Is Your Rabbit Really Sleeping?
Have you ever cradled your rabbit in your arms much like you would hold a baby, and thought she fell right to sleep in that position? This is actually what a lot of people familiar with rabbits describe as "trancing" a bunny.
There are a number of theories about what is really occurring when you hold certain animals in a restraining position. The literature dates back more than three centuries and has called many things, but current investigators prefer to use the more neutral and descriptive term "tonic immobility" as a way to describe the state (Gallup 1974). There is a variety of research related to this phenomenon that examines its behavioral, ecological, chemical, neurological, and genetic aspects. It should also be noted that many different species of animals have been studied, including chickens, guinea pigs, lizards, rats, rabbits, sharks, and more.
Tonic immobility (TI) is defined as a condition "in which an animal is rendered to be immobile when faced with danger" (Overeem 2002). Another set of authors (Leite-Panissi 2002) defines TI as an instinctive defense mechanism "characterized by profound physical inactivity and a relative lack of responsiveness to external stimuli." When confronted by a predator, the prey animal emits a TI response as its last resort to increase its chances of survival. According to the literature, the release of TI during a confrontation increases the prey animal's chances of survival by 50%. It is explained that the struggle of the prey animal may be necessary for the predator to continue attacking. Once the prey appears dead, the predator may lose interest.
Similarly, Misslin (2003) describes TI as one of the immediate responses to fear. Fear is defined as a functional defense behavior system that is a basic instinct for the survival of individuals and species. It functions to protect individuals from constraining and/or dangerous situations, especially from predators. Leite-Panissi (2002) notes that fear is a trigger in the part of the brain called the amygdala for eliciting the TI response. However, if something is blocking the amygdala, TI behavior may be inhibited.
In addition to a threat of fear or danger, tonic immobility can be induced by restricting an animal's movements. In rabbits, TI is induced by turning the rabbit onto her back and restraining her. One article (Porro 1987) seems to suggest that physical restraint can cause a stressful reaction in some animals, and it is accompanied by analgesia and the activation of the body's internal system to release endorphins for relieving pain. The researchers have shown that it is possible to produce TI while a rabbit is experiencing pain. Electroencephalogram (EEG) readings showed activity that is not much different than that observed in animals who are free from pain.
TI has also been described as a form of hypnosis, which is defined as a state of temporary immobility that can be induced by placing an animal on its back and preventing it from moving until it is fully relaxed (Carli 1974). This investigation studied the blood pressure and heart rate rabbits during the various stages of hypnosis. 1) The findings showed that when the initial manipulations prior to hypnosis occurred (such as capturing the rabbit and his subsequent struggle), the rabbit's blood pressure and heart rate showed a series of fluctuations. 2) When the rabbit was put on his back, considerably high blood pressure and heart rate developed, with the highest rates occurring in the initial seconds of the hypnosis. 3) During the actual hypnotic state, blood pressure and heart rate did not differ greatly from the control values, which were described as "quiet wakefulness" of the animal. 4) In longer hypnotic states, blood pressure and heart rates could gradually and slowly decrease. 5) When hypnosis ended, and the rabbit was returned to the upright position, there was a temporary increase in blood pressure, accompanied at times by a slow heart rate.
Another article (van Dijk 2003) investigates whether animals have simply fainted. Fainting is described as unconsciousness due to insufficient circulation or an inadequate flow of oxygen to the brain. The author notes that prey animals in extreme danger can become immobile to escape death, but this is achieved without shutting down their circulation. EEG studies provide evidence that the brains of animals experiencing TI are clearly active, so apparently they are not fainting according to this definition.
As you can see, there are a variety of theories available to help explain what is happening to animals experiencing tonic immobility. Keep in mind that this review is not comprehensive, and it certainly does not give you the one "right" answer. Though not a very recent publication, Gallup (1974) does a good job of comparing the theories and even refuting some of the findings. Perhaps you will draw your own conclusions from further study on your own.
Bibliography
Carli, G. "Blood Pressure and Heart Rate in the Rabbit During Animal Hypnosis." Electroencephalography and Clincical Neurophysiology. 1974; 37: 231-237.
Gallup, G. "Animal Hypnosis: Factual Status of a Fictional Concept." Psychological Bulletin. 1974; 81 (11): 836-853.
Leite-Panissi, C.; Menescal-de-Oliveira, L. "Central Nucleus of the Amygdala and the Control of Tonic Immobility in Guinea Pigs." Brain Research Bulletin. 2002; 58 (1): 13-19.
Misslin, R. "The Defense System of Fear: Behavior and Neurocircuitry." Clinical Neurophysiology. 2003; 33 (2): 55-66.
Overeem, S; Lammers, G. J.; van Dijk, J.G. "Cataplexy: 'Tonic Immobility' Rather than 'REM-sleep Atonia'" Sleep Medicine. 2002; 3 (6): 471-477.
Porro, C.A.; Carli, G. "Immobilization and Restraint Effects on Pain Reactions in Animals." Pain. 1988; 32 (3): 289-307.
van Dijk, J. G. "Fainting in Animals." Clinical Autonomic Research. 2003; 13 (4): 247-256.
