The tonic immobility reaction of the domestic fowl: a review

Abstract

SummaryTonic immobility (TI) is an unlearned state of profound but reversible motor inhibition and reduced responsiveness which is induced by physical restraint. It is thought to represent the terminal reaction in a chain of anti-predator responses and is considered positively related to fear because procedures intended to elicit fear prolong the response whereas fear-reducers attenuate it. It can be induced in a wide variety of species but has been mainly studied in the domestic fowl which gives a particularly robust and easily discernible reaction. The changes in heart-rate, respiration and temperature which accompany the response are reviewed together with its neural and neurochemical correlates.Numerous techniques have been used to induce the tonic immobility response but a U-shaped wooden cradle is recommended as the most reliable method for adult birds. The age at which TI can be first elicited is likely to be a function of the strain of bird and/or methodology used. The visual presence of the observer is thought to prolong the reaction particularly if he maintains eye contact with the bird or changes his appearance but a regular handling or taming regime depresses tonic immobility. The immobility reaction is open to genetic manipulation and selection for high and low responsiveness to novelty in domestic chicks and quail was accompanied by the potentiation or attenuation of TI respectively. Sudden isolation from a social environment prolonged tonic immobility whereas the TI responses of socially-reared birds were reduced by the presence of other chicks though the social status of the companion bird influenced this effect in adults. Possible explanations for the observation that dominant hens showed longer immobility reactions than their subordinate counterparts are discussed. Tonic immobility was longer in birds housed in a) cages than in pens, b) large rather than small groups and c) the top tier of a three-tier battery system than in those from the lower tiers but few concrete conclusions can be drawn at this stage regarding the effects of different housing and husbandry systems on TI and fear in general.This review is not intended as an exhaustive catalogue of the mechanisms involved in or of the extraneous factors affecting tonic immobility. Indeed, substantial gaps remain in the understanding of the nature and control of the immobility reaction. However, it was concluded that the TI reaction, applied in conjunction with a battery of other tests, is a useful behavioural tool for assessing a bird's reactivity and/or fearfulness. The results of such studies may provide guidelines for future research on the welfare aspects of the most appropriate housing systems for the domestic fowl.