I'm going to say with four known exceptions, snake sex is determined genetically. However, temperature CAN influence the secondary sex ratio (the ratio at hatching).
How is the gender of some reptiles determined by temperature?
Alex Quinn, a Ph.D. candidate at the Institute for Applied Ecology at the University of Canberra in Australia, sorts this quandary out for us.
Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination (GSD) and temperature-dependent sex determination (TSD).
Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types. Many species—such as several species of turtle and lizards, like the green iguana—have X and Y sex chromosomes (again, like mammals), with females being "homogametic," that is, having two identical X chromosomes. Males, on the other hand, are "heterogametic," with one X chromosome and one Y chromosome.
Other reptiles governed by GSD have a system, similar to one found in birds, with Z and W sex chromosomes. In this case—which governs all snake species—males are the homogametic sex (ZZ) and females are the heterogametic sex (ZW).
In temperature-dependent sex determination, however, it is the environmental temperature during a critical period of embryonic development that determines whether an egg develops as male or female. This thermosensitive period occurs after the egg has been laid, so sex determination in these reptiles is at the mercy of the ambient conditions affecting egg clutches in nests. For example, in many turtle species, eggs from cooler nests hatch as all males, and eggs from warmer nests hatch as all females. In crocodilian species—the most studied of which is the American alligator—both low and high temperatures result in females and intermediate temperatures select for males.
A widely held view is that temperature-dependent and genotypic sex determination are mutually exclusive, incompatible mechanisms—in other words, a reptile's sex is never under the influence of both sex chromosomes and environmental temperature. This model indicates that there is no genetic predisposition for the embryo of a temperature-sensitive reptile to develop as either male or female, so the early embryo does not have a "sex" until it enters the thermosensitive period of its development.
This paradigm, though, has been recently challenged, with new evidence now emerging that there may indeed be both sex chromosomes and temperature involved in the sex determination of some reptile species. Apparently, in animals where both occur, certain incubation temperatures can "reverse" the genotypic sex of an embryo. For example, there is an Australian skink lizard that is genotypically governed by X and Y sex chromosomes. A low incubation temperature during the development of this lizard's egg reverses some genotypic females (XX) into "phenotypic" males—so that they have only functioning male reproductive organs. Therefore, in this species, there are both XX and XY males, but females are always XX. A slightly different example of this temperature-induced sex reversal is found in an Australian dragon lizard, which has the ZW system of sex chromosomes. In this species, high incubation temperature during egg development reverses genotypic males (ZZ) into phenotypic females; so females can be ZZ or ZW, but males are always ZZ.
Reptiles in which both incubation temperature and sex chromosomes interact to determine sex may represent "transitional" evolutionary states between two end points: complete GSD and complete TSD. It is quite possible that there are other species of reptiles with more complicated scenarios of temperature reversal of chromosomal sex. There are certainly many known examples of fish and amphibians with GSD, in which both high and low incubation temperatures can cause sex reversal. In these cases, all genotypes (from ZZ and ZW to XX and XY) are susceptible to reversal by extremes of incubation temperature.
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EFFECTS OF INCUBATION TEMPERATURE ON SEX RATIOS IN
PINE SNAKES: DIFFERENTIAL VULNERABILITY
OF MALES AND FEMALES
JOANNA BURGER AND R. T. ZAPPALORTI[/B]
(I don't know if you can view this if you are not in an academic setting)
In this discussion, we use primary sex ratio as the ratio at fertilization secondary sex ratio as that at birth( or hatching), and tertiary sex ratio as that at sexual maturity
Most reports indicate that gender in snakes is determined genetically (Voront- sov 1973), with a resultant 50:50 primary and secondary sex ratio (Shine and Bull 1977).
Known exceptions are four male-biased secondary sex ratios (
Agkistrodon contortrix, Fitch 1960a;
Elaphe quadrivirgata, Fukada 1960;
Notechis scutatus, Shine and Bull 1977;
Pituophis melanoleucus, Gutzke et al. 1985),
and one female- biased secondary sex ratio (E. climacophora, Fukada 1956). Gutzke et al. (1985) found a higher proportion of males in both hatchlings and adults over a 2-yr period. Because some mortality resulted from the range of laboratory incubation temperatures, they excluded clutches with any mortality from their sex-ratio analysis. By excluding clutches suffering mortality, they may have missed differ- ential pre-hatching mortality. Furthermore, the biases in secondary sex ratios reported above may be a result of pre-hatching mortality, rather than biased primary sex ratios. Biased adult sex ratios occurred in 19 colubrid and viperid species; males were favored in populations of 9 species; females in 9; and in one species, for which data on two populations are available, a different sex was favored in each population (Parker and Plummer 1987).
Sex Ratios of Experimental Clutches
For hatchlings incubated under cycling or constant controlled temperatures, secondary sex ratios were related to incubation temperature. At low tempera- tures, secondary sex ratios were biased toward females; at high temperatures they were at unity or biased toward males.
However, the primary sex ratio (embryos that hatched and those that failed to hatch) did not differ significantly for different temperatures. Thus, incubation temperature does not determine sex in pine snakes.
At low temperatures, some males did not hatch; and at high temperatures, some females did not. When embryos were dissected, they contained partially or fully formed snakes with complete color patterns. There are developmental expla- nations for the failure of fully formed snakes to cut through the shell, but not for sexual differences in mortality as a function of incubation. Many reptilian em- bryos die if cooled at the beginning of incubation, or they develop cephalic or other abnormalities (Hubert 1985).
