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Hunter. D. valgum dung beetles forgo feces for a diet of live millipedes. An infrared camera captures an attack (inset).

Trond Larsen

No Feces for This Species

Deep in the Peruvian rainforest, one species of dung beetle has turned from scavenger to killer. Instead of feasting on feces--as all of its brethren do--the tiny insect tears millipedes in half and dines on their innards. This dramatic change in diet is rare in the animal kingdom and is a stunning example of evolution in action.

When dung beetles feed on animal droppings, they're actually after the vast numbers of bacteria that make up excrement. Believe it or not, it's a highly competitive way of life, and in some places 80 species of dung beetles fight for the same feces. That may have been what prompted one species, Deltochilum valgum, to break from the pack, says ecologist Trond Larsen of Princeton University.

Curious about the feeding habits of the many types of dung beetle living in the Peruvian rainforest, Larsen and his colleagues baited traps with dung, carrion, fruits, and fungus. They also tried live millipedes, as Larsen had observed some dung beetles feeding on dead insects. Over 11 months, the team recorded 132 species of dung beetles and more than 100,000 individuals. D. valgum was the only species that exclusively fed on millipedes, the team reports online this week in Biology Letters.

Even more astonishing was D. valgum's method of attack. Using infrared cameras, Larsen observed the 8-millimeter-long beetle wedging its serrated head between the millipede's segments, ultimately splitting its prey's body in two. Afterward, the beetle dismantles the rest of the millipede and eats it up. D. valgum can kill prey up to 13 times its own size thanks to subtle body adaptations, explains Larsen, including its wedged head and hind legs adapted to hold the insect and drag it apart.

Those adaptations create the potential for a rapid explosion of new predatory dung beetle species, Larsen says. Indeed, after publishing his findings, he observed several more Deltochilum species feasting on millipedes.

It's a "pretty spectacular finding," says biologist Armin Moczek of Indiana University, Bloomington. But he points out that millipedes have a high proportion of feces inside them because they feed on rotting plants. So if the dung beetles are eating their guts, he speculates, they're essentially still eating dung.