Penny Cook of Liverpool John Moores University, UK and Nina Wedell of the University of Stockholm, Sweden have found that the P.nap butterflies have two distinct types of sperm: fertile or 'euprene' sperm and the shorter, thinner, non-fertile, 'apyrene' sperm which lack nuclear material and represent around 90% of the total number of sperm in each ejaculate. As they explain in Nature [11 February], although both types are transferred to the female during mating (in a package known as a spermatophore) and both migrate to the site of sperm storage - the spermatheca - their effects once in place are actually quite different.

Apparently a female butterfly's receptivity to mating is related to the number of non-fertile, or apyrene, sperm she has stored in her spermatheca. For when the researchers counted the number of each sperm type in the storage chamber of mated females, they found that re-mating females contained fewer non-fertile sperm than those which did not re-mate.

They also found that virgin males deliver a larger spermatophore than mated males, but that the proportion of fertile - euprene - sperm in the spermatophore of mated males was greater than in that of virgins. An intriguing observation given that females who received smaller spermatophores mated more and sooner than other females.

What all this means is more problematic. "The quantity apyrene sperm is more variable than the numbers delivered [and] it is not clear whether this variation is due to differences in quality or persistence of apyrene sperm in storage," admit Cook and Wedell. Nonetheless, what it does indicate is that male butterflies seem to be using non-fertile sperm - which, by dint of having no nucelii, are less energetically 'costly' to produce - to exploit a female system designed to monitor the number of sperm in storage and thus to delay female re-mating and reduce the potential for sperm competition.