Deep in the Amazon jungle, researchers have discovered a dung beetle that doesn't live up to its name, a sign the insect has undergone speciation. A new study published today (Jan. 20) in Biology Letters reports a dung beetle that shuns its normal muck-eating habits in favor of feasting solely on live millipedes -- the first non-dung-eating dung beetle, say the authors. But not everyone agrees with this claim.
Dung beetles are a worldwide group of insects that feed almost exclusively on animal droppings, which can be a rare commodity. Although certain dung beetles sometimes dine on rotting fruit or fungus, and two species have been spotted preying on ants, no obligate predatory dung beetle had ever been reported.
But now, Trond Larsen, a Princeton University biologist, and his colleagues have discovered a killer dung beetle that pooh-poohs its ancestral dung ball-rolling ways, opting instead to maim and often decapitate large, toxic millipedes. "They're not dung beetles anymore," Dick Richardson, an ecologist at the University of Texas at Austin who was not involved in the study, told The Scientist. "They are morphologically, and that's the way the taxonomist would categorize them," but they've adapted to a new ecological niche by becoming predatory. "That's a good way to get new species," he added.
Larsen's team captured around 100,000 individual dung beetles from the lowland rainforests of Peru using a series of traps baited with all sorts of beetle treats -- dung, carrion, fruit, fungus, and millipedes. Of the 132 different species they caught, only one -- Deltochilum valgum -- was attracted exclusively to millipedes. After watching the scarab beetles hunt using infrared video, they showed that this beetle had an elongated hind leg compared to other beetles of the same genus -- the better to grasp and drag its wriggly prey -- and slightly modified head and teeth, which helped the beetle saw open and feed inside the millipedes' exoskeletons.
Considering that dung is often in short supply in the rainforest, the study shows how competition for resources can drive the evolution and diversification of species-rich groups such as insects, said Ilkka Hanski, a population ecologist at the University of Helsinki in Finland who was not involved in the study. "It's another example of the consequences of very intense specific competition," he told The Scientist.
Clarke Scholtz, an entomologist at the University of Pretoria in South Africa and also not a co-author, agreed that the study provides an interesting tidbit of natural history, but he doubted the paper's claim of identifying the "first" non-dung-eating dung beetle. "I'm afraid this is nonsense," he wrote in an email. "The phenomenon has been well described for African species," such as dung beetles of the Sceliages genus, which are obligate millipede-feeders, too.
But study co-author Adrian Forsyth, the vice president of programs at Blue Moons Fund, a conservation organization in Charlottesville, Virginia, noted that Sceliages beetles only consume dead or dying millipedes. "This thing [D. valgum] is not just a passive consumer of millipedes," he said. "It's the first observation of a dung beetle actually actively pursuing millipedes and killing them with a specialized kind of behavior." Hanski agreed: "Other dung beetles also use millipede carcasses, but this one is actually attacking live millipedes."