Scent-marking by male mice under the risk of predation

The use by predators of scent marks made by potential prey is a largely unexplored potential cost of olfactory signaling. Here we investigate how animals that differ in their investment in scent-marking respond to simulated predation risk, by comparing the willingness to approach and counter-mark the 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 scentmark at high frequency (high markers) approach the competitor's scent marks more quickly and spend more time in countermarking than those which initially invest relatively little (low markers). In a sib-sib experimental design, simulated presence of predation risk (urine of ferrets, Mustela putorius furo) caused both kinds of individual to approach the competitor's marks more slowly, but high markers approached more quickly than low markers and spent more time in the vicinity of the competitor's marks. Only high markers significantly reduced their overmarking of the competitor's scent. These results suggest (1) that there is a unique danger inherent to scent-marking at high frequencies and (2) that high-marking males were prepared to accept increased costs of intrasexual competition in order to reduce the risk of predation. Further tests using the scent of naked molerats, Heterocephalus glaber, showed that these effects were not elicited simply by an unfamiliar odor. We discuss reasons for the observed difference in response to predation risk between the groups, and the implications of these results for counter-selection on scent-marking strategies