AVOIDANCE OF SCENT-MARKED AREAS DEPENDS ON THE INTRUDERS 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 signal ler, should depend on the benefits of intruding (using the defended re sources) less the costs (risk of detection, and of damage if attacked) . Costs are partly dependent on the intruder's competitive ability, an d, assuming a link between competitive ability and body size, we teste d the prediction that male house mice of low body weight should be mor e 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 isola tion before testing, a state that promotes high androgen levels. Subje cts had never met the donors and so the effect of previous contests, i ncluding any with the signaller, could be excluded. As predicted, ligh t males avoided the artificially scent-marked area, both in a sample o f 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 sign aller, marks indicate a resource that is worth defending.