Competition for mates drives the evolution of many of the exaggerated male traits, such as the bright plumage of tropical birds or the intricate horns of dung beetles, that are so easily appreciated (and photographed). However, the elaborate consequences of competition for mates continue even after mate choice and copulation has taken place, inside of the female reproductive tract. Much of the research on sperm competition has focused, for obvious reasons, on the quantity and morphology of the sperm produced by males with results that are no less fascinating or extreme. For example, the fruit fly, Drosophila bifurca, produce sperm that are around 2.3 inches long, which is more than 20 times their body length and 1000 times the length of human sperm!
However, sperm constitute only a small portion of the male ejaculate transferred to females during mating. The rest, up to 90%, is composed of a myriad of proteins and other compounds that constitute the seminal fluid. In addition to being produced in abundance, seminal fluid proteins are also diverse. For example, scientists have found that males, of many species, produce dozens, if not hundreds, of different types of seminal fluid proteins. So, what then, do all these proteins do? It turns out that these protein are involved in many different processes that indirectly influence male reproductive success, including influencing female physiology and interacting with a male’s own, as well as rival, sperm.
A recent study by Locatello et al., published in Proceedings of the Royal Society B, has taken advantage of some useful characteristics of the mating system of the grass goby, Zosterisessor ophiocephalus, (pictured above) to investigate how alternative male mating strategies may be paired with alternative sperm competition strategies to help level the playing field between males that vary in their ability to directly compete for mates.
The researchers, part of the Rasotta lab group at the University of Padova, studied the mating behavior and sperm competition strategies of the grass gobies that inhabit the Venetian Lagoon. Grass gobies exhibit alternative mating strategies. The larger males directly compete for access to, and guard, female mates. In contrast, less competitive males avoid direct competition and instead sneak copulations with females already guarded by other males. This variation in behavioral strategies allowed Locatello et al. to ask whether guarding and sneaking males may also employ alternative sperm competition strategies. They hypothesized that the two types of males may differ in the degree to which they invest in high quality seminal fluid or high quality sperm.
Grass gobies are external fertilizers, meaning that both sexes release their gametes into the aquatic environment where fertilization takes place. This characteristic allowed the researchers to separate sperm and seminal fluid and set up a reciprocal experimental design in which sperm performance is tested in different seminal fluid environments (Table 1). Using this design, they tested sperm performance (swimming speed) in the presence of a single type of seminal fluid from either males with the same or different mating strategies. They also performed a second experiment in which sperm from either ‘guard’ or ‘sneaker’ males was exposed to a mixture of seminal fluid from both type of males, which more closely mimics the environment of competing sperm.
The results of the experiments demonstrated that, while both ‘guard’ and ‘sneaker’ sperm performed equally well in seminal fluid from males with the same behavioral strategy, the sperm of sneaker males had increased performance in the seminal fluid of guarding males. Conversely, the sperm of guarding males had reduced preformance in the seminal fluid of sneaking males. Furthermore, when sperm was exposed to a mixture of seminal fluid from both types of males, the sperm of ‘sneaker’ males again outperformed that of ‘guard’ males. All together, these results indicate that grass goby males also exhibit alternative sperm competition strategies, such that ‘guard’ males invest in higher quality seminal fluid at the expense of sperm quality (measured by swimming speed) and that ‘sneaker’ males invest in higher quality sperm at the expense of seminal fluid quality.
These fascinating results suggest many exciting avenues for future research. For example, from a mechanistic perspective: How exactly does seminal fluid composition vary between ‘guard’ and ‘sneaker’ males? Do many different proteins to differ in presence or concentration? Or does variation in one or very few seminal fluid proteins produce the observed differences in sperm performance? Furthermore, from an evolutionary perspective, we can ask: Why do the males differ at all? What constraints keep males from producing both high quality sperm, like the ‘sneakers’, and high quality seminal fluid, like the ‘guards’?
As an added bonus, I (bravely) searched YouTube for videos of Goby mating behavior and found this lovely video of a pair of Trimma Gobies, set to Adele’s Crazy for You.
Locatello, L., F. Poli, and M.B. Rasotto (2013) Tactic-specific differences in seminal fluid influence sperm performance. Proc R Soc B 280: 20122891.