Removing individual rats affects indicators of welfare in the remaining group members Oliver Burman a, ⁎ , Diane Owen b , Usama AbouIsmail a,c , Mike Mendl a a Centre for Behavioural Biology, University of Bristol, UK b Central Science Laboratory, York, UK c Department of Hygiene, Husbandry and Zoonoses, Faculty of Veterinary Medicine, Mansoura University, Mansoura, Egypt Received 22 August 2006; received in revised form 26 July 2007; accepted 2 August 2007 Abstract The removal of individuals from social groups, e.g. in order to maintain appropriate stocking densities in groups of rapidly growing young laboratory rats, is often necessary. However, such removals may be stressful and few studies have investigated their effects on the behaviour, physiology and welfare of the remaining group members. In this study we investigated this issue for rats housed at different stocking densities by observing behaviour and recording faecal corticosterone metabolite levels both before and after removal. We found that, irrespective of stocking density, the rats remaining in the home cage significantly increased agonistic behaviour, audible vocalization, aggressive grooming, bar-chewing and climbing behaviour following removal of their cage-mates, and that these behavioural changes were associated with a highly significant post- removal increase in their faecal corticosterone metabolite levels. Taking the behavioural and physiological results together, it appears that the removal of individuals from groups of young laboratory rats resulted in social stress, and thus an apparent impairment of welfare. © 2007 Elsevier Inc. All rights reserved. Keywords: Animal welfare; Laboratory rat; Social disruption; Social stress 1. Introduction The social environment of a gregarious species such as the laboratory rat [1] is likely to have a major influence upon its welfare. Social isolation [2], over-crowding [3] and group composition [4] all appear able to affect behavioural and physiological indicators of welfare. It is therefore possible that standard husbandry procedures that disrupt the social environ- ment, for instance through the disturbance of social odours (e.g. cage cleaning), the introduction of stressful procedures (e.g. handling) or via change to the composition of individuals within a cage, are also able to impact upon rat welfare [2]. Yet, whilst there has been some investigation into the potentially disruptive effect on the social environment, and thus welfare, of both cage cleaning [5] and handling [6,7], there has been little research into the effect of changing group composition on laboratory rat welfare. A Council of Europe proposal to provide newly weaned rats with smaller space allowances when housed in larger cages will result in animals being raised at higher stocking densities in larger groups comprised of more litters [8]. In order to maintain the appropriate stocking densities in groups of rapidly growing wean- ling rats, such housing systems will inevitably require the removal of a proportion of individuals at particular time intervals. The removal of individuals from social groups will also frequently occur as young rats are shipped out to laboratories from breeding establishments, and when rats are removed for either experimental procedures or due to illness. During the removal process, individual rats are arbitrarily selected to be removed. We were therefore interested to see whether or not such removal – incorporating both the process of removal itself and the resulting change to group size – affects the social behaviour of the remaining rats, and whether it impacts on behavioural and physiological indicators of their welfare. Previous research into the effect of changing group com- position on laboratory rat welfare has focused on either Physiology & Behavior 93 (2008) 89 – 96 ⁎ Corresponding author. Division of Farm Animal Science, Department of Clinical Veterinary Science, University of Bristol, Langford House, Langford BS40 5DU, UK. Tel.: +44 1179289571; fax: +44 1179289582. E-mail address: oliver.burman@bristol.ac.uk (O. Burman). 0031-9384/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.physbeh.2007.08.001