The Why Files The Why Files --

Evolution: The never-ending story

Doggone dung beetles

Let us devote a few words to the dung beetle, a cleanliness-freak insect that rolls up a hunk of dung, lay eggs inside, and then buries the reeking, rotund reproductive unit in the soil, so the beetle's larvae can hatch. It's a win-win, as far as we are concerned: The beetle survives, while simultaneously recycling offal and clearing the landscape.

Five beetles, with horns resembling rhino horns, deer antlers, and triceratops horns
Courtesy Armin Moczek, Indiana University.
Horned beetles illustrate a marvelous range of diversity in both shape and size.Photo: Clockwise from top left: Phanaeus imperator, Eupatorus gracilicornis, Onthophagus watanabei, Golofa claviger, Trypoxylus (Allomyrina) dichotoma.

Dung-wise, dung beetles are often phenomenally choosy, says Armin Moczek of Indiana University. "There is a group of species that is specialized to follow arboreal monkey troops. Some fly after the monkeys, but a subset with modified front legs clings to the fur around the anal region. When the monkey defecates, the beetle drops with the dung and buries it in the ground."

A dung-seeking missile!

When Australia's cattle industry boomed during the 20th century, the local beetles that dined on marsupial manure turned up their noses (or the beetle equivalent) at the mammalian dung. Then, as heaps of cattle manure began hatching swarms of flies, dung beetles began to get some respect, and in the 1960s Australia imported about 50 species, including members of the genus Onthophagus.

Three male dung beetles show off their horns and copulatory organs. As one gets larger, the other gets smaller. From top: Onthophagus watanabei, O. Gazella, O. sagittarius:
Three horned, gold beetles appear alongside their scorpion stinger-like copulatory organs.
Courtesy Armin Moczek, Indiana University.

Onthophagus is Earth's most diverse genus of animals, containing more than 2,400 species. "They show an extraordinary abundance and diversity of horns used as weapons in male combat," says Moczek. But studies he performed with graduate student Harald Parzer on several Onthophagus species in Australia found something else: For unknown reasons, as the horns get larger, the copulatory organ shrinks.

Horns, like other features that males use to fight over mates or simply attract their attention, play a key role in deciding whose genes get reproduced, which explains the elk's giant antlers and the peacock's magnificent tail.

But the shape of the copulatory organ is also a serious matter to female beetles, and so if it changes in tandem with the horn, that would multiply the selective impact of a change in the horn, Moczek says. And that linkage could explain why the genus Onthophagus evolved so many species.

Chatting with Charles: What would you say to Darwin?

In "The Origin of Species," Darwin noted that developing animal embryos tend to reproduce the evolutionary history of their ancestors. Human embryos, for example, look reptilian at one stage of development. This old notion is getting new respect, Moczek told us.

Armin Moczek: "Until recently, most evolutionary biologists did not care much about how traits formed during development, but over the past 20 years, evolution-development -- evo-devo -- has provided mounting evidence that we can learn much about the quirky path of evolution, why it goes this way and not another, from development. Now we can see tremendous conservation of genes and developmental pathways across large phylogenetic distances. The same pathways that helped build you also exist in a mouse, a zebra fish, a fruitfly. Organismal diversity arose largely due to evolutionary changes in the regulation between conserved pathways. This is the main rationale for why we can learn about human heart disease by studying the 'same' genes and developmental processes in fruitflies."

Heard about a cricket without a song?

Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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