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Bring itIn females of the beetle Onthophagus sagittarius, competition for dung to wrap eggs and feed young favors the evolution of exaggerated horns. Sean Stankowski

So many moms, so little fresh excrement.

Though male animals are usually the ones to sport horns and other weapons, in one species of beetle battle armor comes in handy for the ladies, who use their oversized horns in fights over dung.

Females of the species Onthophagus sagittarius who had heftier horns won control of more available dung and thus laid more eggs, evolutionary biologists Nicola Watson and Leigh Simmons of the University of Western Australia in Crawley found in lab tests. Competition for quality dung is the evolutionary force selecting for feminine weaponry in this species, the researchers conclude in a paper to be published online the week of March 2 in Proceedings of the Royal Society B.

“It’s a rare example of this type of evolutionary event for sure,” says biologist Ted Stankowich of the University of Massachusetts, Amherst.

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Venus and MarsBoth of these dung beetles belong to the same species, but the horns of the female (right) lie centrally positioned like a rhino's and probably evolved independently from the side-by-side horns of the male (left).Sean Stankowski

Horns on bulls, antlers on stags and other guy weapons have preoccupied scientists who study evolution, Stankowich says. Darwin proposed that male weaponry arose from the struggle between rivals for access to females, and later work has found plenty of examples that fit that scenario.

But females have their own reasons to grow personal arsenals. In gemstock antelope, for example, defense against predators probably drove the evolution of female horns, Stankowich says. In other species, such as small antelopes called duikers, territorial battles probably did the same.

The dung beetle's horns are special, Stankowich says. Female duiker horns generally look like the males', but female dung beetles grow another type of horn altogether.

The female beetles sprout a large central horn with a smaller stub behind it, Watson says. The arrangement looks like a miniature version of a rhino’s arsenal. Males grow a pair of stubbier horns, more like a bull’s, that sprouts from a different place on the body.

Because male and female horns are so different, Watson and Simmons dismiss the possibility that the female horns grow simply as some kind of genetic spillover from male horns. Instead, Stankowich says, “it’s an independent evolutionary event from male horn evolution.”

Another possible explanation for female horns is that males find them alluring. Probably not, Watson and Simmons report in the March–April Behavioral Ecology. In lab tests, male beetles showed no extra ardor for females with greater horn endowments.

To test the idea that horns help females compete more effectively, the researchers bred beetles in the lab and counted their offspring under a variety of circumstances. When females of different sizes had to compete for dung, the larger ones flourished while smaller ones produced very few dung-swaddled eggs. And when researchers paired females of about the same body size, those with proportionately heftier horns had more offspring.

Outside the lab, these beetles have reproduced well in Australia, where they were introduced to clean up after cattle and other imported livestock. The native dung beetle species had evolved with marsupial dung, Watson says, and weren’t inclined to switch.

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