INTRODUCTION 
The phenomenon of environmental sex determination (ESD) in reptiles has been highly 
publicized in recent years. However, the underlying mechanism(s) that control this process are 
still poorly known. Additionally, the distribution of ESD within the Reptilia is poorly known, 
with only 93 of the approximately 6500 species of extant reptiles having been examined for the 
presence of ESD (Janzen and Paukstis, Quart. Rev. Biol.). 
This paper provides a summary of much of the research conducted on ESD in reptiles from 
laboratory studies that have employed constant temperatures of incubation (Table 1). Table 1 is an 
extension of and appendix to a review article on ESD in reptiles (Janzen and Paukstis, Quart. Rev. 
Biol.). As a result of the recent proliferation of publications concerning various aspects of ESD in 
reptiles, this table originated as a tool to provide an overview of laboratory results that were 
currently available. As the data accumulated, we were impressed not by their consistency, but by 
the amount of variability that existed among different studies. Although each of these studies 
individually provided important new data on ESD, when many papers were viewed simultaneously 
it became very difficult to compare results. 
The potential sources of this variability in sex ratios among different studies are many but, in 
general, fall into two broad categories--biological and artifactual. Among biological sources of 
variability are such factors as inter- and intrapopulational genetic differences, nongenetic maternal 
influences, and different regulatory mechanisms that may vary taxonomically within the Reptilia. 
Artifacts (nonbiological and experimental) include differences in experimental design and 
implementation (e.g., how closely temperature was monitored or regulated, randomization of eggs 
across experimental treatments, small sample sizes) and different techniques in sexing hatchlings 
(e.g., presence/absence of oviducts, histological confirmation). 
Another factor that may reduce the value of results from some studies is the manner in which 
the results are presented (e.g., actual sample sizes as opposed to just the sex ratios of hatchlings, 
data on embryonic mortality). An additional problem is inconsistency and lack of definition of 
terminology. For example, what is the actual relationship between the morphology of a given 
gonad and the terminology used to describe it in hatchlings from different studies, when these 
hatchlings have been described as "hermaphrodites", "intersexes", "unsexable", or "unsexed"? 
Even though the answer to this question may be obvious, unless the terminology in each of these 
papers has been adequately defined, it is difficult to ascertain precisely how the gonads of these 
hatchlings may or may not differ. To properly understand ESD in reptiles, it is very important to 
differentiate between these biological and artificial/experimental sources of variation and to strive to 
minimize those sources of variation that may mask actual biological effects. 
In this paper we provide a compilation of much of the published empirical research on ESD in 
reptiles. Specifically, we summarize information on incubation temperatures, sex ratios, and 
sample sizes from laboratory studies that have used constant temperatures of incubation. 
Comments are provided in those instances where they may lend insight into variability of sex ratios 
or to denote information that may be of particular interest. The arrangement of the major taxa used 
in Table 1 follows that of Janzen and Paukstis (Quart. Rev. Biol.). Families are listed 
alphabetically within the major taxa and genera are presented alphabetically within families. 
We thank E. D. Brodie, III, L. E. Brown, J. J. Bull, S. O'Steen, P. A. Verrell, and M. J. 
Wade for support and discussion during the preparation of this manuscript. This work has been 
supported in part by an NIH Pre-Doctoral Training Grant in Genetics and Regulation (GM-07197) 
and by an NSF Doctoral Dissertation Improvement Grant (BSR-8914686) to FJJ. 
