Female dung beetles use their weight and horns to fight competitors for cow dung and breeding sites, say Australian researchers.
PhD researcher Nicola Watson and Professor Leigh Simmons from the University of Western Australia in Perth publish their findings today in the journal Proceedings of the Royal Society B.
"If you're bigger or have bigger horns, it's easier to bully your opponent out of your way," says Watson.
Biological weaponry, such as horns or antlers, evolves in males that compete for territory or access to females. But rarely do females evolve such traits.
Watson and Simmons say the evolutionary pressure to grow weaponry on female dung beetles comes from competition for breeding grounds.
Watson studied female dung beetles of the species Onthophagus sagittarius, in which the female has much larger and differently-shaped horns than the males.
The females use their horns, which look a bit like the twin horns of a rhinoceros, to fight other females for desirable patches of cow dung and for access to the tunnels underneath the dung where they lay their eggs.
"In a tunnel the beetles interlock these horns like two prongs," says Watson.
"The [beetle] that has the longer horn can keep the other at a distance and use the horn for leverage to evict the other female."
In two separate studies, the researchers split 181 female dung beetles into three groups based on their body size, and found that larger females fared better in terms of their success at reproducing.
They then compared females of the same body size but with different horn sizes and found that beetles with bigger horns also had a greater reproductive success.
Reproductive success was measured by the number of 'brood balls' - pieces of the dung compacted into the end of the tunnel - they were able to create.
Females create a 'brood ball' for each egg, so that when the larva hatches, it has an instant source of nutrients from the dung.
The more dung in the brood ball, the bigger the larvae will grow. So females with more dung and who are able to secure tunnels can ensure they are producing more and bigger offspring.
But competing females can take over or destroy other females' brood balls.
This competition for tunnels and dung favours the evolution of the female's unique horns, says Watson.
The researchers say females produce significantly fewer broods when in competition with other breeding females, indicating that competition and/or interference from other females reduces access to dung for brood ball construction.
Since one brood ball represents one offspring, the acquisition of dung directly determines reproductive success, they say.