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Behavioural mechanisms underlying the spatial dispersion of commensal
 Mus domesticus
 and grassland
 Mus spretus
 


Mus domesticus usually exists commensally with humans, often reaching high densities in areas containing concentrated food resources. Mus spretus exists in vegetational habitats with generally scattered and seasonal food supplies. Two behavioural mechanisms could be responsible for determin- ing their spatial dispersion: (1) resident territory holders aggressively exclude intruders from attractive areas (aggressive exclusion hypothesis), or, (2) animals spread out to exploit scattered resources and avoid dominant competitors occupying preferred sites (avoidance of dominant competitors hypothesis). Previous results from M. spretus supported the avoidance of dominant competitors hypothesis. An identical series of tests were repeated on commensal M. domesticus to allow direct comparison of the competitive behaviour of the two species. WhileM. spretusare strongly attracted to nest sites containing conspecific odour and, subsequently, are more aggressive towards the owner than towards another unfamiliar conspecific, M. domesticus showed no such attraction or discrimination. In neutral (clean) enclosures, M. domesticus dyads were more aggressive than M. spretus dyads, while the opposite was true when one opponent had been resident for 18 h. In a resident's enclosure, resident status determined which M. domesticus individual was aggressive, resulting in significant diVerentiation in competitive behaviour between resident and intruder. No such diVerentiation was found within M. spretus dyads, where relative weight was the important factor determining aggressive behaviour. Results from M. domesticus support the aggressive exclusion hypothesis, suggesting that there may be important diVerences between Mus species, or between environments, in the social behaviour that underlies their spatial dispersion. ? 1997 The Association for the Study of Animal Behaviour