Excluding neighbours from territories: effects of habitat structure and resource distribution

Small animals vulnerable to predation, such as rodents, have a strong preference for sites that provide physical protection from predators. This is likely to affect not only their use of space and activity but also the ease with which they can defend a territory, since the likelihood of encountering (or losing) intruders and their willingness to compete are affected by the quality and distribution of resources and the structural complexity of the habitat. To examine how these different habitat factors interact to influence territorial behaviour in male house mice, Mus domesticus, which inhabit environments with very different levels of complexity and resource distribution, we housed male-female pairs in enclosures representing one of eight habitat types varying in ground-level structure (open/complex), overhead cover (present/absent) and distribution of protected nest sites and food (resources clumped together/scattered). Neighbouring pairs were allowed to interact five times over 3 days and we examined behaviour during the first (unfamiliar) and fifth (familiar) periods. Initially, encounter rates were two to three times higher in open habitats with overhead cover than in either complex habitats or open habitats without cover, and higher when resources were scattered than when they were clumped. Aggressive interactions between unfamiliar males were more prolonged in habitats with open ground-level structure, where pursuits followed restricted pathways. The effects of overhead cover on aggression among unfamiliar neighbours unexpectedly depended on the origin of the mice. Once neighbours learnt the outcome of their interactions, aggressive interactions were most prolonged in habitats with scattered resources and complex ground-level structure, making these habitats the most difficult to defend.