Why don鈥檛 bats get dizzy when they hang upside down? Or do they?
鈥� Dizziness is a sensation humans describe when they feel a sense of motion, even when not moving. It can be associated with queasiness or nausea, and sometimes vomiting. Other types of dizziness include motion sickness and vertigo, which often manifests itself as a spinning feeling, or other sensations such as light-headedness or heavy-headedness.
It is impossible to know for sure whether or not an animal is dizzy because it cannot communicate such feelings. However, it is possible to infer an animal is dizzy from how it behaves. For example, if an animal is aimlessly walking in circles, it is probably dizzy.
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Motion sickness occurs when there is excessive stimulation of the inner ear or from a conflict between sensory information from different sources, such as from the inner ear and the eyes. The balance mechanism of the inner ear is complicated, and includes sensors that detect both movement and orientation with respect to gravity, even when an individual is not moving. Bats have such a balance mechanism, and in addition use echolocation.
The parts of the inner ear that are important for orientation with respect to gravity are called the : the utricle and the saccule. It is these parts of the inner ear that would be activated while the bat was hanging upside down. Stimulating these parts of the inner ear, however, would not necessarily lead to dizziness, especially in a dark cave where there is no conflict between information from the inner-ear balance mechanism and vision.
The bottom line is that bats are used to hanging upside down without showing any behavioural changes that would suggest dizziness or motion sickness. But because we cannot ask a bat directly whether or not it is dizzy, we can鈥檛 be certain about the effects of hanging upside down.
Joe Furman, Editor of the Journal of Vestibular Research, University of Pittsburgh, Pennsylvania, US
鈥� When you think of bats, you usually think of them in one of two conditions: hanging upside down resting, or flitting about pulling high-g turns in the dark. So why don鈥檛 they get dizzy?
Bats have evolved a number of adaptations to allow them to hunt and hang without the problems that humans would face.
First, some bats have specialisations in the vestibular portion of their inner ears 鈥� the portion that generates sensory signals for controlling balance. Their sacculus, which in humans acts as a gravity sensor to help us stand upright, is slightly rotated forwards. This lets it act more as a pitch detector, which is more useful in flight.
Second, their semicircular canals, which sense rotation of the head, have an internal structure more like a bird鈥檚 than a human鈥檚. This probably allows them to make high-speed turns without the fluid in the canals sloshing back and forth too much.
Lastly, if you photograph bats in flight with a high-speed camera, you notice that they keep their heads very stable except in the most violent turns.
鈥淚f you photograph bats in flight with a high-speed camera, you see they keep their heads very stable鈥�
But it is how bats sense the world that probably gives them immunity to dizziness. All the vestibular system does is tell you about changes in acceleration of your head. It requires other senses to pin down your position and motion in the outside world. We primarily use vision to do this, but vision is very slow. Anything you look at that takes a second or less to cross 30 degrees of your vision appears smeared. , while not blind, rely more on biosonar, an especially precise form of hearing that lets them build up 3D images from echoes.
Echolocating bats emit brief sonar chirps from 30 to more than 150 times per second, and respond to changes in echoes of less than a microsecond. These bats integrate echolocation with their vestibular system, so they are working with a faster, more precise positioning system than humans do with vision. Because dizziness and motion sickness usually arise when signals from the vestibular system conflict with those from other sensing systems, bats are less likely to show motion sickness than other mammals.
Seth Horowitz, Assistant professor, neouroscience, Brown University, Rhode Island, US