Among reptiles that show temperature-dependent sex determination, sex ratios vary across constant incubation temperatures in ways sufficiently predictable to allow classification into patterns. One common pattern shows low temperatures yielding only males and high temperatures yielding only females. Another common pattern has low as well as high temperatures yielding only or mostly females and some intermediate temperatures yielding mostly males. Patterns tend to be associated with the direction of sexual dimorphism in adult size, especially for species with strong dimorphism.

Pivotal temperatures (those yielding 1:1 sex ratios) within the best-documented species and genera tend to increase with both latitude and longitude across central and southern North America. These geographic trends probably reflect factors that affect nest temperatures (duration of growing season, insolation, and prevailing amounts of shading by vegetation).

Data from a population of the alligator snapping turtle (Macroclemys temminckii) suggest that some embryos are temperature-independent females because these individuals become females even when they are shifted among male-producing temperatures during development. These individuals are also more frequent in clutches of small eggs. In this and several other species, no constant incubation temperatures yield more than 75% males. © 1994 Wiley-Liss, Inc.