In the competition for a partner, males typically have to vie with each other – be it with a colorful plumage, a large set of antlers or a seductive courtship dance. The females of some species, however, copulate with several males, so that rivals even after mating are still not defeated. So their sperm become rivals. Because greater size can increase the chance of fertilization, in some species truly giant sperm cells have evolved – some grow to be even larger than the male that produced them. (…)
This has led to some true giants evolving along the way. A human sperm would have to be 40 meters long in order to measure up against Drosophila bifurca, for example: the males of this fruit-fly are only a few millimeters in size, but produce giant sperm around six centimeters long. Also other insects, as well as some primates, birds and worms are known for the production of giant sperm. Another example is one group of ostracods, whose sperm are up to ten times as big as the animals themselves. These aquatic crustaceans typically grow to only a few millimeters, and are – much like mussels – surrounded by a bivalve-like calcareous shell.
We all know that it’s only the genes which matter biologically, with the organism just being a vehicle for them to get around. I like the illusion of being the boss, though. If the mechanism for combining my genes with those of another organism can beat me in a fight, that illusion is gone.