In the animal world, if several males mate with the same female, their sperm compete to fertilize her limited supply of eggs. Longer sperm often seem to have a competitive advantage. However, a study conducted by researchers from the Universities of Zurich and Stockholm now reveals that the size of the animals also matters. The larger the animal, the more im-portant the number of sperm is relative to sperm length. That's why elephants have smaller sperm than mice. 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 Depart-ment 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 sug-gested 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.