Reflective Saharan silver ant hairs thermoregulate, create bright color

The body hairs of the Saharan silver ant cause total internal reflection of light to make the ants almost ten times more reflective, preventing overheating and yielding their silver sheen, according to a study published April 13, 2016 in the open-access journal PLOS ONE by Quentin Willot from the University Libre de Bruxelles, Belgium, and colleagues.

The Saharan silver ant is one of the terrestrial living organisms best-adapted for high temperatures, and can forage in the desert even when temperatures exceed 50 degrees Celsius. It has previously been shown that its dorsal body hairs are responsible for its silver color and help prevent overheating. However, the details of the hairs' optical properties had not been elucidated.

The authors of the present study used a Scanning Electron Microscope (SEM) to examine the ant hairs and trace the path of incoming light rays. They also compared normal 'hairy' ant samples with ants that had been shaved, measuring how light was reflected and how quickly ants heated under simulated sunlight.

The researchers found that hairy ants were almost ten times more reflective than shaved ants, and were able to stay up to 2 C cooler under simulated sunlight. They also confirmed that the triangular cross-section and corrugated surface of each hair act like a prism to reflect light, where the light rays entering each ant hair undergo total internal reflection, bouncing back off the bottom plane of the hair instead of transmitting through it. This mirror effect gives the ant its bright silver sheen and reduces heat absorption from sunlight, preventing the ant from overheating.

Most arthropods have cylindrical or plate-like body bristles, so these results may be specific to this ant species. However, the authors state that this study improves our understanding of the Saharan silver ant's unusual color and ability to survive in the desert heat.

Co-author Serge Aron notes, "The ability to reflect solar radiation by mean of total internal reflection is a novel adaptive mechanism in desert animals, which gives an efficient thermal protection against the intense solar radiation. To the best of our knowledge, this is also the first time that total internal reflection is shown to determine the color of an organism."