Picture of the Week: Saharan Silver Ant

Summer in the Sahara is scorching—sand temperatures can range between 149-158 degrees Fahrenheit. While skittering across the African desert at high noon might sound like a death wish, it’s only natural for the Saharan silver ant (Cataglyphis bombycina). The insect emerges from its nest in the ground to forage midday, capable of withstanding body temperatures up to about 127°F. 

Known as “thermophilic scavengers,” these ants “collect corpses of other insects [mostly flies] that have succumbed to the heat stress,” says Rüdiger Wehner, a professor at the Brain Research Institute at the University of Zürich, Switzerland, who’s been studying heat-tolerant ant species for nearly four decades. “The higher the temperature is, the more food for them is around,” he says.

The silver ant has evolved several adaptations to cope with the extreme heat, including a trait that inspired its name: hairs that glint silver in sunlight, and that densely coat its back and sides. The ant looks like “a ball of mercury rolling on the ground, ‘cause they’re really glittering,” says Wehner. In a study recently appearing in Science, Wehner and colleagues discovered how these hairs help keep the ant cool, and found that they can contribute to a five-degree drop in ant body temperature under windless conditions.

First the team investigated hair shape and configuration. By scrutinizing ant specimens through various imaging techniques, they found that each hair looks like an elongated prism, with two corrugated sides and one flat side. The hairs bend out from their roots at 90-degree angles, leaving an air gap between the ant’s body and the flat side of the follicles along their length.

The researchers then used spectrometers to measure light reflection in ant specimens with their hairs intact, as well as specimens whose hair had been removed. They found that the hirsute ants reflected 67 percent of incoming light in the near-infrared and visible spectrum, while hairless ants scattered only 41 percent of that light.

Subsequent computer modeling showed that the hair’s prismatic structure and arrangement—that is, paralleling but not touching the body—enhance reflection by trapping incoming light and scattering it outward in all directions. That reflection also lends the hairs their metallic sheen. (The researchers still aren’t sure what role the corrugation on the two sides of each hair plays.)

In another part of the study, the team found that the hairs don’t just keep the ants cool by reflecting light—they also allow the insects to offload heat into cooler surrounding air by emitting radiation in the mid-infrared spectrum (in this case, around 6-16 microns) more efficiently than ants sans hair.

“[The radiation] propagates with very little reflection through the hair coating and then into the air,” says Nanfang Yu, an assistant professor of applied physics at Columbia University in New York, and an author on the study. But why don’t the hairs reflect the ant’s thermal radiation back into its skin? Turns out, the wavelength of that radiation is much bigger than the cross section of follicles that it passes through. As a result, in the mid-infrared, the hair actually functions as an anti-reflective coating, says Yu—one that enables the radiation to move from the ant’s body to the air relatively uninterrupted. 

Silver ants make the most of this phenomenon by getting high—that is, moving to elevations on top of stones or vegetation where the air is even cooler, and heat can be transferred more effectively. Once the insect has lowered its body temperature, it scampers back down and forages along the sand some more until climbing back up again, repeating the process over and over, for about 30 minutes on hot days. “They can spend up to 70 percent of their whole nest-out time—that is, when they’re outside the nest—in respiting, in cooling off,” says Wehner. “It’s a thermal tight rope on which these ants are.” (Healthy body temperatures during foraging range from about 118-124°F.)

The silver hairs’ cooling effect works in concert with other adaptations that help the ants beat the heat. For one, their legs are disproportionately long compared to their body, which hovers four millimeters above ground, according to Wehner (the average ant body is raised only half that distance). At “ant height,” the air temperature can be 27°F lower than the sizzling sand, he says.

The ants can also run like the dickens, clocking maximum speeds of 70 centimeters per second (normally, they run 20-30 cm/sec). Their scurrying generates a wind that likely helps cool them down through convection, whereby the hot layer of air surrounding the insect rises because it’s less dense than cooler air. (Wehner notes that this effect hasn’t been systematically investigated in the lab.)

While several other ant species can tolerate extreme heat, Wehner says that the silver ant is potentially unique among its kin for sporting metallic-looking tresses. He and Yu plan to further study the shiny species in the field using infrared cameras to measure the insects’ body temperatures as they go about their day.

The team is also interested in applying what they’ve learned about the silver hairs to developing optical coatings that could be used to passively cool down objects and materials. “Maybe one day you will have a fur coat with silver hairs,” muses Wehner. 

*This article was updated on August 11, 2015, to reflect the following change: An earlier version stated that, in the mid-infrared, the ant's hair “actually functions as a vacuum medium.” The phrase has been updated to indicate that the hairs function as an anti-reflective coating (similar to what you might find on eyeglasses to reduce glare or reflection).