Rabbits and Tonic Immobility (TI)

We have recently been made aware of the paper below on Tonic Immobility (TI) and the discussion about it on social media. The paper is freely available as an open access item and we would encourage people to read it in order to form an opinion.

http://www.medycynawet.edu.pl/images/stories/pdf/pdf2021/062021/2021066541.pdf?fbclid=IwAR1VvtmthLajMJOFTv8l86X9pgw2jxMKo5MHWIKNMVQ1ETsb5LuM3CzSY_4

This was a small study, limited to small, young rabbits in a veterinary clinic setting, and the authors acknowledge that more needs to be done in order to draw firm conclusions. There are few papers which have been published on this subject so far.

TI has been used both historically and currently in many settings and species. It is postulated, variously, that this induces calm in an animal as it mimics feeding behaviour as an infant, that it is a “playing dead” position to help evade predation, or that it forms part of mating behaviour. The method of induction varies between species but it generally involves turning the animal on its back.

Rabbits have been “hypnotised” like this for many years. They tend to go very still, and it was thought to calm the rabbit, which is why it was recommended for handling. There had even been the suggestion of carrying out painful procedures like this, although with the addition of local anaesthetic for surgery.

However, a study by McBride et al in 2006 (http://eprints.hud.ac.uk/id/eprint/11361/) demonstrated signs of stress in rabbits subjected to TI: increases in heart rate and breathing rate, increased cortisol, a “stress hormone”, and behaviour associated with stress.

Trancing Rabbits: Relaxed hypnosis or a state of fear? – University of Huddersfield Repository – Welcome to the University of Huddersfield Repository – University of Huddersfield Repository Tonic immobility (TI) is a transitory and reversible state of profound motor inhibition that can be induced in susceptible species (Klemm 1976). Episode duration can be from several seconds to several hours, even within a single species (Gallup, 1975) The most susceptible species are those which are highly preyed upon. Described by Darwin as the death feint (Darwin 1839), it is believed that … eprints.hud.ac.uk

There are several papers where there is an initial increase in heart rate associated with positioning rabbits on their backs. These are cited in a study from 2014 (Giannico AT, Lima L, Lange RR, Froes TR, Montiani-Ferreira F. Proven cardiac changes during death-feigning (tonic immobility) in rabbits (Oryctolagus cuniculus). J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 2014 Apr;200(4):305-10. doi: 10.1007/s00359-014-0884-4. Epub 2014 Feb 11. PMID: 24515628.) which shows that heart rate reduces during TI.

The new paper is specifically about rabbits in a veterinary clinic setting. It shows that heart and respiratory rates stay the same or increase in rabbits when they are examined on their fronts, but decrease when they are examined on their backs. Pupil size, another measure of stress, increases when examined on their fronts, but not their backs. Blood glucose remained at a very similar level throughout in both groups. Blood cortisol goes up with handling in both groups, but increases less in those on their backs than their fronts. Rabbits entering a vet clinic are, by definition, already very stressed, so the baseline here is somewhat skewed (this is not a criticism of the paper, just an observation), with heart rates and, in particular, respiratory rates, well above those commonly suggested as “normal”.

In summary, during TI in this study:

  • Heart and breathing rate drop
  • Pupil size decreases
  • Glucose does not significantly change
  • Cortisol increases (but less than during examination on their fronts)
  • There is no suggestion that there is any kind of “natural” pain relief induced by TI.
  • These are averages, and some rabbits show very different responses to others.

Conclusions we can draw from this:

  • The changes seen during TI are not those of a “fight or flight” response.
  • Some rabbits respond with decreases in signs, some with increases, suggesting different individual susceptibilities.
  • We cannot say if a rabbit “likes” being turned on its back.
  • The decrease in heart and respiratory rate suggest lowered physiological stress but this doesn’t necessarily equal less emotional stress.

And the main concern: there is a difference between lying on their back and “true” TI. Different ways to hold and restrain rabbits have different effects and may induce TI or may not.  In her social media posts on this, Dr Harcourt-Brown mentions this too, drawing a distinction between TI and trancing.The author describes carefully putting rabbits into a position on their backs, which is how Dr Harcourt-Brown also puts it. The exact nature of the difference needs further exploration, with some suggestions previously that where rabbits have had TI used in the past are more susceptible to it in future, and whether there is an element of “learned helplessness” present in at least some cases.

This study was limited to a veterinary clinic setting. As vets, we need to incorporate these results into a contextualised care approach, and consider:

  • Why we are doing it, for example to facilitate a life-saving diagnostic or therapeutic procedure like obtaining an x-ray.
  • If the position is appropriate. For example, it is a good position to take a urine sample from the bladder but not for syringe feeding because of aspiration risk.
  • How are we are lying them on their backs, and that it is in a stable and supported way.
  • Whether we are inducing a physiologically relaxed response or inducing a state where the rabbit is tense and poising to jump up and run for its life.
  • If it is appropriate for that rabbit. While the rabbits in this study appear to undergo a relaxation response, not all rabbits will do that. If the technique doesn’t work for an individual rabbit, there could be serious consequences if they jump and twist.
  • That there is no suggestion that this process has any analgesic benefit at all.

The only conclusions we can draw from this study are limited to a veterinary clinic setting and our advice for rabbits in all other settings remains the same. The RWAF aims to give the most up-to-date care advice for rabbits. As they are studied more, there is more evidence, and our advice may therefore change in response.

 Richard

Richard Saunders (he/him) BSc (Hons) BVSc FRSB CBiol DZooMed (Mammalian) DipECZM(ZHM) MRCVS 

Rabbit Welfare Association and Fund Veterinary Advisor