When combat isn't an option, some males resort to far subtler, but no less effective, tactics to dispose of sexual rivals. The male green-veined white butterfly, which is widespread across Europe and Asia, injects methyl salicylate along with its sperm into the female during copulation. The strong scent of this pheromone--also called oil of wintergreen and found consumer goods including Listerine mouthwash and chewing gum--wards off other potential mates, helping to ensure that no other males fertilize the female's eggs.

Because females have several opportunities to mate, male size also becomes a factor. A large male, which produces bigger sperm packets, has a better chance of successful fertilization. Scientists have also found that the larger the ejaculate, the longer a female needs to wait before she can mate with another suitor.