Hair Helps Cool the Body

close up photograph of reddish-brown ants crawling on sand

Biological Strategy

Saharan silver ant

Alexandra Ralevski

Image: Pavel Krasensky / Shutterstock / Some rights reserved

Sense Light (Non-visible Spectrum) From the Environment

Living systems interact with each other and with their environments to gain information. Sometimes that information is in the electromagnetic spectrum. Wavelengths in the electromagnetic spectrum are also called the non-visible spectrum, because humans can’t detect them with the naked eye. These include ultraviolet (UV) light, infrared (IR) light, radio waves, and other wavelengths. Detecting within these spectra requires strategies beyond those used for visible light, so many living systems that depend on these signals have specialized organs to do so. For example, beetles that feed on burned trees have sensory organs that detect infrared radiation emitted by fires, enabling them to quickly locate a burned area.

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Sense Light (Visible Spectrum) From the Environment

Living systems constantly receive signals from their environment that help them survive. Light (in the visible spectrum) can come from other living systems (such as fireflies) or from non-living sources (such as the sun). Survival often depends on sensing and responding to challenges like low light conditions or light that has been altered in some way. Because basic survival is at stake, living systems must excel at meeting those challenges. A well-known phenomenon is how water bends light. A stork trying to catch a fish underwater can compensate for this bending effect so that when it strikes at the fish, it has a good chance of catching it.

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Transform Thermal Energy

Heat is necessary for survival, but too much of it can be damaging to an organism. Explore the ways different living things have adapted to use or transform different types of thermal energy.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Insects

Class Insecta (“an insect”): Flies, ants, beetles, cockroaches, fleas, dragonflies

Insects are the most abundant arthropods—they make up 90% of the animals in the phylum. They’re found everywhere on earth except the deep ocean, and scientists estimate there are millions of insects not yet described. Most live on land, but many live in freshwater or saltwater marshes for part of their life cycles. Insects have three distinct body sections: a head, which has specialized mouthparts, a thorax, which has jointed legs, and an abdomen. They have well-developed nervous and sensory systems, and are the only invertebrate that can fly, thanks to their lightweight exoskeletons and small size.

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The hair of the Saharan silver ant keeps it cool by efficiently reflecting light and heat energy while also dissipating excess heat.

Saharan silver ants (Cataglyphis bombycina) have a remarkable ability to thrive in one of the hottest regions on the planet—the Sahara Desert. Unlike the sleek skin, shells and scales of most high-temperature dwellers, these ants have a unique exterior defense mechanism to beat the heat: hair.

The hair of Saharan silver ants reduces heat absorption by efficiently reflecting sunlight and dissipating heat. This enables the ant to stay cool in the midday Sahara, where temperatures can reach up to 47°C (117°F). The hairs, with their unique shape, achieve this through three distinct mechanisms:

First, the hairs reduce heat absorption by maximizing the amount of light that is reflected off their surface, a process known as total internal reflection (TIR). This is achieved through the unique prism shape of the hairs, which have a flat base that lies against the body of the ant, while the other two sides are grooved (see gallery images). When light enters through one of the grooved sides, it reflects off of the base, and then exits from the other grooved side. A groove concentrates a ray of light onto a certain part of the hair in order to optimize its path of exit and thus maximize TIR. Multiple grooves enable multiple rays of light to be reflected within the same hair simultaneously. The hairs are densely packed to ensure that a minimal amount of sunlight reaches the ant’s body underneath, keeping it 2°C cooler than it would be without the hairs.

Second, the hairs also reduce heat absorption by reflecting sunlight (solar radiation) from both visible and non-visible parts of the electromagnetic spectrum, including the near-infrared (NIR). It is in these two regions that solar radiation is at its most powerful.

And third, the silver ant has a particularly effective method of offloading excess heat. The shape of the hairs increases the ant’s ability to radiate heat—known as emissivity—in a specific range of the electromagnetic spectrum, the mid-infrared (MIR). Within this range, the ant’s warm body can most effectively give off excess heat energy to cooler surrounding air via thermal radiation. Ant researcher Nanfang Yu explains: “To appreciate the effect of thermal radiation, think of the chilly feeling when you get out of bed in the morning. Half of the energy loss at that moment is due to thermal radiation, since your skin temperature is temporarily much higher than that of the surrounding environment.” This mechanism enables the ant to maximize the amount of heat it emits and cool itself off. The last two mechanisms cool the ant off by an additional 5-10°C, enabling the silver ant to withstand temperatures up to 53.6°C (128.48°F).

Image: Bjørn Christian Tørrissen / CC BY SA - Creative Commons Attribution + ShareAlike

Electron microscopy images of the hairs of the Silver ant

Electron microscopy close-up of the hairs of the silver ant.

Last Updated March 25, 2019

References

“The [ant] hairs have a triangular cross-section with two corrugated surfaces allowing a high optical reflection in the visible and near-infrared (NIR) range of the spectrum while maximizing heat emissivity in the mid-infrared (MIR). Those two effects account for remarkable thermoregulatory properties, enabling the ant to maintain a lower thermal steady state and to cope with the high temperature of its natural habitat.”

Journal article

Total internal reflection accounts for the bright color of the Saharan silver ant

PLoS One | 01/01/2016 | Willot Q, Simonis P, Vigneron JP, Aron S

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Reference

Book

Dryland Climatology.

Cambridge University Press. | 01/01/2011 | Nicholson, SE

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Reference

“[The hairs] enhance not only the reflectivity of the ant’s body surface in the visible and near-infrared range of the spectrum, where solar radiation culminates, but also the emissivity of the ant in the mid-infrared. The latter effect enables the animals to efficiently dissipate heat back to the surroundings via blackbody radiation under full daylight conditions.”

Journal article

Keeping cool: Enhanced optical reflection and heat dissipation in silver ants.

Science | 01/01/2015 | Nan Shi N, Tsai C, Camino F, Bernard, GD, Yu N, Wehner R.

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Reference

Journal article

Passive radiative cooling below ambient air temperature under direct sunlight.

Nature | 01/01/2014 | Raman AP, Anoma MA, Zhu, L, Rephaeli E, Fan, S

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Reference

“To appreciate the effect of thermal radiation, think of the chilly feeling when you get out of bed in the morning. Half of the energy loss at that moment is due to thermal radiation, since your skin temperature is temporarily much higher than that of the surrounding environment.”

Web page

Staying cool: Saharan silver ants

Columbia Engineering | 01/01/2015 | The Fu Foundation School of Engineering and Applied Science

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