Whether baby lizards will turn out to be male or female depends on the size of the eggs - at least for one lizard species, says a new study.
"We were astonished. Our studies on small alpine lizards have revealed another influence on lizard sex: the size of the egg. Big eggs tend to give girls, and small eggs tend to give boys," said Richard Shine of the University of Sydney.
"And if you remove some of the yolk just after the egg is laid, it's likely to switch to being a boy, even if it has female sex chromosomes; and if you inject a bit of extra yolk, the egg will produce a girl, even if it has male sex chromosomes," he added.
Shine and his colleagues found in hatchlings of the alpine-dwelling Bassiana duperreyi that large lizard eggs were more likely to produce daughters and small eggs to produce sons.
Despite the correlation, Shine said he had assumed that the association was indirect.
In fact, his colleague Rajkumar Radder conducted studies in which he removed some yolk from larger eggs, more likely to produce daughters, to confirm that assumption.
"We were confident that there would be no effect on hatchling sex whatsoever. When those baby boy lizards started hatching out, we were gob-smacked," Shine said.
Shine believes that there will be much more to discover when it comes to lizard sex determination.
"I suspect that the ecology of a species will determine how it makes boys versus girls, and that our yolk-allocation effect is just the tip of a very large iceberg," he said.
The study is published online on June 4th in Current Biology, a Cell Press publication.