THRESHOLD TEMPERATURES AND THE THERMAL CYCLE OF A HELIOTHERMIC LIZARD

Body temperatures of the Puerto Rican teiid lizard Ameiva exsul were m easured throughout the activity period by transmitters implanted in th e abdominal cavities of eight lizards held in outdoor enclosures. Body temperature at moments of passage from sun to shade and shade to sun and the consequent heating and cooling rates were calculated. Environm ental temperatures were also recorded. Lizards moved from sun to shade at an average body temperature of 39.5 C (CI = 1.42, N = 8), the uppe r threshold temperature. Lizards moved from shade to sun at 32.5 C (CI = 1.25, N = 8), the lower threshold temperature. For individual lizar ds, standard deviation of the upper threshold was smaller than that of the lower suggesting close regulation of the upper threshold. The hea ting rate in the first five to ten minutes following a move to the sun (0.54 C/min [CI = 0.13, N = 8]; was of greater absolute magnitude tha n the cooling rate following a move to the shade (0.30 C/min [CI = 0.0 9, N = 8]). During the average thermal cycle significantly more time w as spent in the shade, where free-ranging lizards forage intensively i n the leaf litter, than in the sun. We suggest that the restricted ele vational distribution of this species, first attributed to thermoregul ation by Heatwole and Torres, may result from inability to maintain bo dy temperature above the lower threshold while foraging in shade.