At the foot of certain bushes in the Mojave Desert, fluffy white specks glide like weeds in the sand. Some are fruits: harmless and fuzzy orbs dropped by the creosote bush. Others are non-white wasps of the Dasymutilla gloriosa species, which have painful stingers and luxurious, silky hair.
The fruit of creosote and the female D. gloriosa, Also called velvet thistle ant, which is a wasp, not an ant, they are almost perfect doppelgängers. So entomologists long assumed that wasps had developed their white locks to camouflage themselves as fallen creosote fruits. The resemblance was so obvious that no one questioned it. Until Joe Wilson.
Ten years ago, Dr. Wilson, a biologist at Utah State University, was working on a dissertation on how deserts influenced the evolution of velvety ants when he was fascinated by fluffy white thistles – the wasp Bichons Frises . The species was ancient, with white ancestors living in North America 5 million years ago. But fossil records suggest that the creosote bush evolved in South America, traveling north sometime during the Ice Age and only established in the Mojave about 100,000 years ago, the blink of an eye on a scale of evolutionary time, and just long enough for wasps to evolve mimetic coloration.
“I had to know,” said Dr. Wilson. “Why was this wasp white?”
A decade later, Dr. Wilson’s team believes they have solved the puzzle. Staining the wasp may have emerged as a strategy to stay cool in the boiling hot sand, according to a study published Wednesday in Biology Letters. The colder a female thistle becomes, the more time they can spend on the true purpose of their life: wandering the desert in search of sand wasp burrows to parasitize.
The first clue was the color of the wasp. Dr. Wilson’s laboratory studies Müllerian mimicry, a defense mechanism by which different species develop similar traits, with the result that predators learn to avoid the entire group. Thistle belongs to a fuzzy, off-white velvet ant ring, many of which resemble creosote fruit, at least to the human eye. But humans and animals perceive the world differently, particularly with respect 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 different from each other in the eyes of, for example, a bumblebee.
Dr. Wilson suspects that there must have been a strong selective advantage to wasp coloration. After all, a snow-white insect sitting on the red-brown sand is asking to be eaten. He wondered if the wasp’s whiteness could be a way to regulate his body temperature in the scorching heat of the desert. Measuring the body temperature of a rippling, live velvet ant seemed like an insurmountable challenge, so the researchers placed temperature probes in the abdomen of the preserved white thistle velvet ants and their close relatives, Dasymutilla vestita, which is orange. When placed under an incandescent heat lamp, white wasps remained several degrees cooler, internally and externally, than orange wasps.
The specimens came from Utah’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 rub and the wasp looks a little bald,” he said.
As the team examined the phylogenetic history of the velvety ant, they discovered that the wasps developed their white fuzz at the same time that deserts formed in the southwest, further supporting their hypothesis. “If you’re white and fluffy, they won’t cook you in the sun,” said Dr. Wilson. He added that the creosote fruit may have developed its pale, furry tendrils for a similar reason.
Donald Manley, an emeritus professor of plant and environmental science at Clemson University and an expert on velvet ants, did not question Dr. Wilson’s finding, but said the wasp’s similarity to creosote extended beyond mere appearance. .
“On many occasions I chased what appeared to be a D. gloriosa female only to discover that it was a creosote fruit that was flapping in the wind,” said Dr. Manley. “They move almost exactly like a creosote fruit.”
Dr. Wilson said the arrival of creosotes likely reinforced the wasp’s coloration, encouraging the species to grow even whiter and fluffier.
He hopes that his beloved wasps will inspire other researchers to reexamine the 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 should look like a leaf for everything,” said Dr. Wilson.
