In the animal kingdom, sperm usually is considerably smaller than eggs, which means that males can produce far more of them.
This is due to the competition among sperm from different males to fertilize the few eggs increases as sperm become more abundant.
This sperm competition spurs sexual selection after mating, favoring the best sperm in the female reproductive tract.
“The record holder is Drosophila bifurca: Although this fruit fly is only a few millimeters long, its sperm reach an impressive length of almost six centimeters,” says Stefan Luepold, an evolutionary biologist at the University of Zurich.
Such examples contradict the common understanding of sexual selection because fewer sperm vying to fertilize an egg should relax the selection pressure on sperm to be more successful than their competitors.
If only a few sperm are transferred to females as in fruit flies, selection – and thus also the evolution of longer sperm – is expected to be weakened or even halted.
A study lead by Stefan Luepold and published in the journal Nature now provides the first explanation for the development of such giant sperm.
Together with Scott Pitnick and other scientists from Syracuse University and George Washington University in the USA, he managed to demonstrate that sexual selection has a significant influence on the evolution of sperm length in fruit flies.
The study revealed that, on the one hand, different characteristics and processes of sperm uptake, storage and use in the female reproductive tract favor longer sperm.
Each mating creates an opportunity for sexual selection via sperm competition.
“In fruit flies, longer sperm are really good at displacing their competitors from the female reproductive tract, which gives them an advantage in the competition for fertilization. Sexual selection thus favors longer sperm,” explains Stefan Luepold, first author of the study.
Small males can invest less in sperm production and therefore use up their sperm reserves after only a few copulations.
Only large, healthy males that are favored by females can afford to produce more sperm despite the increased energetic costs of longer sperm.
Thanks to this increased reproductive success, the genes for longer sperm can spread in the population, which ultimately drives the evolution of longer sperm.
These complex relationships can maintain, if not intensify, sexual selection even when only very few sperm end up competing for fertilization.
“Compared to these and numerous other exaggerated sexual traits, fruit-fly sperm is probably the most extreme example in the animal kingdom,” finds Luepold.