Camp. Biochem. Physic& 1973, VoI. 46A, pp. 799 to 826. Pergmwn Press. Printed in Gmzt Britain PANTING THRESHOLDS OF LIZARDS-I. SOME METHODOLOGICAL AND INTERNAL INFLUENCES ON THE PANTING THRESHOLD OF AN AGAMID, AMPHIBOLURUS MURICATUS HAROLD HEATWOLE, BRUCE T. FIRTH and GRAHAME J. W. WEBB Department of Zoology, University of New England, Armidale, N.S.W. 2351, Australia (Received 20 November, 1972) Abstract-l. Amphibolurus muricatus undergoes thermal panting at body temperatures of approximately 40°C. 2. Body size, site of temperature measurement, sex, moulting and rate and method of heating did not significantly influence the panting threshold. 3. Rapid heating resulted in greater variability of panting threshold. 4. Non-radiant heat caused greater variation in panting threshold than radiant heat. 5. Repeated testing with short intervals between tests decreased panting threshold, whereas long intervals had no effect. 6. Panting was weakly developed in &inks and snakes tested. 7. The major source of variation in panting threshold is a day-to-day shift within individuals. INTRODUCTION ALTHOUGH exceptions are known, many species of lizards open the mouth widely when subjected to heat stress (see Table 1). In some, this response occurs only at the point that breathing nearly or completely ceases, heart beat precipitously drops and the animal approaches heat-induced spasms (Veron & Heatwole, 1970; Webb et al., 1972); such gaping can scarcely be of thermoregulatory significance. How- ever, in other species the mouth is open during periods of high ventilation and would seem to be of use in enhancing evaporative cooling (Langlois, 1902; Cole, 1943 ; Dawson & Templeton, 1963 ; Heath, 1965 ; Murrish & Vance, 1968 ; Heatwole et al., 1969; Heatwole, 1970; Webb et al., 1972) and indeed has been demonstrated to function in dissipating the heat produced by thermally imposed high metabolic rates, or even to reduce body and/or head temperature (Templeton, 1960; Dawson & Templeton, 1963; Warburg, 1965a; Dewitt, 1967; Crawford & Kampe, 1970; Case, 1972; Crawford, 1972; Webb et al., 1972); in other species that ventilate during mouth-opening, or even in the same species under other conditions, thermoregulatory function is less pronounced (War-burg, 1965b; Dawson & Templeton, 1966; Crawford & Kampe, 1971). 799