Summer 2017

True Colors

By Cameron Walker
Photograph by © RUSHTON IMAGES PHOTOGRAPHY

What can the rapidly evolving white lizards of White Sands National Monument tell us about how animals can survive environmental change?

Ten thousand years ago, as the last Ice Age was ending and temperatures rose, an enormous glacial lake in New Mexico’s Tularosa Basin dried up, revealing something unusual: bright-white gypsum sand. As the years passed, wind began to sculpt the sand into the largest gypsum dunefield in the world. Then about 5,000 years ago — around the time that pharaohs began to rule Egypt — the lizards arrived.

At first these early reptilian colonizers of what is now White Sands National Monument had dusky scales to match the dark soils of the surrounding desert. But now, the three species of lizards that call the monument home are a different color: white.

How were these bleached lizards able to evolve so quickly? What clues can they provide about how life elsewhere on the planet might respond to environmental change?

These are some of the questions that drew evolutionary biologist Erica Bree Rosenblum to this striking environment 15 years ago. She believes the lizards can help researchers better understand what evolutionary success looks like, and why some species are thriving in the face of climate change, habitat loss and other ecological shifts around the globe.

“Environmental change has always happened, but it is happening so fast right now,” Rosenblum said. “Some species are going to adapt to environmental change, but many are not.”

The species Rosenblum and her colleagues have been focused on are the eastern fence lizard, the lesser earless lizard and the little striped whiptail. The milky scales on the lizards’ backs help them blend into the pale sand, protecting them from predatory birds and larger reptiles. But each species has surprising bursts of color, too, from the bright blue of the fence lizard’s belly to the pink and orange patches on the earless lizard’s throat. These colors flash as they perform pushups during their mating dances. Rosenblum thinks of the reptiles as tiny, charismatic dinosaurs.

SWIMMING IN THE DESERT

The endemic White Sands pupfish, a threatened species that lives in the monument’s Lost River, is the only fish in the Tularosa Basin and has lived there for more than 12,000 years. (The nearby city of Alamagordo, New Mexico, is home to a professional baseball team that is named after the fish.)

The creatures aren’t just charming. “Lizards are really important to the ecosystem, because they’re right smack in the middle of the food chain,” she said. As biodiversity disappears, the loss of lizards can have an especially detrimental effect, breaking the link between top predators and smaller prey.

To study the monument’s lizards, researchers catch them by trolling the sands with fishing poles and hooking lizards with a loop on the end of the line. (It’s harder than it sounds. Silvia Lopez Segura, who spent a summer as an undergraduate field assistant, said on a good day, they might catch eight. “These little guys are super intelligent,” she said. Lizards would dart off under vegetation or bite at the loop to escape. “Sometimes it would take three of us to get them.”) Once they’ve caught a lizard, researchers measure everything from its length to the size of its jaws. They’ve also grouped the lizards in enclosures to observe how they respond to predators and to each other, watching the lizards in the field and on hours of video recordings to understand their behavior.

Over the years, Rosenblum and her colleagues have found that the pale lizards have evolved in multiple ways. “You change one thing in an ecosystem, and you’re going to have a cascade of other changes,” she said.

The White Sands lizards have larger heads and bigger, stronger jaws than their darker counterparts, which allows them to munch on heftier, tougher bugs. These lizards also have longer limbs that help them run smoothly on the gypsum sand. (Unlike beach sand, the gypsum crystals stay cool under the hot Southwestern sun, so even visitors can run barefoot.) Lizards here act differently, too: They’re less wary and skittish than lizards in the nearby desert, likely because fewer predators lurk in the gypsum dunes.

What’s most interesting to Rosenblum is the genetics behind the lizards’ eye-catching color change. Using scale samples from the lizards’ tails, she and her colleagues have identified the specific gene that has driven the shift from dark scales to light. This gene — called melanocortin-1 receptor gene — is the same one that shapes human pigmentation and influences color variation in some mice and bird species. Having one gene that controls color change may have allowed these lizards to adapt more quickly; a species that needs many coordinated genes to change its coloring might take longer to develop camouflage that helps its members survive in a new environment.

The fact that the same gene is responsible for the color change in all three lizard species suggests that creatures confronted with environmental challenges are likely to adapt in similar ways. This finding could help others — researchers might be able to predict how multiple species in an ecosystem respond to change based on a few successful adapters. In addition, advances in genetic sequencing will allow researchers to identify the genes that trigger other changes in the bleached lizards, such as their length and head size, and they can look for these same genes in other species.

Luckily, the White Sands ecosystem supports many other potential candidates for study. “Most people don’t realize there’s anything else here besides sand,” said David Bustos, the chief of resource management for the national monument. “There’s a lot of really incredible life if you look a little closer.”

Rosenblum is turning next to the sand treader cricket and the jumping spider. Both species are lighter in color than their brethren outside White Sands. Looking at a wider range of critters in the same environment can help the researchers solidify their ideas about what makes animals successful there — and someday, apply those ideas elsewhere. As many as half of the world’s species may face extinction by the end of the century, and that is the sort of statistic that weighs on Rosenblum.

“Are there ways to increase the probability that more species are going to make it?” she asked. “That’s more and more a central question about biodiversity on our planet.”

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