Of mice and sperm

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Abstract
When females are sexually promiscuous, the ejaculates of different males may compete for the fertilization of a set of ova, a process known as sperm competition. In internally fertilizing species, potential for sperm competition occurs whenever the ejaculates of different males cooccur in the female reproductive tract at the time of ovulation. Sperm competition is now recognized as a powerful agent of evolutionary change, and the signal of its selective pressure is being detected on a number of male physiological and behavioral traits (1, 2). One of the major determinants of the outcome of sperm competition is the relative numerical representation of different ejaculates at the time of fertilization. All else being equal, the male that inseminates more sperm into a female has a fertilizing edge over his competitors (1, 3). The rate of spermatogenesis is determined by gonadal mass, and under more intense sperm competition we would expect males to grow relatively large testicles to inseminate more sperm. Evidence that sperm competition favors the evolution of larger testes relative to body mass (gonadosomatic index) comes from multiple comparative and experimental evolution studies across a number of taxa (1, 4–7). In addition to the number of sperm inseminated, the fertilizing efficiency of sperm also plays a critical role in sperm competition (8). Therefore, traits associated with the fertilizing performance of an ejaculate are also expected to evolve in response to sperm competition. The work by Gomendio et al. (9) in this issue of PNAS reveals that sperm competition may also have shaped the evolution of sperm function in eutherian mammals. Fertilization in eutherian mammals is a complicated business. After ejaculation, spermatozoa face two hierarchical challenges. First, sperm must undergo a process of maturation in the female uterus and oviduct. This process is known as capacitation …