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Ultrasonic vocalizations
and other behavioural responses to carbon dioxide euthanasia in laboratory
rats
L. Niel and D.M. Weary
University of British
Columbia, Vancouver, Canada
The objectives of this experiment were to determine 1) whether rats euthanized with carbon dioxide display behaviors indicative of distress, and 2) whether these behaviours are due to oxygen deprivation. Rats were either euthanized with carbon dioxide or exposed to argon, an inert gas used only to displace a similar level of oxygen (n = 8 for both gases). The animals were acclimatized to the chamber for 15 minutes, and then the treatment gas was added at a rate of 20% of the chamber volume per minute. Argon treated animals were removed unharmed from the chamber after 2.5 minutes, which is the approximate time that it took for the carbon dioxide treated animals to become fully recumbent. The rats that were exposed to carbon dioxide were more active (3.8 ± 0.65 versus 1.0 ± 0.57 movements ± SEM; p < 0.01), and showed increased levels of escape and gas avoidance related behaviours including rearing (11.6 ± 1.30 versus 4.6 ± 1.25 rears ± SEM; p < 0.01), nose to ceiling contact (28.6 ± 5.38 versus 8.0 ± 3.34 sec ± SEM; p < 0.01), and scratching and pushing at the chamber lid (8.0 ± 4.14 versus 0.1 ± 0.12 events ± SEM; p < 0.01). Although the argon treated animals showed some increase in nose to ceiling contact from baseline (p < 0.05), they did not show a significant increase in any other variables. Furthermore, six of the eight carbon dioxide treated animals produced a broad frequency ultrasonic vocalization, and this behaviour was only recorded in one of the argon treated animals (p < 0.05). These ultrasonic sweeps reached a maximum frequency of 102.6 kHz, and on average had a frequency range of 41.5 ± 8.60 kHz ± SD. The vocalization duration ranged from 5.2 msec to 116.4 msec, with an average length of 36.88 ± 26.38 msec ± SD. These results indicate that carbon dioxide euthanasia results in distress in rats, and that this distress is due to the properties of carbon dioxide rather than oxygen deprivation.