You’re standing in the Tularosa Basin of New Mexico during the last ice age—let’s say about 30,000 years ago. A huge lake shimmers in the background. Herds of mammoth and camels squelch through the mud leaving long lines of tracks. Packs of saber-toothed cats and dire wolves follow, waiting for the opportunity to hunt.
The lake is not unusual for this time period, nor this geologic setting. The Tularosa Basin is completely enclosed, bound by mountains to the east, west, and south; and Chupadera Mesa to the north. Water that drains into the basin has nowhere to go, so—depending on the climate—it either accumulates or it evaporates.
During the last ice age, water accumulated. Temperatures were cooler, evaporation wasn’t as rapid, and there was simply more water. At least eight lakes dotted the basins of the northern Chihuahuan Desert in Arizona, Texas, and New Mexico.
But the giant lake of the Tularosa Basin (now known as Lake Otero) is different. Although the water sparkles and shimmers, it’s not “pure.” The mountains surrounding the basin are partially composed of gypsum deposits hundreds of feet thick. As rain or snowmelt flows across the surface, the water slowly but surely dissolves the minerals of the rocks, and carries them away in solution to eventually accumulate in the lake.
Now, jump forward about 20,000 years. The climate is changing. Temperatures are rising and there is less precipitation. The mammoths and camels no longer roam the lake shores. The dire wolves and saber tooth cats have disappeared. The lake is drying up.
As the free water evaporates, millions of tons of dissolved minerals become concentrated in less and less water. Knife-like crystals of selenite begin to grow in the supersaturated muds of the lake bed. Eventually, the water of the lake is gone and the crystals lie exposed. Battered by the wind, cracked during freeze/thaw cycles, broken as animals walk across them, tiny chips and flakes of gypsum from the crystals are ground down into pure white, sand.
The wind picks up the sand grains and bounces them across the landscape. They begin to accumulate around rocks, bushes, and anything stable enough to stop their movement. Sand dunes begin to form.
Take another giant step through time. Pay your admission and enter White Sands National Monument, an amazing system of gypsum sand dunes. These gypsum dunes are quite rare. They require a set of specific conditions to form: a source of gypsum, arid conditions (so the gypsum won’t just dissolve away), and the wind to pile everything up. The three largest gypsum dune systems in the world are in the Chihuahuan Desert (White Sands, Guadalupe Mountains National Park, and Cuatro Cienegas, Mexico).
Although the dunes seem barren, they’re actually teaming with life. But you need to be on your toes (or more likely, your knees) to see much. Many of the animals spend their days dug into cool burrows in the sand or tucked into the shade of a bush.
But the main reason that you may not see anything is because the animals have changed color to blend with their environment. Instead of the normal browns and tans of the desert, the animals of white sands are bleached blondes.
This color adaptation is exciting to researchers. After all, the dunes are only about 7,000 years old, so to find white versions of common animals shows speciation moving along at a fairly rapid clip.
The Bleached Earless Lizard (Holbrookia maculata ruthveni) shows the greatest adaptation to its environment of all the lizards at White Sands. On the dark, volcanic rock surrounding White Sands National Monument, the earless lizard is a dark brown. The populations at White Sands are nearly pure white.
This adaptation was probably originally a matter of natural selection. A dark animal on white sand is an easy target for a predator. Since the Bleached Earless Lizard is active during the day and prefers open habitats, the darker lizards would have been removed from the environment, leaving lighter and lighter animals to breed.
Today, predators are probably still picking off darker lizards, but the lizards themselves have changed their way of looking for a suitable mate. Field and experimental studies have shown that White Sands males will display preferentially to the lighter White Sands females when given a choice, thus continuing the selection for lighter animals. Although the Bleached Earless Lizard is considered a subspecies of the general population of Holbrookia maculata, these behavioral changes may indicate that a new species is in the making.
While other reptiles and animals such as pocket mice and crickets all show adaptation to the white sand, perhaps the most astounding group (evolutionarily speaking) are the moths.
Not much was known about the moths of gypsum dunes until 2006 when lepidopterist Eric Metzler was invited to conduct a long-term study at White Sands National Monument. He established a three km-long transect that cut across the four habitat types within the dune field: open dunes with no vegetation, interdunal spaces, the edge of the dunes, and open habitat outside the dune field.
Using light traps, Metzler and his colleagues collected moths at 11 sample sites along the transect. What they found was astounding. Among the thousands of moths collected were 24 undescribed species in 7 families. As you would expect, many of the new moth species are white or very pale in color.
As Metzler’s study shows, there is still much to be learned in the gypsum dune fields of the Chihuahuan Desert. So the next time you visit White Sands National Monument or venture out into the gypsum dunes at Guadalupe Mountains National Park, take a moment to wander. Watch for movement and holes in the ground. Imagine the life under your feet and admire the resourcefulness of the plants and animals that live here. Perhaps you, too, will discover a new species.