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...