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Egg Size, Temperature and Genes Determine Lizard Hatchlings’ Sex

I remember well my shock when the news concerning temperature dependant sex determination (TDSD) among reptiles first broke.  Imagine…a turtle or crocodile egg can develop as either a male or female, depending upon the incubation temperature!  Well, it now seems that at least one species of lizard, the three-lined skink (Bassiana duperreyi), complicates matters even further.


Egg Size and Sex

Writing in the June, 2009 issue of Current Biology, University of Sydney biologists have revealed that large skink eggs develop into females, while small eggs become males.  Furthermore, removing yolk from a large egg resulted in a male hatchling, despite the fact that female genes were present; adding yolk to a small egg over-rode the effect of the male genes already in the egg, producing instead a female.


Temperature and Sex

Extreme nest temperatures, it seems, can also affect the hatchlings’ sex, regardless of its genetic makeup, resulting in xx chromosome males (males are usually xy) and xy females (females are typically xx).


Survival and Conservation Implications

The question that begs answering, of course, is why has this lizard evolved such a unique and complicated reproductive strategy?  Researchers are now looking into its ecology to determine the answer.

Understanding such matters has greatly assisted those working to conserve reptiles and amphibians.  Knowing the parameters of TDSD has allowed us to pick and choose the sexes of highly endangered species being bred in zoos, so that a favorable sex ratio can be maintained.

Of course, there were some problems early on.  I was working with green sea turtle head-start programs in Costa Rica when TDSD first came to light.  The organization I was with had been gathering sea turtle eggs for 30 years, incubating them, and then releasing the young when they had grown past the vulnerable stage.  Great in theory…but by using a single incubation temperature, we may have produced only female green turtles for the entire 3 decades!

Further Reading

You can read more about the natural history of the three-lined skink and its relatives at http://www.jcvi.org/reptiles/species.php?genus=Bassiana&species=duperreyi.



About Frank Indiviglio

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Being born with a deep interest in animals might seem unfortunate for a native Bronxite , but my family encouraged my interest and the menagerie that sprung from it. Jobs with pet stores and importers had me caring for a fantastic assortment of reptiles and amphibians. After a detour as a lawyer, I was hired as a Bronx Zoo animal keeper and was soon caring for gharials, goliath frogs, king cobras and everything in-between. Research has taken me in pursuit of anacondas, Orinoco crocodiles and other animals in locales ranging from Venezuela’s llanos to Tortuguero’s beaches. Now, after 20+ years with the Bronx Zoo, I am a consultant for several zoos and museums. I have spent time in Japan, and often exchange ideas with zoologists there. I have written books on salamanders, geckos and other “herps”, discussed reptile-keeping on television and presented papers at conferences. A Master’s Degree in biology has led to teaching opportunities. My work puts me in contact with thousands of hobbyists keeping an array of pets. Without fail, I have learned much from them and hope, dear readers, that you will be generous in sharing your thoughts on this blog and web site. For a complete biography of my experience click here.
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