This study investigated how voles (Microtus pennsylvanicus) determine which scent is the top scent of an over-mark and how this information influences memory for the top scent. This series of experiments shows that a region of overlap is necessary for both a preference and a differential memory for 1 of the 2 scents, and a difference in the area covered by one of the two overlapping scents is not sufficient by itself to cause these effects. These results suggest that voles have specialized perceptual and evaluative mechanisms for the analysis of scent over-marks. Thus, voles may attach a greater importance to the odor of the top-scent donor than that of the bottom-scent donor of an over-mark.Animals often have to deal with a complex array of social signals to differentiate between conspecifics. For example, meadow voles, Microtus pennsylvanicus, and golden hamsters, Mesocricetus auratus, both have at least five different sources of individually or sexually distinctive odors (Ferkin

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