Avoidance of scent-marked areas depends on the intruder's body size

By themselves, the scent-marks in the territories of male mammals can provide only general information about the signaller (such as sex and androgen levels). Whether intruders use this information and avoid the area, or only leave after obtaining more information about the signaller, should depend on the benefits of intruding (using the defended resources) less the costs (risk of detection, and of damage if attacked). Costs are partly dependent on the intruder's competitive ability, and, assuming a link between competitive ability and body size, we tested the prediction that male house mice of low body weight should be more likely to avoid scent-marked areas than males of high body weight. A modified Y-maze was used to examine the males' initial decision about whether or not to enter an artificially scent-marked tunnel. Subjects and donors of urine for the artificial scent marks were kept in isolation before testing, a state that promotes high androgen levels. Subjects had never met the donors and so the effect of previous contests, including any with the signaller, could be excluded. As predicted, light males avoided the artificially scent-marked area, both in a sample of young (88-108 days) and older (140-160 days) adult mice. Heavy mice were attracted to the artificially marked substrate, possibly because, to males that are less likely to be damaged in contests with the signaller, marks indicate a resource that is worth defending.