Sexual selection and the rodent baculum: an intraspecific study in the house mouse (Mus musculus domesticus)

Ramm SA, Khoo L, Stockley P. Sexual selection and the rodent baculum: an intraspecific study in the house mouse (Mus musculus domesticus). Genetica. 2010;138(1):129-137.The rapid divergence of genitalia is a pervasive trend in animal evolution, thought to be due to the action of sexual selection. To test predictions from the sexual selection hypothesis, we here report data on the allometry, variation, plasticity and condition dependence of baculum morphology in the house mouse (Mus musculus domesticus). We find that that baculum size: (a) exhibits no consistent pattern of allometric scaling (baculum size being in most cases unrelated to body size), (b) exhibits low to moderate levels of phenotypic variation, (c) does not exhibit phenotypic plasticity in response to differences in perceived levels of sexual competition and (d) exhibits limited evidence of condition dependence. These patterns provide only limited evidence in support of the sexual selection hypothesis, and no consistent support for any particular sexual selection mechanism; however, more direct measures of how genital morphology influences male fertilization success are required

Baculum morphology predicts reproductive success of male house mice under sexual selection

Stockley P, Ramm SA, Sherborne AL, Thom MDF, Paterson S, Hurst JL. Baculum morphology predicts reproductive success of male house mice under sexual selection. BMC Biology. 2013;11(1): 66

Genital morphology linked to social status in the bank vole (Myodes glareolus)

Lemaitre J-F, Ramm SA, Jennings N, Stockley P. Genital morphology linked to social status in the bank vole (Myodes glareolus). Behavioral Ecology and Sociobiology. 2012;66(1):97-105.Since genital morphology can influence the outcome of post-copulatory sexual selection, differences in the genitalia of dominant and subordinate males could be a factor contributing to the fertilisation advantage of dominant males under sperm competition. Here we investigate for the first time if penile morphology differs according to male social status in a promiscuous mammal, the bank vole (Myodes glareolus). In this species, dominant males typically achieve higher reproductive success than subordinates in post-copulatory sexual selection, and male genital morphology is complex, including both a baculum (os penis) and penile spines. Our results show that despite no difference in body size associated with male social status, baculum width is significantly larger in dominant male bank voles than in subordinates. We also found evidence of positive allometry and a relatively high coefficient of phenotypic variation in the baculum width of male bank voles, consistent with an influence of sexual selection. By contrast, baculum length and three measures of penile spinosity did not differ according to male social status or show evidence of positive allometry. We conclude that dominant male bank voles may benefit from an enlarged baculum under sperm competition and/or cryptic female choice and that differences in penile morphology according to male social status might be important but as yet largely unexplored source of variation in male reproductive success