The Hairy Optics of Saharan Silver Ants

April 26, 2016

Foraging for lunch in temperatures exceeding 120°F is no picnic.

Saharan silver ants don’t just survive in the scorching heat, they depend on it. The small silver ants come out of their colony for just a few minutes one time each day to quickly scavenge animal carcasses for food.*

Saharan silver ants disassemble a beetle to take it back to the nest.
A group of Saharan silver ants at work dismantling a beetle to bring it back to their nest. Image Credit: Bjørn Christian Tørrissen (CC BY)

By emerging during the hottest period of the day, the ants can scavenge without fear of their main predator, a lizard that can’t withstand such extreme temperatures. But surviving the heat requires a body well adapted for the harsh conditions found at midday in the deserts of the Saharan, the Sinai, and the Arabian Peninsula.

Reflecting Hair

  

The unforgiving sun beats down on Saharan silver ants just like it does on the other desert dwellers, but the ants have a creative way of reflecting most of the sunlight away from their body—through their hair.

Their hair is one of the most unique features of Saharan silver ants, officially known as Cataglyphis bombycina. Zoom in on one of these ants and you’ll see that its head, neck, and abdomen are covered in a dense layer of hair.

The silver ant Cataglyphis bombycina. Colonies contain a caste of spindly workers and a caste of soldiers with large heads and saber-shaped mandibles.
The silver ant Cataglyphis bombycina. Colonies contain a caste of spindly workers and a caste of soldiers with large heads and saber-shaped mandibles. (a) In full sunlight, workers and soldiers show a metallic silvery sheen. Photo copyright: P. Landmann. (b, c) The hairs covering the dorsal side of the workers' head, thorax (b) and abdomen (c). Image Credit: Willot Q et all (CC BY)

Zoom in even more and you’ll see that a strand of this hair has a triangular cross-section. Two sides of each hair are covered in grooves, and the third side is flat.

 Electron Microscopy images of the ants' hairs.
Electron Microscopy Images of the hairs. (a) Scanning electron microscopy of the hairs, which have a triangular shape and end in a sharp point. (b) Transmission electron microscoy (TEM) image of the cross-section of a hair. The two upper sides of the triangular shape are corrugated (have grooves) and the base is flat. (c) Close up on the grooves. (d) TEM image of hairs shows that they all adopt the same orientation, with their flat side parallel to each other and to the surface. Image Credit: Willot Q et al (CC BY)

This unique hair was first seen in 2015 by a collaboration of scientists from Columbia Engineering, the University of Zürich, and the University of Washington. Their work showed that the hair plays two key functions that help keep the ants’ body temperature below their critical limit. The hairs are great at reflecting light in the visible and near-infrared range, and are great at emitting light in the mid-infrared as heat.

Building off of this research, evolutionary biologists Quentin Willot and Serge Aron at the Université Libre de Bruxelles and physicists Priscilla Simonis and Jean-Pol Vigneron at the University of Namur recently used geometric optics to explore how light interacts with the ant hair in more detail. Their results, published in PLOS ONE, show that the hair reflects most of the incoming sunlight away from the body through a process called total internal reflection.

When total internal reflection occurs, light traveling in one material hits a boundary with another material and all of the light is reflected. This is in contrast to what occurs most of the time at the boundary between two materials—some of the light is reflected and some travels into the second material. The amount of light that is reflected and that gets through depends on the optical properties of the two materials, and on the angle at which the light hits the boundary.

Total internal reflection of a laser beam in an acrylic block.
Total internal reflection of a laser beam in an acrylic block. Image Credit: Public Domain  

The researchers found that the optical and physical properties of the ant hair favor total internal reflection. These properties include the hair’s triangular shape, the grooves running along the upper two sides of the hair, and the shape of the ant’s abdomen. All of the hairs lay in the same orientation, with the flat side parallel to the ant’s body.

Under most conditions, when light enters the hair through one of the grooved sides it is totally internally reflected off of the flat side and then exits from the other grooved side. In this way the hair keeps the light, and therefore heat, away from the ant’s body. The grooves allow more light to enter the hair, increasing the amount that can be totally internally reflected.

A diagram depicting total internal reflection in a triangular prism.
A diagram depicting total internal reflection of a laser beam in a triangular prism. Image Credit: Public Domain

Each hair essentially acts like a tiny mirror that reflects nearly all of the light that hits it. This helps to regulate the ant’s temperature by limiting the amount of light that is absorbed by the body. To verify this experimentally, the researchers compared the light reflected by a regular Saharan silver ant to the light reflected by a shaved one. They show that hairy ants reflect 10 times the amount of light as shaved ants in the visible range, leading to an internal temperature difference of up to 2° C.

The team showed that the ants’ bright silvery sheen also results from the total internal reflection. When visible wavelengths are reflected in random directions off of a surface, we see the surface as white. When those wavelengths are reflected in the same direction, as results from the total internal reflection in the hair, the surface appears silvery or mirror-colored. The scientists used a technique that suppresses total internal reflection to support this theory experimentally.

Other Cooling Measures

 

Hair alone doesn’t ensure the Saharan silver ant’s survival at such high temperatures. This adaptation works in conjunction with many others to support its way of life, some that probably haven’t been uncovered yet. But here are a few that we do know about:

  • Saharan silver ants have longer legs than other ants, which keep their bodies further from the hot sand.
  •  An internal navigation system based on the position of the sun that helps the ants minimize time spent outside the nest.
  • The ants produce heat shock proteins prior to going out in the heat. These proteins help the body continue functioning at high temperatures. In most animals they are released during exposure to heat, not prior to exposure, but this early production is essential for the Saharan silver ant.

More Hairy Lessons?

 
Most animals have some kind of hair, bristles and fur included. Hair takes many forms and serves a variety of purposes: camouflage, communication, temperature sensing, temperature regulating, and capturing prey, among others.  We now know that the hair of the Saharan silver ant helps to keep the ants cool in two main ways, and that its optical properties give the ant its color. Might the hair serve others purposes as well? Can its physical and optical properties provide guidance on creating materials to withstand blazing heat? Scientists are working together on these questions and more, seeking to learn all we can from this small insect about thriving under extreme conditions.
 
* To see the ants in action, check out this video clip from the BBC.
—Kendra Redmond