13 OCTOBER 2005
One of nature's stranger evolutionary puzzles centers around one of the most unglamorous members of the animal kingdom: a bug that exclusively eats, well, poop.
Dung beetles -- of which there are thousands of species worldwide-feast on the dung of practically every animal under the sun, from tree-swinging howler monkeys to packrats living in underground burrows.
Courtesy E. Greene
It's an odd choice of cuisine, but one that serves a crucial ecological function -- dung beetles work like natural septic systems and help to churn and enrich soil.
Males of many dung beetle species also sport 'horn'-like appendages that serve as weapons in violent underground battles for female mates and territory. What evolutionary scientists have found puzzling, though is why these 'horns' come in such a stunning variety of shapes and sizes.
Beetle horns can be huge, sometimes constituting up to 15 percent of a beetle's body weight. "To a human that would be like having another leg and wearing it around your head for your entire life," says Doug Emlen, an evolutionary biologist at the University of Montana-Missoula.
Courtesy E. Greene
For over a decade, Emlen has been mystified by what he calls the "outrageous diversity" of dung beetle horns, rather unwieldy appendages that seem to be useful only in combat. Some dung beetles have one horn, while others have five; some have short horns at the front of their head while others have long horns at the back; some beetle horns even branch out like the antlers of an elk.
The question, Emlen says, is why?
Harnessing Horn Evolution
Why have the forces of evolution and development given rise to such incredible horn diversity in dung beetles?
Evolutionary biologists have explored similar questions in other species that feature a diversity of 'extreme' traits. Drosopholid flies in Hawaii, for instance, have diverged into more than 900 species, each with unique wing patterns. The gorgeous long tails of male birds-of-paradise, or the bright colors of African Great Lake cichlids are other startling "evolutionary radiations" in nature.
Scientists have suggested that such exaggerated 'ornaments' in the animal kingdom evolved as a result of sexual selection -- where individuals in one sex compete over limited numbers of reproductive mates. Peahens, for example, are most likely to choose males with the flashiest feathers.
But while scores of studies have focused on female choice and the phenomena of ornamental diversification, few have explored how -- or even if -- the sister process of male competition can lead to the diversification of weapon structures. It's a surprising fact, given that horn-like weapons are widespread in nature, in flies, crabs, ungulates and even chameleons.
Emlen works on the beetle genus Onthophagus, a sub-group of the family Scarabidae, to which most of the horned beetles belong. There are at least 2000 known Onthophagus species around the world, with another 2000 or so still waiting to be described.
Dung beetles swarm to fresh dung almost as soon as animals expel it. Many species dive straight into the waste and dig tunnels into the underlying soil. While females dig, the males often help by stashing dung into the tunnels. The females then fashion a sequence of little dung balls and lay eggs into each. Meanwhile, males guard the tunnel entrances, using their horns to fend off intruders. The winner of a fight automatically mates with the resident female in the tunnel.
Working with molecular data from 48 Onthophagus species, Emlen first mapped patterns of dung beetle horn evolution. He found that over 40 million years, the beetles underwent at least 25 changes in the physical location of their horns and evolved 15 novel horn types!
From Emlen, Marangelo, Ball and Cunningham, 2005.
Habitat differences alone are unlikely to have spawned such diversity, Emlen reasoned, because practically all dung beetle species use their horns in the same setting: dark underground tunnels. "It's true that all tunnels are not the same, but tunnels are tunnels, whether in the Australian outback or tropical jungle," says Emlen.
Then in 2001, Emlen discovered something that forced him to reassess his previous notions about the forces of evolution. Dung beetles, he found, must negotiate an evolutionary 'trade-off' with nearby body parts in return for growing large horns. In other words, species with horns in the front of the head are likely to face the 'cost' of growing smaller antennae for example; beetles with horns at the base of the head grow smaller eyes, while those that grow horns on the thorax have proportionately smaller wings.
Courtesy Douglas Emlen
Such evolutionary costs may be driving beetle horn diversity, says Emlen. Following this logic, he found that nocturnal species that rely more heavily on their eyes, for instance, tend to lose horns at the back of their head and replace them with other horn types instead.
Simultaneously studying how beetle horns develop, Emlen has also now identified many of the genes involved in beetle horn formation. In trying to explain the influence of a genetic blueprint on the generation of horn diversity, Emlen's work is starting to resonate with a growing body of research that holds that the forces of evolution and development are much more intertwined than previously thought.
Emlen's work reflects how particular findings in specific species can provide general insight about evolutionary and developmental processes in the animal kingdom, says Sean Carroll, a prominent developmental biologist at the University of Wisconsin-Madison. "Great biological models can come in weird packages," says Carroll,"And sometimes they even eat shit."
-- Paroma Basu
Emlen, D. J. Costs and the diversification of exaggerated animal structures. Science 291: 1534-1536. (2001)
Emlen, D. J., Marangelo, J., Ball, B. and Cunningham, C. W. Diversity in the weapons of sexual selection: Horn evolution in the beetle genus Onthophagus (Coleoptera: Scarabaeidae). Evolution (2005)