A highly unusual new study has uncovered a surprising fact: nocturnal dung beetles use the distant glow of the Milky Way to guide their path as they roll balls of excrement.
The South African beetles will roll spherical balls of dung away from its source to enjoy privately, away from other beetles and scavengers. It is imperative that these lines progress in a straight path, to avoid circling back to the competition.
While dung beetles’ eyes are unable to focus on specific details at a distance, such as individual constellations, they are capable of discerning the direction of ambient polarized light emitted by the luminous band our galaxy describes across the night sky. Researchers even found that the beetles will periodically climb on top of their dung balls and lift their heads in a manner which suggests they are getting their bearings from the stars above.
Researchers from the University of Lund in Sweden traveled to South Africa, home of the species Scarabaeus satyrus, and placed the beetles in a circular area surrounded with high walls, blocking the view of any potential landmarks on the horizon. The beetles were then tested on both speed and directness in reaching the opposite end of the enclosure, on a moonlit night, a moonless night, and under an overcast sky. Cardboard caps obstructing the view overhead were also affixed to the beetles in several trials.
The results suggested that a clear view of the stars overhead was the major factor allowing the beetles to roll a straight path in a timely manner. Next, the researchers repeated the experiment in a planetarium, alternately showing only the brightest stars in the sky, all stars visible from the southern hemisphere, and only a diffuse band of the Milky Way. The beetles clocked the same average time under a diffuse galactic glow and the full starry spread, and a slower time under all other conditions.
In an exclusive interview with The Space Reporter, co-author of the breakthrough study, Professor Eric Warrant of the University of Lund, elaborated on the team’s findings. The following is a transcript of the interview.
The Space Reporter (TSR): What originally gave you the idea for this rather unique study?
Eric Warrant (EW): “We have for many years been studying how day-active beetles use the disk of the sun, and the circularly symmetric pattern of polarized light produced around the sun, as compass cues to travel in straight lines. The polarization pattern is invisible to us, but the type of photoreceptors possessed by insects allow them to see it. Due to its symmetry in the sky it is possible for insects to orient relative to the pattern, and this is what day-active dung beetles use to orient.
“About 10 years ago we started studying the same behavior in nocturnal dung beetles – two years before scientists had discovered that the moon also produced a symmetric pattern of polarized light identical to that formed around the sun, only a million times dimmer. We wondered whether the straight rolling paths of dung beetles resulted from orientation to the moon’s pattern. It turned out that this was the case.
“A couple of years later we repeated the experiments in another species, but on many nights the moon, and its polarized light pattern, was absent from the sky. But the beetles still rolled in straight lines! This got us worried at first but then we began to wonder whether they could use the stars, maybe as a last resort. Well they could!”
TSR: Do you believe dung beetles may be unique in navigating by the Milky Way? What other species might have reason or ability to do so?
EW: “No. There maybe many animals that use the Milky Way, such as migrating birds or moths. Unfortunately however the Milky way is a rather unreliable cue over a long time period because the earth rotates away from it during the night, so even if fully visible at the start of the evening, it might be below the horizon just a few hours later.”
TSR: How might a characteristic like this evolve?
EW: “That’s a good question that I don’t have a good answer for unfortunately. But it is my guess that when beetles needed to roll dung on moonless nights, those rare males capable of orienting to the Milky Way (by chance) were those that were the most successful in getting their balls away from the fury of the dung pile. It was thus those males that were most successful in mating with a female in peace, resulting in having her egg deposited in his ball. The offspring of such males may have inherited the abilities necessary to orient with respect to the Milky Way, thus being equally as successful as their fathers and thus having best possible chances of passing on this ability to their own offspring. And so on.”
The research was published online Thursday, January 24, in the journal Current Biology.