Two modes of sex determination occur in squamates: genotypic sex determination (GSD) and temperature-dependent sex determination (TSD). Within each of these two major modes, there are many different variants, or mechanisms. Male heterogamety, female heterogamety, multiple sex chromosome systems, and homomorphic sex chromosome systems are all types of GSD found in squamates. Two patterns of TSD have been reported.Only three snakes have been investigated for their sex-determining mechanisms, each having GSD, although incubation temperature does cause differential mortality and affects post-hatching physiolom. Less than 50 lizard species have been investigated, but there is considerable diversity in the sex-determining mechanisms reported thus far. Apparently, TSD (and/or GSD) has evolved multiple times within a given taxon. Presently, both GSD and TSD are found in the Agamidae, Eublepharidae, and Gekkonidae, and possibly in the Iguanidae, Lacertidae, and Varanidae as well. Only GSD has been reported for the Scincidae and Teiidae.Correlations within the Eublepharidae suggest an adaptive explanation for the evolution of sexdetermining mechanisms; a shift from male-larger dimorphism to female-larger (or no sexual size dimorphism) is accompanied by a shift in sex-determining mode. These shifts are in agreement with similar correlations observed in turtles. b
Hatchling Sceloporus undulatus elongatus from Washington Co., Utah and S. u. garmani from Woods Co., Oklahoma were raised to maturity and reproduction under identical laboratory conditions with ad libitum food availability. Growth, allometry, age and size of maturity, clutch size and egg mass were compared among lab-raised cohorts from the two populations, among lab-raised and field-caught animals (including their field-caught mothers) and, for growth, with values obtained by previously published field studies on the same or nearby populations. For all traits population differences observed in previous field studies and current field samples resulted from both a plastic response to proximate environmental conditions and intrinsic (possibly genetic) difference. The most plastic traits were growth and age of maturity. Cohorts from both populations expressed the ability to mature in less than 6 months in the laboratory but only the S.u. garmani express early maturity in the field. Allometric differences generated during growth in the lab were not observed in field samples but may reflect an adaptive physiological difference. The least plastic trait was egg mass. The only trait for which the rank order of the difference in the field was reversed in the lab was growth rate. S.u. elongatus grew significantly faster than S.u. garmani in the lab but much slower in the field. The tendency of S.u. garmani females to breed at minimum size of maturity may be greater than that of S.u. elongatus.
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