Effects of novelty on rats’ responses to CO 2 exposure Lee Niel * , Richard D. Kirkden 1 , Daniel M. Weary Animal Welfare Program, Faculty of Land and Food Systems, The University of British Columbia, 2357 Main Mall, Vancouver V6T 1Z4, Canada Accepted 17 June 2007 Available online 31 July 2007 Abstract Laboratory rats are routinely killed using a gradually increasing concentration of carbon dioxide (CO 2 ) gas. However, CO 2 exposure has been shown to cause aversion and to increase exploratory and escape behaviours in rats. We examined whether these responses are due, in part, to novelty, using two different approaches. For the first experimental approach, we used approach-avoidance testing to examine gas aversion by pairing gas exposure with a food reward. Rats were tested with repeated exposure to a gradually increasing concentration of either CO 2 (Experiment 1) or a novel odour (Experiment 2) to determine whether these stimuli cause aversion, and to document whether rats show habituation to these stimuli. We reasoned that aversion associated with novelty would decline with repeated exposure, but that aversion due to other factors would not. During control sessions in Experiment 1, rats were tested with air and they ate for an average of 266 Æ 4 s (meanÆS.E.M.). However, on the four CO 2 test days they stopped eating after only 30 Æ 2 s. Across the first three days of testing with CO 2 , the latency for rats to stop eating and leave the test cage did not change, and the average CO 2 concentrations at these time points were 11 and 14%, respectively. On the final day of testing with CO 2 , after eight additional days of CO 2 exposure, these latencies had increased slightly, but rats were still leaving the chamber at only 18% CO 2 . In contrast, rats in Experiment 2 showed similar latencies to stop eating and leave the test cage with both air and a novel odour (232 versus 235 Æ 14 s), and their responses did not change with repeated exposure. These results suggest that novelty has only a minor effect on rats’ aversion to CO 2 . For the second experimental approach, we examined the behavioural responses of rats during baseline and during exposure to a gradually increasing concentration of either CO 2 or peppermint odour (Experiment 3) to determine which of the behavioural responses observed during CO 2 exposure can be attributed to novelty. In comparison to baseline, rats that underwent forced exposure to either CO 2 or a novel odour showed increased activity and rearing, and spent more time with the www.elsevier.com/locate/applanim Applied Animal Behaviour Science 111 (2008) 183–194 * Corresponding author. Present address: Department of Psychology, University of Toronto Mississauga, 3359 Mississauga Road N., Mississauga, Ontario L5L 1C6, Canada. Tel.: +1 905 569 4889; fax: +1 905 569 4326. E-mail address: lee.niel@utoronto.ca (L. Niel). 1 Present address: IFM Biology, Linko ¨ping University, 581 83 Linko ¨ping, Sweden. 0168-1591/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2007.06.004