Size matters for female dung beetles

Research published today in Proceedings of the Royal Society B shows that although the size of a female dung beetle’s horn may not make her more attractive to males, it can give her an advantage when fighting off other females in competition for a critical resource – dung.

The beetles use dung to build protective “brood balls” for their young, so the more dung a female beetle can collect, the more young she is able to rear.  Now Nicola Watson and Leigh Simmons, based at the University of Western Australia, have discovered that female dung beetles with larger horns tend to produce more young and that this is due to the competitive advantage provided to them by their horns.

It has long been observed that male members of the animal kingdom often possess features that may give them an advantage in breeding without being directly related to reproduction (think of the extravagant feathers of a peacock or the luxuriant mane of a lion).  These features are much more rarely observed in female animals and, up to now, research has almost exclusively focused on those characteristics that make a female more attractive to a mate.

However, the researchers suggest that it’s not so surprising to find females compete against each other: “...while it may be true that...females do not typically compete for males...females [are] much more likely than males to experience intense competition for resources important for reproduction”.  Their study confirms this statement by showing that when dung is limited and females have to compete for it, those with the largest horns tend to produce the most young.   Male beetles don’t appear to show any preference between small and large-horned females, so this reproductive advantage has to be down to larger horns providing a competitive advantage when pitted against other females.

While the authors admit that this kind of competition between females is a fairly rare occurrence, they point out that “it still remains the case that the topic of female competition over reproduction represents a gap in sexual selection theory”.  The authors hope that this study, one of a handful of papers documenting such competition to be published so far, “will stimulate increased effort to document such selection in other taxa”.