Invertebrates
Dung Beetle
Scarabaeus satyrus · also called scarab beetle, tumblebug
The dung beetle makes a living from what other animals discard. It finds fresh droppings by smell within minutes, carves off a portion, shapes it into a ball, and rolls it away in a perfectly straight line — steering, when the night is dark, by the light of the galaxy.
Overview
Dung beetles are found on every continent except Antarctica, wherever large mammals produce droppings, and they are especially diverse in tropical grassland and savanna. There are three broad lifestyles: rollers, which form a ball and move it away from the pile; tunnellers, which bury dung directly below the pat; and dwellers, which live inside it. Scarabaeus satyrus, a southern African roller, is one of the species used to study how they navigate.
The straight line
A fresh dung pile is a frantic, crowded marketplace — thousands of beetles can converge within a quarter of an hour, and finished balls are frequently stolen. A roller’s best strategy is to leave as fast as possible in a straight line, in whatever direction, so it does not blunder back into the competition.
To hold a straight course while walking backwards and pushing a ball bigger than itself, the beetle takes a celestial bearing. In daytime it reads the sun and the polarisation pattern of the sky. At night it uses the moon and its polarisation. And on moonless nights, African rollers steer by the faint band of the Milky Way — the only animal known to use the galaxy as a compass, demonstrated by covering beetles’ eyes and watching them lose the ability to go straight.
Strength
Dung beetles are, pound for pound, among the strongest animals on Earth. In tests, a male horned dung beetle (Onthophagus taurus) pulled a load equivalent to over 1,100 times its own body weight — the proportional equivalent of a person dragging several loaded trucks. The strength is used mainly by males wrestling rivals for control of a tunnel and a female.
Reproduction
Rollers bury a brood ball in an underground chamber, with a single egg sealed inside. The hatching larva feeds entirely on the dung around it, pupates, and emerges as an adult. In some species the parents guard the burrow; in a few, the male and female cooperate to build and provision it.
Why they matter
By rapidly removing and burying dung, beetles suppress dung-breeding flies and livestock parasites, recycle nutrients into the soil, improve soil aeration and drainage, and disperse seeds. Their ecological and economic value is large enough that they have been deliberately moved between continents — famously to Australia, where imported dung beetles cut plagues of bush flies by clearing cattle dung the native fauna could not process. Where big wild grazers vanish, specialist dung beetles decline with them.
Dung Beetle: common questions
Do dung beetles really navigate by the Milky Way?
Yes — it is the first animal shown to do so. African ball-rolling dung beetles work at night, and researchers found that on moonless nights they still roll in near-perfect straight lines by using the faint stripe of light from the Milky Way as a compass reference. Beetles fitted with tiny cardboard caps that blocked their view of the sky lost the ability and wandered in circles. By day they use the sun and polarised light; by night, the moon, its polarisation pattern, and, when the moon is absent, the galaxy itself.
Why do dung beetles roll the ball in a straight line?
Speed and security. A fresh dropping is a crowded, competitive resource — thousands of beetles may arrive within minutes, and larger beetles will try to steal a finished ball. A beetle that has shaped its ball needs to get it away from the pile as fast as possible, and the shortest route is a straight one in any direction. Rolling straight, rather than circling back toward the crowd, minimises the time the ball is at risk.
What do dung beetles do with the ball?
It depends on the beetle. "Rollers" trundle the ball a safe distance, dig a chamber, and bury it either as a food store for the adult or, for a brood ball, with a single egg laid inside — the larva then eats its way through the dung from within. "Tunnellers" dig straight down beneath the dung pat and drag portions underground. "Dwellers" simply live and breed inside the pat itself.
Why are dung beetles important?
They are unpaid sanitation and agriculture workers. By burying dung, they remove breeding habitat for pest flies and parasites, recycle nitrogen and other nutrients into the soil, improve soil structure and water infiltration, and disperse the seeds contained in the dung. When dung beetles were introduced to Australia to deal with cattle dung that native beetles ignored, they dramatically reduced bush-fly numbers. The value of their services to livestock farming runs into hundreds of millions of dollars a year in some countries.