Peripheral auditory processing in the bobtail lizard Tiliqua rugosa - V. Seasonal effects of anaesthesia

Several parameters of the peripheral auditory physiology of the bobtail lizard have been examined with respect to the time of the year. These include gross-potential and neural thresholds across the entire hearing range as well as a standard measure of gross-potential size and an estimate of the success in recording primary auditory afferents. 1. The results do not indicate any pronounced changes in hearing ability across the year. This contrasts with earlier reports by Johnstone and Johnstone (1969 a, b) and Holmes and Johnstone (1984 a, b) who found gross potential size, gross potential thresholds and recordability of auditory-nerve fibres to change markedly with the seasons. 2. An additional examination of the anaesthetic regimes used in the present report, however, revealed seasonal changes in the animals' anaesthetic requirements. The time course of this phenomenon closely matches the previously reported time course of changes in hearing over the year. 4. It is therefore suggested that the previous results represent an artefactual variation in hearing ability due to the use of standard anaesthetic doses causing a variable physiological state of the experimental animals. Evidence supporting the idea of a gradual impairment of hearing by a varying degree of anaesthetic overdose is discussed.