Male monarch butterflies have a sixth sense about where their female mates have been. As New Scientist explains, "[s]ensors on the male monarch butterfly's penis may detect the volume of sperm directly, like the dipstick in a car's oil tank." That is, the male butterflies decide how much of their own sperm to deposit based on the female's mating history. If the male senses that the "oil tank" is nearly empty, it will inject a concoction of fertile sperm along with a good amount of fake sperm (sperm look-alikes with no nuclei) to discourage future male suitors. If the tank is already full, the male injects a more potent mixture, with a higher concentration of fertile sperm, in order to compete with the the sperm already in the female. The ability to assess sperm competition helps the male butterflies hone their reproductive strategy. As a result, later lovers may actually gain a slight reproductive advantage. Males of other species are also known to engage in sperm competition. Some fish species, for example, will release more sperm if there are rivals in sight. Although sperm competition is most common is promiscuous species, it's been noted in humans as well. The "dipstick method," however, appears to be unique to butterflies, says Michelle Solensky, who conducted the new research published in Animal Behaviour [subscription required]. Related Content: DISCOVER: Butterflies Like Mates With a Twist DISCOVER: Sperm Cells Demonstrate Some Brotherly Solidarity

Image: flickr / Mataparda