But a lizard mom doesn't hormonally supercharge her eggs in all cases; her decision depends on the environment and color of neighboring lizards' throats, according to a new study published in the journal Ecology Letters.
Side-blotched lizards live mainly in dry areas in western North America, including Texas, Washington State and California. They have orange, yellow or blue throats and their backs are marked with stripes that are either horizontal (called "bars" by ecologists) or vertical. In a male, throat color is related to behavior: orange-throated male lizards are aggressive and spend most of their time in the open battling each other, according to Lesley Lancaster, lead author of the study. Their yellow-throated compeers generally hide in tall grass and furtively steal the orange-throats' mates while they are off battling or searching for food. Blue throats, meanwhile, are mostly low-key, quietly keeping an eye out for the sneaky yellow-throated males.
"The social environment will determine how visible you are to predators," says Lancaster, an ecology graduate student at the University of California, Santa Cruz, whose finding is the first known instance in which a mother's hormones were found to change a fundamental aspect of her offspring's appearance.
Because orange- and yellow-throated males hang around in different locations with either rocks or grass, the markings on their backs can help them better avoid becoming dinner by camouflaging them. Orange-throated lizards, for instance, are better off with vertical stripes on their backs that confuse predators as the lizards run away in plain sight, whereas bars help yellow-throated lizards blend into their grassy hideouts.
A lizard mother who puts more estradiol in the yolks of her eggs makes her yellow progeny barred and her orange progeny striped. If the female lives among yellow-throated males, giving her eggs an extra dose gives her yellow progeny of both sexes vibrant bars on their backs. If she is in an area populated with more orange-throated lizards, an estradiol boost gives her orange-throated baby boys a sharp set of stripes. The difference in the hormone's effect for male and female progeny is related to the interaction of the estradiol and the genes determining the sex and throat color of the lizards.
Scientists are not sure how the mothers know when to give extra estradiol, but they note that side-blotched lizards have excellent color vision—better than humans—so it is easy for them to tell the color of their lizard neighbors' throats. Researchers also do not know whether moms' estrogen levels are high in certain circumstances and if some leaches into her eggs or if the environment automatically triggers her to deliberately swath her eggs with the hormone.
"It could be a passive or an active mechanism," Lancaster says.