On a recent Marine Biology trip to Turkey, where we were studying turtles, the question of how they slept arose. A few ideas, such as “with one eye open”, were tossed around, before the question was dismissed. However it got me thinking about how they, and other animals, sleep.
Turtles
Turtles don’t sleep so much as rest; they will do this under
ledges or rocks where possible, presumably to stop themselves floating off.
However, if they are in deeper water, they will float near the surface. While resting, they close their eyes like humans, but are conscious enough to
know to surface in order to breathe when necessary.
Horses
Although many horses have become domesticated, and so are
rarely preyed on, they still have many adaptions of a prey animal; by being
able to sleep standing up, horses are able to escape danger more quickly, as
they don’t have to waste time struggling up. Horses have a network of ligaments and tendons
called the stay apparatus that allows them to sleep standing up, by locking
their legs so that they are able to relax their muscles. The stay apparatus in horses’ front and back legs differ; while the front leg
just needs to relax to use the stay apparatus (it is always in place), a horse
needs to literally hook one bone over a knob on another bone to engage the stay
apparatus in its hind legs. However, usually once a day, a horse will lie down for a deeper sleep, as it
seems to be unable to achieve this while standing.
Dolphins
Dolphins, along with other cetaceans (whales, orcas and
porpoises) sleep in a similar way to birds – a type of sleep called
unihemispheric slow-wave sleep.
This means that when a dolphin “sleeps”, half of its brain (one
hemisphere) shuts down, and it closes the opposite eye. This then leaves the
other half of the brain and the other eye to control breathing, observe
surroundings and maintain muscle movement to keep the dolphin warm. While in
this sleep-like state, the dolphin will either swim slowly or stay still nearthe water’s surface.
Human babies, like many other animals, spend a significant
amount of time sleeping, but dolphin babies (calves) don’t sleep at all in the
first few months after they are born and when they become older, they sleep while being towed along in their
mother’s slipstream (echelon swimming).
Bats
Most people know that bats sleep hanging upside down, but
not many know why. The main reason is that, unlike birds, their wings don’t
produce enough lift to take off from the ground, so by roosting upside down,
they can spread their wings and “fall” into flight. It also helps them hide
from predators. In a somewhat similar way to horses, bats have evolved a
mechanism to enable them to lock their talons closed, so that they can relax
while hanging upside down; their tendons in their talons are connected to their
upper bodies, meaning that their body weight pulls on the tendons, locking the talons closed.
Bats are also known for being nocturnal, so they sleep
during the day.
Swifts
The swifts found in the UK have normally migrated from South
Africa, a distance of over 6000 miles. Swifts rarely land (sometimes they won’t land for 6 months), other than at nest
sites, and routinely fly at 10,000ft at night-time. This leads us to presume
that swifts must sleep “on the wing”, although it has not been proved, as no electroencephalograms (EEG) have ever been performed.
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Emily Lauterpacht
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