Silver hairs protect desert ants from scorching heat

A scanning electron microsocope (SEM) view of the Saharan silver ant’s head densely covered in hairs. (Credit: Norman Nan Shi and Nanfang Yu/Columbia Engineering)

The Saharan silver ant forages for food in one of the hottest terrestrial environments on Earth. Covered in tiny silver hairs, the ant looks like a ball of mercury skittering across the desert sand.

Scientists have discovered two key strategies that enable Saharan silver ants (Cataglyphis bombycina) to survive in blistering temperatures of up to 158 degrees Fahrenheit.

Saharan silver ants dismantle a beetle
The ants search for corpses of insects and other arthropods that have succumbed to the thermally harsh desert conditions. (Credit: Bjørn Christian Tørrissen/Wikimedia Commons)

In a study published online in the journal Science, the researchers demonstrate how the ant’s uniquely shaped silver hairs work across an extremely broad range of the electromagnetic spectrum to reflect sunlight and shed heat.

The findings could lead to biologically inspired coatings for passive radiative cooling of objects such as buildings, vehicles, or even clothing.

2 ways the hairs keep ants cool

The silver hairs reflect visible and near-infrared light, including sunlight that pounds the Saharan desert. They also help emit body heat in the mid-infrared spectral band.

“This silvery ant is the champion of the desert, the species that does the best job of surviving in these difficult conditions,” says Gary Bernard, a University of Washington affiliate professor of electrical engineering who has spent decades studying insects’ use of the electromagnetic spectrum. “We thought the silver hairs help control heat and wanted to understand in detail how they worked.”

“The silver hairs reflect sunlight at the shorter wavelengths while in the mid-infrared range they help the ant’s body exchange its heat to a cooler area, the sky,” adds Bernard, a coauthor of the study.

Other bugs bake in the sun

Bernard, whose expertise lies in insect vision, has enjoyed a decades-long collaboration with coauthor Rüdiger Wehner, director of the University of Zürich’s Brain Research Institute, who first marveled at the heartiness of desert ants on his honeymoon in Tunisia 40 years ago.

During many years of working with desert ants, Wehner discovered that Saharan silver ants forage in the desert in the full midday sun when surface temperatures reach up to 70 C (158 F), and must keep their body temperature below their critical thermal maximum of 53.6 C (128.48 F) most of the time.

In their wide-ranging journeys, the ants search for corpses of insects and other arthropods that have succumbed to the thermally harsh desert conditions, which they are able to endure more successfully. Being most active during the hottest moment of the day also allows these ants to avoid predatory desert lizards.

To measure the ant’s capacity to control heat, the insect researchers needed someone with expertise in the infrared band of the electromagnetic spectrum. They asked Nanfang Yu, an assistant professor of applied physics at Columbia Engineering, to collaborate.

His team found that the answer to how the ants’ conspicuous silvery coat kept them keep cool was much broader once they conducted experimental and imaging work to investigate the important role of mid-infrared light.

“While there have been many studies of the physical optics of living systems in the ultraviolet and visible range of the spectrum, our understanding of the role of infrared light in their lives is much less advanced. Our study shows that light invisible to the human eye does not necessarily mean that it does not play a crucial role for living organisms,” says Yu.

The hairs up close

Using electron microscopy and ion beam milling, lead author Norman Nan Shi and the Columbia team discovered that the ants are covered on the top and sides of their bodies with a coating of uniquely shaped hairs with triangular cross-sections that keep them cool in two ways. These hairs are highly reflective under the visible and near-infrared light—in the region of maximal solar radiation.

The hairs are also highly emissive in the mid-infrared portion of the electromagnetic spectrum, where they serve as an antireflection layer that enhances the ants’ ability to offload excess heat via thermal radiation, which is emitted from the hot bodies of the ants to the cold sky.

SEM image of ant hairs
The hairs grow parallel to the skin and are separated from the skin by a small air gap. The hairs have triangular cross-sections with two corrugated top facets and a flat bottom facet facing the ant’s body. (Credit: Norman Nan Shi and Nanfang Yu/Columbia Engineering)

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“The fact that these silver ants can manipulate electromagnetic waves over such a broad range of spectrum shows us just how complex the function of these seemingly simple biological organs of an insect can be,” says Shi, a doctoral student who works with Yu at Columbia Engineering.

The researchers found the enhanced reflectivity and radiative efficiency have comparable contributions to reducing the body temperature of silver ants by 5 to 10 degrees, compared to if the ants were without the hair cover.

The Columbia team is currently working on adapting the engineering lessons learned from the study of Saharan silver ants to create flat optical components, or “metasurfaces,” that consist of a planar array of nanophotonic elements and provide designer optical and thermal radiative properties. They also plan to extend the research to other animals and organisms living in extreme environments, trying to learn the strategies these creatures have developed to cope with harsh environmental conditions.

Other co-authors are Cheng-Chia Tsai of the Department of Applied Physics and Applied Mathematics at Columbia University and Fernando Camino of Brookhaven National Laboratory.

The National Science Foundation and the Air Force Office of Scientific Research funded the work.

Source: University of Washington