This makes relaxed sleeping in a cold ocean quite dangerous for a relatively small animal like a dolphin.

Staying in place in a moving ocean, keeping an eye out for friend and foe, and moving to stay warm are all likely factors that have led to the evolution of unihemispheric sleep in dolphins. Unihemispheric sleep is a solution for the problem to the problem of sleep in under demanding circumstances, although it is not exactly ideal. Other small sea-dwelling mammals, the fur seals, have found a similar solution: they sleep unihemispherically while paddling to stay afloat at sea, but they strongly favor sleep with their entire brain when they get the chance to stay on dry land. Dolphins just don’t have that choice.

References:

LM Mukhametov, AY Supin and IG Polyakova (1977), Interhemispheric asymmetry of the electroencephalographic sleep patterns in dolphins; Brain Research, 134: 581-584 (link)
P Pillay and PR Manger (2004), Testing thermogenesis as the basis for the evolution of cetacean sleep phenomenology; J. Sleep Res. 13:353–358 (link)
SR Ridgway (2002), Asymmetry and Symmetry in Brain Waves from Dolphin Left and Right Hemispheres: Some Observations after Anesthesia, during Quiescent Hanging Behavior, and during Visual Obstruction; Brain Behav Evol 60:265–274 (link)
JG McCormick (2006)  Behavioral observations of sleep and anesthesia in the dolphin: implications for bispectral index monitoring of unihemispheric effects in dolphins. Anesth Analg. 103(3):626-32 (link)
NC Rattenborg, SL Lima, CJ Amlaner (1999), Facultative control of avian unihemispheric sleep under the risk of predation. Behav Brain Res. 105(2):163-72. (link)
OL Lyamin, PO Kosenko, JL Lapierre, LM Mukhametov, JM Siegel (2008) Fur seals display a strong drive for bilateral slow-wave sleep while on land. J Neurosci 28(48):12614-21 (link)
PD Goley (1999) Behavioral aspects of sleep in Pacific white-sided dolphins (Lagenorhynchus obliquidens, Gill 1865). Mar Mamm Sci 15:1054–1064
S Sekiguchi and S Kohshima (2003), Resting behaviors of captive bottlenose dolphins (Tursiops truncatus). Physiol Behav 79(4-5):643-53 (link)

2 Comments

Filed under behavior, dolphin, sleep

2 responses to “Why only do it with half a brain?

  1. Pingback: Days of continuous vigilance: researchers keep dolphins awake, dolphins don’t care | Taking a cat apart

  2. Pingback: The problem of the ever-awake babies | Taking a cat apart

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