Of prime importance in evolutionary biology are the description of pattern and explanations of process. Frequently, however, multiple processes can explain a given pattern. Such cases require experimental protocols or research criteria to distinguish among alternatives so pattern can be critically assigned to process. Noteworthy examples of this approach include evaluating adaptations and identifying character displacement (Gould and Lewontin 1979; Schluter and McPhail 1992). The field of vertebrate sex determination similarly requires such criteria. The sex of organisms is determined by two distinct mechanisms. In genotypic sex determination (GSD), sex is determined at conception by genes usually contained in sex chromosomes. In environmental sex determination (ESD), sex is determined permanently after fertilization by environmental factors (Bull 1983). In ESD (unlike in GSD), there is little if any genetic difference between the sexes (Solari 1994), so sex cannot be predicted by zygotic genotype (Bull 1983). In many ESD vertebrates, sex is determined after fertilization by incubation temperature (TSD). Though ESD’s biological significance seems clear for various taxa (Bull 1983; Conover 1984; Michaud et al. 1999), TSD evolution in vertebrates remains unexplained (Shine 1999). To complicate matters, temperature can influence sex ratios in a multitude of ways other than TSD.