Scent-marking by male mice under the risk of predation

The use by predators of scent marks made by potential prey is a largely une xplored potential cost of olfactory signaling. Here we investigate how anim als that differ in their investment in scent-marking respond to simulated p redation risk, by comparing the willingness to approach and counter-mark th e scent marks of a competitor in the presence or absence of predator odor. We aimed to test whether animals that invest heavily in scent-marking, and which may thus be more vulnerable to eavesdropping predators, will either ( 1) take greater risks to counter-mark the competitor's scent or (2) reduce or abandon scent-marking. Using outbred male laboratory mice, Mus musculus, we show that, in the absence of predators, individuals which initially sce nt-mark at high frequency (high markers) approach the competitor's scent ma rks more quickly and spend more time in counter-marking than those which in itially invest relatively little (low markers). In a sib-sib experimental d esign, simulated presence of predation risk (urine of ferrets, Mustela puto rius furo) caused both kinds of individual to approach the competitor's mar ks more slowly, but high markers approached more quickly than low markers a nd spent more time in the vicinity of the competitor's marks. Only high mar kers significantly reduced their overmarking of the competitor's scent. The se results suggest (1) that there is a unique danger inherent to scent-mark ing at high frequencies and (2) that high-marking males were prepared to ac cept increased costs of intrasexual competition in order to reduce the risk of predation. Further tests using the scent of naked mole-rats, Heteroceph alus glaber, showed that these effects were not elicited simply by an unfam iliar odor. We discuss reasons for the observed difference in response to p redation risk between the groups, and the implications of these results for counter-selection on scent-marking strategies.