Sperm are probably the most diverse cells in terms of size and shape and have been a continual source of fascination since their first discovery nearly 350 years ago. But why are sperm so incredibly different between species? After all, they all have the same job: to fertilize the female’s eggs. As we know from many studies, sperm competition plays a key role in the evolution of sperm. This contest occurs when a female mates with several males and their numerous sperm compete to fertilize her eggs. Longer sperm are often more competitive. Interestingly, this is more common in small rodents, such as mice and rats, than in larger animals. The rodents’ sperm are also often twice as long as those of the considerably larger carnivores, ungulates, primates or even whales. The reason for this, however, is disputed.
Taking sperm number and length into account
A new study could now help shed light on the matter. Stefan Lüpold, a new member of the Department of Evolutionary Biology and Environmental Studies, University of Zurich, and his colleague John Fitzpatrick, University of Stockholm, compared the influence of sperm competition on the evolution of sperm in 100 mammalian species. Unlike previous studies, however, they didn’t just consider sperm length, but also the number of sperm per ejaculate, which is important as the resources available for sperm production need to be shared between sperm size and number. In other words, the longer every individual sperm, the fewer of them a testicle of a certain size can produce. Earlier studies suggested that the number of sperm might be just as important as sperm length, if not even more so. After all, the more sperm a male fields against his competitors, the greater the likelihood that one of them will win.
Animal size is relevant
Based on their joint consideration of sperm size and number, and with the aid of new meta-analytical methods, the two researchers now reveal that species facing intense sperm competition invest more in their ejaculates on average than their monogamous counterparts. Moreover, they discovered that whether the length or the number of sperm is more important actually depends on the size of the animals. The bigger the animal, the greater the selection pressure on the overall investments in ejaculates and the more important the number of sperm becomes compared to sperm length. This is due to the more voluminous female reproductive tract, in which the sperm tend to get lost or become “diluted”.
In larger species, sperm length or speed probably comes into effect only if a sufficient number of sperm manage to get near the egg. In smaller species, however, the distance for sperm to cover is shorter and the risk of loss much smaller, allowing the advantage of longer sperm to manifest itself. As a result, you tend to find the most complex sperm forms in small species, not in large ones. For instance, small fruit flies have the longest sperm ever described, not whales, whose sperm are less than a tenth of a millimeter long and almost a thousand times shorter than those of the flies.
Lüpold S. & Fitzpatrick JL. Sperm number trumps sperm size in mammalian ejaculate evolution. Proceedings of the Royal Society of London B. Doi: 10.1098/rspb.2015.2122