Reptiles do all sorts of crazy things when it comes to sex determination. Some use good old fashioned sex chromosomes like mammals do, but in other species the temperature that the egg is incubated at determines the sex of the offspring. For instance in the tuatara, eggs that are incubated at 23 degrees are uniformly male, and eggs incubated at 18 degrees always turn out female. Some reptiles seem to use a combination of both – they have sex chromosomes, but these are overridden when the eggs are incubated at extremely high or low temperatures. Now it seems it could be even more complicated for some species, as a recent study out of Rick Shine’s lab at the University of Sydney has found that egg size is also a determining factor. In their study of the Eastern three-lined skink (Bassiana duperreyi), they found that sex chromosomes, temperature and egg size interact to determine the sex of the offspring, with larger eggs incubated at low temperatures producing females, regardless of their sex chromosomes.
Ed Yong from Not Exactly Rocket Science has a nice write-up about this paper – go check it out!
The paper citation is: Radder, Pike, Quinn and Shine (2009) Offspring Sex in a Lizard Depends on Egg Size. Current Biology in press doi:10.1016/j.cub.2009.05.027