Males with the longest and spiniest genitalia propagate their genes better than their less endowed rivals, a new study says.

Several species, especially among insects, are known to physically harm their mates in reproduction, but scientists aren't sure why these traits evolved.

The new research offers the first proof that dangerous genitalia in males can represent a reproductive advantage. The resulting wounds in the females, however, are likely just an "unfortunate side effect" of the strategy, Arnqvist said, and do not provide a reproductive benefit.

For instance, 20 to 30 seed beetle species have arisen whose males look nearly identical, but have wildly divergent sexual parts. "The male copulatory organ is the single [physical] trait that evolves very rapidly across animals," Arnqvist said.