Rabbits can easily be injured if they aren't handled carefully, but laying a rabbit on his back doesn't instantly paralyze him. A rabbit can be put into a trance-like state of paralysis, however, by placing him on his back and flexing his neck. This is often referred to as "trancing" a rabbit, and some owners believe that a rabbit is trusting and relaxed in this state.
Paralysis is defined as a loss of muscle function. It can be permanent or temporary, although when most people think of paralysis they think of a permanent loss of the use of limbs. Placing a rabbit on his back in the right position and under the right conditions can cause a sort of temporary paralysis. This isn't due to physical injury, however, but part of a survival mechanism.
In the simplest of terms, tonic immobility is "playing dead." While people might teach their dogs to play dead as a fun trick, for a rabbit it's an extreme means of surviving an attack. When a rabbit has been caught by a predator, tonic immobility allows him to become limp and unresponsive. His blood pressure and respiration rate drop as well. This often convinces the predator that the rabbit is dead and gives him one last chance to escape.
Purposely inducing tonic immobility by laying a rabbit on his back can cause him serious stress. While he may appear relaxed during the trance-like state, afterward his heart and respiration rate is likely to be very high. A study led by Dr. Anne McBride of the University of Southampton found that rabbits show signs of extreme stress after an episode of tonic immobility. Another danger of placing a rabbit on his back is that he could break his back if he struggles against being turned over.
While Dr. McBride's study points out that putting a rabbit into a state of tonic immobility is stressful, there may be some purpose in it for medical treatment. For example, Dr. Dana Krempels of the University of Miami suggests placing a rabbit on his back in order to take his temperature to evaluate whether or not he's having a medical emergency. Some rabbit owners also use tonic immobility to clip their pet's nails, examine him or give other home treatments. Whether the stress following an episode of immobility outweighs the risks of a rabbit struggling against having his nails clipped or being examined should be discussed with a rabbit-savvy veterinarian.
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