When it come to fruit fly sperm, does size matter?
As anyone with a passing interest in sexuality knows, exaggerated and outlandish displays by the male of the species are the norm -the strut of the peacock, the twelve pointed buck’s antlers, all the way down to that dude’s cologne you can still smell a block away- whatever it takes to attract the female of the species, it seems that males are up for it.
But even outlandishness has to meet up with biological truths once in a while, hence the problem biologists have dubbed the “big sperm paradox.”
In a nutshell, as they say, here it is. The common assumption has been that to increase its odds of passing on its genetic lineage, anygiven male of a species needs to not only find a mate but to make sure that its sperm win the raceto fertilize the female’s egg. For most species, the chosen tactic has been all about strength in numbers -produce a large quantity of sperm so as to guarantee that of those that actually make it to the egg, at least some will be viable, functional and successful at fertilization.
Yet, consider Drosophila bifurca, the fruit fly who turns that logic on its head. A scant few millimetres in length, the male of the species produces sperm six centimeters long! The problem of the big sperm paradox comes in once it’s realized that with limited space and resources at its disposal, the male fruit fly can physically only produce and house a few of these massive sperm at one time, thus seemingly going against the maxim that sexual selection is weakened by having fewer sperm available. How is it possible that in applying its “go big or go home” approach the fruit fly can succeed in reproduction?
A research team led by Stefan Lupold of the Center for Reproductive Evolution at Syracuse University has come up with a solution. By manipulating Drosophila sperm length as well as number in order to gauge their effect on reproductive success, the researchers found that in being attracted to larger rather than smaller males, female fruit flies are effectively choosing those males that have the resources to produce bigger sperm. Further, it turns out that the unique characteristics of the female fruit fly’s reproductive system actually favour longer to shorter sperm.
Both of these features allow the male fruit fly to succeed with fewer but bigger sperm, which shows that for this species at least, what’s more important than following a general rule of sexual selection is cleaving to the particular preferences of the female of that species. “For many species, what may matter most in post-copulatory sexual selection is not simply transferring the most sperm or the best sperm, but rather the greatest number of sperm that are designed to survive and compete best given the specific female reproductive environment,” say the study’s authors.
Thus, instead of strength in numbers or size matters – the most important rule turns out to be, keep your eyes on the prize.
The new study is published in the journal Nature.