The long and the short of it: new insights on sperm length help demystify the complexities of sexual selection

H sperm are approximately 6000th of a centimeter long, a small fraction of a man’s total body length. By contrast, fruit fly (Drosophila spp.) sperm can reach nearly 6 cm, roughly twenty times the total length of their bodies. This dramatic variation in male reproductive biology is explored in a recent paper from the journal Nature.1 While the literature on sperm competition has for decades emphasized the fitness benefit males of many species accrue by producing small gametes in large quantities, understanding species whose males produce large gametes in small quantities while remaining competitive for fertilizations has until recently proven more difficult. Stefan Lüpold and colleagues suggest a solution to this “big‐sperm paradox.” Most sexually reproducing organisms exhibit two discrete sexes, defined by the type of gamete they produce: males produce many small sperm while females produce fewer, but larger, ova. As such, the number of ova constrains the possible number of offspring that can be conceived. In turn, males must often compete for mates2 and consequently exhibit greater variance than do females in the number of offspring they are able to produce; some males manage many mating partners, others none.3 This competition drives sexual selection,4 the type of evolution via natural selection favoring traits that aid their bearers in winning mating opportunities, thus increasing their reproductive fitness. An outcome of this process is the well-documented cross-species sex difference of greater male The long and the short of it: new insights on sperm length help demystify the complexities of sexual selection