Color of light sets dung beetles straight

Here’s a reason to keep eyes on the skies while dealing with dung on the ground: Variations in the color of light in different parts of the sky can act as a compass, at least if you’re a dung beetle.

Greenish and ultraviolet variations in daylight could be enough for the beetles to steer by.

In lab tests, Scarabaeus lamarcki beetles manage to keep moving along a relatively straight path when given either or both colors of LED lights as guides in an otherwise dark room, Basil el Jundi of Lund University in Sweden and his colleagues report. 

These beetles don’t use landmarks to keep their bearing; they look to the sky for clues, such as sun position. The new lab tests, the first exploring color as a possible beetle compass, suggest that the beetles have an emergency backup system for orienting on cloudy days, the researchers say November 4 in Biology Letters.

Biologists have not tested many species for an ability to find directions using color gradations in the sky. But Rüdiger Wehner of the University of Zurich suspects that the capacity could turn out to be widespread in insects. His research group has found that desert ants respond to the color gradient. And his group and others have shown that honeybees can use color cues.

Dung beetles make another good test case. Theyroll a freshly collected ball of their sustenance in a relatively straight line away from the frenzy of beetles competing for chunks of a fresh deposit. It’s impossible to maintain a straight course without some kind of external compass. “Even humans can’t do it,” says study coauthor Emily Baird, also of Lund University, speaking from experience. “We got a bit bored, so we decided to test ourselves,” she says. Wearing blindfolds and headphones and concentrating ferociously, “we really walked in circles.”

Research on nocturnal and dusk-loving dung beetles has found that they use a wide repertoire of celestial clues for orientation, including polarization of moonlight (SN: 7/5/03, p. 4) and, according to tests in a planetarium, the Milky Way (SN: 2/23/13, p. 15). Day-active beetles not only use the sun but can orient themselves from seeing just a small patch of sky’s polarized daylight.

To test for another possible compass clue, the researchers set dung beetles one by one in the middle of an indoor arena illuminated by a green light on one side and a UV one on the opposite side. That lighting roughly mimicked a simplified version of the way the sun illuminates the Earth. There’s more greenish light where the sun shines down directly and increasingly less across the rest of the sky. In the sky opposite the sun, light takes a longer trip through the atmosphere and disproportionately loses lower-energy colors. There, high-energy UV light accounts for a greater proportion of the illumination than does the weaker green.  

With both green and UV lights on, researchers checked which way each beetle rolled its dung. Once a beetle had established a preference for a particular dung batch, the beetle roughly maintained its heading when researchers moved it back to its mid-arena starting point.

Beetles that had started rolling with both lights on kept moving in the same general direction when researchers turned off the UV source. Beetles, however, might not have perceived, or cared, that the light was green. It might have acted as some beacon the beetles kept at a particular angle while rolling along.

For a stronger test, researchers then turned off the green light and switched on the UV one. Dung beetles still maintained their original bearing, treating the UV light as something that should be opposite to the now-off green light. Baird says the UV light may even serve as an “anti-sun,” something that belongs on the far side of a sunny sky.