Scientists were scratching their heads while looking for the evolutionary explanation as to why the males of a butterfly species possessed predominantly non-fertile sperm.
What the heck is going on?
Promiscuous mating regimes are great to research if you're interested in the coevolutionary arms race between male and female reproductive interaction. Usually it is evident that males will have evolved to manipulate female receptivity and females to resist the manipulation. An interesting example lies in the green-veined white butterfly (Pieris napi). Male butterflies transfer fertile and non-fertile sperm to their female. The non-fertile sperm requires less energy to produce and is used because females possess a sperm-storage organ that, when full, switches off the receptivity of the female. This enforces female monogamy because she cannot successfully mate with another male until she finishes the period during which she is not receptive (due to the full sperm storage organ). This period of time is called her refractory period.
However, females have also evolved to combat the standard lengthy refractory period by shortening it significantly. It is advantageous because it allows females to mate more and therefor have a higher reproductive output. Researchers found a positive genetic correlation between non-fertile sperm transfer and female refractory period. In other words, in the context of a promiscuous mating system, sexual conflict yeilds short female refractory periods and high proportions of non-fertile sperm in males. They also found that these traits were heritable and when the mating system was manipulated, the traits would also fluctuate.
This finding is consistent with previous findings related to male-female sexual conflict: selection on female reproductive traits directly affects male traits and vice versa.
Nina Wedell, Christer Wiklund, and Jonas Bergström. 2009. Coevolution of non-fertile sperm and female receptivity in a butterfly. Biology Letters.