Most male mammals have a penis bone, called a baculum. In fact, humans are the odd ones out for not having one. It’s well known that different mouse populations vary in average baculum size, and it has previously been shown that when a female mouse breeds with multiple males in the same fertile phase (mouse polygamy), the males with thicker baculae sire more offspring. This correlation suggests that penis, and baculum, morphology is under sexual selection. By this we mean that because mice with thicker baculae have more offspring, copies of genes that give rise to larger baculae would become more frequent in the population, and average baculum girth would increase. But, correlation is not causation, and so to test this hypothesis, scientists bred two mouse populations for 27 generations. One population they forced to remain monogamous, and in the other they forced polygamy by breeding the same female mouse to four males in rapid succession during each fertility phase. Only in the second population did they see the average baculum thickness increase, indicating that polygamy does select for thicker penis bones. So why do males with thicker penis bones sire more offspring? Nobody knows… yet!
“Male genitalia exhibit a taxonomically widespread pattern of rapid and divergent evolution. Sexual selection is generally believed to be responsible for these patterns of evolutionary divergence, although empirical support for the sexual selection hypothesis comes mainly from studies of insects. Here we show that sexual selection is responsible for an evolutionary divergence in baculum morphology among populations of house mice Mus domesticus. We sourced mice from three isolated populations known to be subject to differing strengths of postcopulatory sexual selection and bred them under common-garden conditions. Mice from populations with strong postcopulatory sexual selection had bacula that were relatively thicker compared with mice from populations with weak selection. We used experimental evolution to determine whether these patterns of divergence could be ascribed to postcopulatory sexual selection. After 27 generations of experimental evolution, populations of mice subjected to postcopulatory sexual selection evolved bacula that were relatively thicker than populations subjected to enforced monogamy. Our data thereby provide evidence that postcopulatory sexual selection underlies an evolutionary divergence in the mammalian baculum and supports the hypothesis that sexual selection plays a general role in the evolution of male genital morphology across evolutionary diverse taxonomic groups.”