Size doesn't matter here!
Elephants tend to have smaller sperm than mice because the larger the animal, the more important the number of sperm is relative to sperm length, a new study suggests. In the animal world, if several males mate with the same female, their sperm compete to fertilise her limited supply of eggs. Longer sperm often seem to have a competitive advantage.
However, the study conducted by researchers from the Universities of Zurich and Stockholm now shows that the size of the animals also matters. The larger the animal, the more important the number of sperm is relative to sperm length. That is why elephants have smaller sperm than mice, researchers said. Stefan Lupold from the University of Zurich and his colleague John Fitzpatrick from University of Stockholm compared the influence of sperm competition on the evolution of sperm in 100 mammalian species.
Unlike previous studies, however, they did not 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. Based on their joint consideration of sperm size and number, and with the aid of new meta-analytical methods, the researchers now show 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, researchers said. 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 millimetre long and almost a thousand times shorter than those of the flies.