Behavioral and corticosterone responses to carbon dioxide exposure in reptiles.

The use of carbon dioxide (CO2) exposure as a means of animal euthanasia has received considerable attention in mammals and birds but remains virtually untested in reptiles. We measured the behavioral responses of four squamate reptile species (Homalopsis buccata, Malayopython reticulatus, Python bivitattus, and Varanus salvator) to exposure to 99.5% CO2 for durations of 15, 30, or 90 minutes. We also examined alterations in plasma corticosterone levels of M. reticulatus and V. salvator before and after 15 minutes of CO2 exposure relative to control individuals. The four reptile taxa showed consistent behavioral responses to CO2 exposure characterized by gaping and minor movements. The time taken to lose responsiveness to stimuli and cessation of movements varied between 240-4260 seconds (4-71 minutes), with considerable intra- and inter-specific variation. Duration of CO2 exposure influenced the likelihood of recovery, which also varied among species (e.g., from 0-100% recovery after 30-min exposure). Plasma corticosterone concentrations increased after CO2 exposure in both V. salvator (18%) and M. reticulatus (14%), but only significantly in the former species. Based on our results, CO2 appears to be a mild stressor for reptiles, but the relatively minor responses to CO2 suggest it may not cause considerable distress or pain. However, our results are preliminary, and further testing is required to understand optimal CO2 delivery mechanisms and interspecific responses to CO2 exposure before endorsing this method for reptile euthanasia.