At the foot of certain shrubs in the Mojave Desert, fluffy white motes skitter like tumbleweeds across the sand. Some are fruit: harmless, fuzzy orbs dropped by the creosote bush. Others are white wasps — not that kind — of the species Dasymutilla gloriosa, which have painful stingers and luxuriously silky hair, or setae.
The creosote fruit and the female D. gloriosa, also called the thistledown velvet ant — which is a wasp, not an ant — are near perfect doppelgängers. So entomologists long assumed the wasps had evolved their white tufts to camouflage themselves as fallen creosote fruits. The resemblance was so obvious that no one ever questioned it. Until Joe Wilson.
Ten years ago, Dr. Wilson, a biologist at Utah State University, was at work on a dissertation on how deserts influenced velvet ant evolution when he found himself entranced by the white and fluffy thistle-downs — the Bichons Frises of wasps. The species was old, with white ancestors living in North America 5 million years ago. But fossil records suggest the creosote bush evolved in South America, traveling north sometime during the Ice Age and only becoming well-established in the Mojave approximately 100,000 years ago — a blink of an eye in an evolutionary time scale, and hardly enough time for the wasps to evolve mimicry coloration.
“I had to know,” Dr. Wilson said. “Why was this wasp white?”
A decade later, Dr. Wilson’s team thinks they have solved the conundrum. The wasp’s coloration may have emerged as a strategy to stay cool on scalding hot sand, according to a study published Wednesday in Biology Letters. The cooler a female thistledown stays, the more time she can spend on her life’s true purpose: wandering the desert in search of sand wasp burrows to parasitize.
The first clue was the wasp’s color. Dr. Wilson’s lab studies Müllerian mimicry, a defense mechanism whereby different species develop similar traits, with the result that predators learn to avoid the whole group. The thistledown belongs to a mimicry ring of whitish and fuzzy velvet ants, many of which resemble the creosote fruit, at least to the human eye. But humans and animals perceive the world differently, particularly with regard to ultraviolet light. When the researchers measured the spectral reflectance of the wasps and the fruit, the two reflected different amounts of UV light, meaning they would look distinct from one another in the eyes of, say, a bumblebee.
Dr. Wilson suspected that there must have been a strong selective advantage for the wasp’s coloration. After all, a snow-white bug sitting on red-brown sand is asking to be eaten. He wondered if the wasp’s whiteness could be a way of regulating its body temperature in the desert’s scorching heat. Measuring the body temperature of a live, wiggling velvet ant seemed like an insurmountable challenge, so the researchers placed temperature probes in the abdomens of preserved white thistledown velvet ants and their close relatives, Dasymutilla vestita, which is orange. When placed under an incandescent heat lamp, the white wasps remained several degrees cooler, internally and externally, than the orange wasps.
The specimens came from Utah State’s dizzying collection of velvet ants, one of the largest in the world. Dr. Wilson selected only the fluffiest wasps. “When you store them for too long, the hairs can be rubbed off and the wasp looks a little bald,” he said.
As the team examined the velvet ant’s phylogenetic history, they found that the wasps evolved their white fuzz around the same time as deserts formed in the southwest, further supporting their hypothesis. “If you’re white and fluffy, you won’t get cooked in the sun,” Dr. Wilson said. He added that the creosote fruit may have evolved its pale, hairy tendrils for a similar reason.
Donald Manley, a professor emeritus of plant and environmental sciences at Clemson University and an expert on velvet ants, did not dispute Dr. Wilson’s finding, but said that the wasp’s similarity to the creosote extended beyond mere appearance.
“On many occasions I chased down what appeared to be a female D. gloriosa only to discover that it was a creosote fruit tumbling in the wind,” Dr. Manley said. “They move almost exactly like a creosote fruit.”
Dr. Wilson said that the arrival of the creosotes likely reinforced the wasp’s coloration, encouraging the species to grow even whiter and fluffier.
He hopes that his beloved wasps inspire other researchers to re-examine assumptions they have made about animal camouflage that deceives the simple human eye but not necessarily the complex eyes of the animal kingdom.
“Just because something looks like a leaf doesn’t mean it must look like a leaf to everything,” Dr. Wilson said.