Off the gypsum, in the brown scrub of the Tularosa Basin, the fence lizard is brown and gray with a bright blue belly, the little striped whiptail is dark with pale stripes, and the lesser earless lizard is speckled tan. On the dunes of White Sands, all three are white or nearly so.
A young desert
The dunes are young. They formed within the last ten thousand years, and by most estimates within the last six thousand, as wind piled up gypsum from the dry lakebed to the west. Lizards moved onto them from the surrounding desert, and on bright white sand the paler ones were harder for birds and other predators to see. Over a few thousand years, each species produced a white dune population.
One gene, three ways
In 2010 the biologist Erica Bree Rosenblum and her colleagues reported what lay behind the color. In each species the white lizards carry a different single change in the same gene, Mc1r, which helps the body make dark pigment. In the fence lizard and the whiptail the change partly cripples the gene's receptor, but by different routes: in the fence lizard less of the receptor reaches the surface of the cell, while in the whiptail it reaches the surface and then signals poorly. In the earless lizard the change showed no measurable effect in the laboratory, so its whiteness may come about some other way. Even the inheritance differs; the fence lizard's white version is dominant, so a single copy is enough. The fence lizard and the whiptail last shared an ancestor something like 175 million years ago, and on these dunes they arrived at the same answer separately.
Bolder lizards
The dune lizards differ in more than color. They are less wary than lizards in the surrounding desert, probably because fewer predators hunt the open gypsum, and researchers have found differences in their body size and head shape as well. Other dune animals here have pale forms too.
Finding them
The lizards are most active on warm mornings from spring into fall. The best places to look are the edges of the dunes where plants take hold, and along the boardwalk early in the day at White Sands National Park, before the heat sends them under cover.