Species specific differences of house mouse social behavior compared to its closest relatives (aboriginal species Mus macedonicus, Mus spicilegus, and Mus spretus) have recently been suggested. However, substantial variation of behavioral traits between mouse populations has been also evidenced. Agonistic behavior of laboratory-born house mice from five commensal populations (Mus musculus musculus: central Czech Republic, Mus musculus domesticus: Bulgaria, Greece, Turkey, and natural Mus m. musculus/domesticus hybrids from the Czech part of the hybrid zone) and five non-commensal populations of M. m. domesticus (C. Syria, E. Syria, Jordan, Iran, Libya) was studied. Dyadic interactions in a neutral cage were performed and the effects of sex and population on time spent by agonistic behavior evaluated. In all studied populations, the male-male interactions were more agonistic than the female-female ones. Male-male behavior gradually increased from the least agonistic population of M. m. musculus from Central Europe to the Near East populations of M. m. domesticus exhibiting the highest scores of agonistic behavior. Between-population differences were even stronger when female-female encounters were considered. While females of commensal populations belonging to both M. m. musculus and M. m. domesticus were tolerant of each other, those coming from non-commensal populations of M. m. domesticus were highly agonistic, reaching even the level of aggression between the females of some aboriginal mouse species. This phenomenon may be attributed to increased competition for food in non-commensal populations when compared to commensal ones supplied by superabundant resources. Social behavior of house mice, therefore, appears to be pliable rather than rigid and species specific. It can be changed rapidly according to ecological needs and such adaptability allows house mice to colonize various habitats. Aggr. Behav. 31:283-293, 2005. r

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We studied testes size in free-living and laboratory-born commensal and non-commensal populations of various Mus species (M. musculus musculus, M. m. domesticus, M. spicilegus, M. spretus, M. macedonicus, and laboratory mice). We found no apparent differences between wild-caught and laboratory-born individuals, or among commensal, non-commensal, and laboratory populations of M. musculus. There were, however, considerable differences among the species studied. The highest values of relative testes size were found in the aboriginal species M. spicilegus (4.4% and 2.9% for wild and laboratory populations, respectively), followed by those of M. macedonicus (from 1.7% to 0.9% for various samples) and M. spretus (1.5%). All thirteen samples representing various populations of Mus musculus exhibited smaller testes (0.7-1.0%), and finally the three lowest mean values came from laboratory mice (0.5-0.7%). It is very surprising that aboriginal species, in particular M. spicilegus, which is widely considered to be monogamous, have relatively larger testes than the polygynous/promiscuous M. musculus. This result is in apparent contradiction to the current views on evolutionary forces affecting testes size, and suggests that there could be another uncontrolled factor obscuring the relationship between testes size and multiple paternity. This raises a question concerning the proper interpretation of social organisation in the genus Mus.