Are differences in variation and allometry in testicular size of two sibling species of the genus Mus (Mammalia, Rodentia) caused by female promiscuity?

Body and testes size can significantly affect male reproductive success under pre- and post-copulatory sexual selection. Testicular relative mass and/or volume are often used as a characteristic of sperm competitive ability in a comparison of phylogenetically close mammal species. Mus spicilegus males have the largest testes relative to body mass of any Mus species, which is often an indicator of high sperm competition. These findings suggest that these mound-building mice are probably not strictly monogamous. Here, we show the quantitative characteristics of testicular size, variation, and allometry of two sibling species with a different social and mating system, the house mouse (Mus musculus) and the mound-building mouse (M. spicilegus) from the Western Carpathian (Slovakia). We investigated whether testicular size (testicular length/width) was correlated with the head-and-body length and body weight, which are not involved in reproduction. Our results confirmed higher testicular values in M. spicilegus than in M. musculus. Similarly, the high phenotypic variance and positive allometry in testicular growth confirmed the suggestion that males with larger testes and a higher production of testosterone may be more competitive and more successful in post-copulatory selection.