Female meadow voles, Microtus pennsylvanicus, respond differently to the scent marks of multiple male conspecifics.

Many nonhuman animals are capable of discriminating a group or entity containing more objects from one containing less of the same objects. The capacity for making judgments of numerousness may also allow individuals to discriminate between potential mates. Female meadow voles (Microtus pennsylvanicus) may use judgments of relative numerousness to distinguish between potential suitors by selecting males that signal their interest by depositing more scent marks relative to other males. We used a familiarization-discrimination paradigm in the absence of training to test the hypothesis that female voles will discriminate between the different numerosities of scent marks of two male conspecifics that are similar in features of their phenotype and quality. During the exposure phase, we presented female voles with different ratios of feces scent marks from two males. During the test phase, we presented females with a single, fresh fecal scent mark from each of the two male donors, whose marks they had previously encountered during the exposure phase. In both phases, females spent more time investigating the scent mark(s) of the male that deposited more scent marks than that of the male that deposited fewer scent marks provided the difference in the ratio of scent marks provided by the male donors in the exposure phase was ≥2. Our results are consistent with studies on a variety of taxa, suggesting that numerosity discriminations are evolutionarily ancient and spontaneously available to nonhuman animals and humans.