Temperature-dependent sex determination in the frillneck lizard, Chlamydosaurus kingii (Agamidae)

Frillneck lizards (Chlamydosaurus kingii) are large (0.75 m total length) agamid lizards from tropical Australian savannas. We incubated 56 eggs from six clutches of C. kingii at four thermal regimes in the laboratory: constant temperatures of 26, 29 and 32 C, and a diurnally cycling regime (33 +/- 5 C) designed to mimic extreme natural nest temperatures. Incubation temperatures determined hatchling sex; all hatchlings from the hottest and coldest thermal regimes were female, with males produced only at intermediate temperatures. Thermal regimes also determined incubation periods and significantly affected hatchling sizes. The sexes differed in size (mass, snout-vent length, tail length) at hatchling, but the influence of incubation temperature on hatchling morphology was similar in the two sexes. Several attributes of C. kingii suggest that it offers an ideal opportunity to investigate the adaptive significance of temperature-dependent sex determination in reptiles.