Unlike birds and mammals, in many reptiles the temperature experienced
by a developing embryo deter mines its gonadal sex. To understand how
temperature-dependent sex determination (TSD) evolves, we must first
determine the nature of genetic variation for sex ratio. Here, we anal
yze among-family variation for sex ratio in three TSD species: the Ame
rican alligator (Alligator mississipiensis), the common snapping turtl
e (Chelydra serpentina), and the painted turtle (Chrysemys picta). Sig
nificant family effects and significant temperature effects were detec
ted in all three species. In addition, family-by-temperature interacti
ons were evident in the alligator and the snapping turtle, but not in
the painted turtle. Overall, the among-family variation detected in th
is study indicates potential for sex-ratio evolution in at least three
reptiles with TSD. Consequently, climate change scenarios that are po
sited on the presumption that sex-ratio evolution in TSD reptiles is g
enetically constrained may require reevaluation.