SEXUAL SIZE DIMORPHISM IN FREE-LIVING POPULATIONS OF MUS MUSCULUS: ARE MALE HOUSE MICE BIGGER?

We studied sexual size dimorphism (SSD) in captive house mice derived from free-living commensal (inhabitants of buildings and stores) and non-commensal (field and/or semidesert dwellers) populations. While females of commensal populations and most strains of laboratory mice are more or less tolerant to each other, females from non-commensal populations are as highly aggressive as their male conspecifics. As body size considerably contributes to fighting success, we addressed the question whether sexual size dimorphism in commensal mice, with larger males, can be attributed to the switch to the commensal way of life. For this purpose, we performed a laboratory common garden experiment in which non-commensal populations of Mus musculus domesticus from Jordan and SW Iran were compared with Greek commensal mice belonging to the same subspecies. M. m. musculus and natural hybrids of these subspecies from the Czech Republic were also included. Growth was recorded for 102 litters and 592 juveniles born during the experiments, and SSD calculated on the basis of within litter comparisons between the sexes. Males were considerably larger (SSD = 1.05) at the age of 35 days. Newborn males tended to be larger than newborn females but this tendency rapidly disappeared during the early postnatal period. Starting from a nearly monomorphic state at the age of two weeks, size difference between the sexes was established gradually up to the end of the experimental period. We found no significant differences in SSD among the studied populations at any age. Hence, we suppose that the present SSD in the house mouse could be explained by selective forces operating in the non-commensal way of life, which is ancestral with respect to the commensal one.