TEMPERATURE-DEPENDENT SEX DETERMINATION IN CROCODILIANS

Half of the 22 extant crocodilians show evidence of temperature-depend ent sex determination (TSD). We examine evidence for TSD in 11 species by reviewing reports on five and presenting new data for six. The fem ale-male pattern (FM; females at low temperature, males at high temper ature) attributed to Alligator mississippiensis and Caiman crocodilus are here revised to be female-male-female (FMF; males at intermediate temperature, females at low and high temperatures). A similar pattern characterizes Crocodylus palustris, C. moreletii, C. siamensis, and Ga vialis gangeticus based on new data; published accounts establish a FM F pattern in Crocodylus porosus, C. johnstoni, and C. niloticus. TSD a pparently occurs in Paleosuchus trigonatus and Alligator sinensis, but patterns are not yet documented. In the well-studied species, the inc ubation temperatures for FM transitions are congruent, but MF transiti on temperatures differ among species. In A. mississippiensis, 100% mal es are produced over a range of constant incubation temperatures, wher eas in C. johnstoni, only low proportions of males are produced at any constant temperature. The thermosensitive period (TSP) for A. mississ ippiensis occurs during stages 21 to 24 (days 30-45 at intermediate te mperatures) and coincides with gonadal differentiation. A similar scen ario is suggested in other species. The TSP in A. mississippiensis (an d possibly other crocodilians) encompasses the third quarter of develo pment and occurs later than in turtles and a lizard. In A, mississippi ensis as in turtles, the duration (cumulative effect) and/or the magni tude (potency effect) of incubation temperatures during the TSP predic tably alter sex ratios. TSP chronologies and features which are shared among TSD reptiles suggest common, underlying mechanisms; A. mississi ppiensis is an appropriate model for further study. In crocodilians, c lutch effects are a significant source of variation in TSD response. H atchling sex ratios previously reported for A. mississippiensis are re considered in light of our new data. (C) 1994 Wiley-Liss, Inc.