Male and female dung beetles (Photo: Sean Stankowski)

Horny beetles battle for dung

  • BY Courtenay Rule |
  • March 17, 2010

Competition between horned female dung beetles promotes evolution in their weaponry, a study has shown.

IF YOU'RE A MOTHER dung beetle looking to get your offspring off to a successful start, it pays to be well armed, says a new study which found that females lock horns in battles for dung.

This is the conclusion of evolutionary biologists Nicola Watson and Professor Leigh Simmons of the University of Western Australia in Perth, who reported their findings recently in the journal Proceedings of the Royal Society B.

For the research, Nicola investigated the evolution of female ornamentation, focusing on a Southeast Asian species of dung beetle (Onthophagus sagittarius), in which females have bigger and differently-shaped horns to males. While male horns are short and at the front of the head, female horns are arranged more like those of a rhinoceros, with a long, curving prong.


Horn-to-horn combat

Nicola’s work found that the female beetle’s body size and the length of her horns gave her a crucial advantage in competing with other females for breeding sites and supplies of dung for their larvae.

These beetles lay eggs in tunnels under patches of cow dung. To feed each hatchling larva, the mother makes a ball of compacted dung. But when supplies are limited, they compete to swipe tunnels and dung from other females, by locking horns in battles. A longer horn makes it easier to shove a rival back out of the tunnel.

Nicola and Leigh also compared female beetles of different body sizes and found that larger beetles with longer horns were able to produce the most offspring.

“Our findings indicate that even though females may not compete to attract males, female competition for breeding resources can favour the evolution of female weaponry,” says Nicola.

According to the researchers, male beetles show no significant preference for longer-horned females over shorter-horned ones, so the horns are unlikely to be used for winning males over.


Redressing a gender bias


Entomologist Chris Reid, from the Australian Museum in Sydney, comments that the study is “a fascinating examination of why some insects have the lumps and bumps that they do.” Even so, he adds, it’s still a mystery why females of other known dung beetle species haven’t evolved similar horns.

In the animal kingdom, when it comes to good looks or impressive weapons, females often miss out. Males much more frequently display characteristics that increase attractiveness to females (such as the peacock’s tail) or help them win in battles with other males (the antlers of a stag, for example).

Because of this, most research in this area has focused on males. Nicola hopes that her study will begin to redress the imbalance.

“This study is significant because it’s a very rare case,” Nicola told Australian Geographic  –  “a female characteristic that relates directly to reproductive selection.”