Tonic immobility (TI) is a state of motor inhibition that can occur in prey animals as a last defence against a predator. The rabbit will lie motionless, thereby giving the impression of being already dead and encouraging the predator to release its grip, giving the rabbit a last chance to escape. This state of temporary hypnosis can be induced in domestic rabbits by laying the rabbit on its back (dorsal recumbancy) and flexing the head against the neck; as long as the head remains flexed the rabbit remains in the same position.
Rabbits in dorsal recumbancy show a drop in blood pressure, heart rate and respiratory rate along with a depression of spinal reflexes and the abolition of the righting reflex. There is also a reduced response to noise and painful stimuli. According to a study by Danneman et al, 1988, tonic immobility cannot be induced in approximately 25% of rabbits.
TI is commonly referred to as "trancing" or hypnotising rabbits and in the past has been commonly used by vets and rabbit owners as an easy way of examining a rabbit or clipping its nails. It was previously thought to relax the rabbit but this has been discredited in recent years, with studies (in particular, a 2007 study by McBride et al) showing increased heart rate and respiration following an episode of TI . In addition, after TI a rabbit's behaviour may be adversely affected in that it hides away more, grooms itself more and shows less inclination to explore. The more often it is done and the longer the rabbit is "tranced" for, the worse the effects.
Although trancing is still a debated subject, with many rabbit owners advocating it as a useful tool, it is commonly accepted that TI should be avoided where possible as it is detrimental to the rabbit's health. In the UK most rabbit experts, welfare organisations and vets caution against the use of TI or recommend it is only used as a last resort; for example, when a vet is examining a very nervous rabbit and other means of restraint have failed.