The eleven different functions for which mammals use urine marking are reviewed in this paper, and the urine marking behavior of the red fox (Vulpes vulpes) is described in detail. A new hypothesis is advanced that urine marking may serve as a "book keeping system" in the red fox's scavenging behavior. Foxes consistently investigate and urine mark inedible food remnants (e.g., bones, bird wings, and dried out pieces of hide). When a fox re-investigates a marked remnant, the urine mark signals "no food present," and the fox investigates this object for only a brief period of time. This use of urine marking may increase the efficiency of its scavenging behavior, i.e. more food-items found per hour of scavenging. This efficiency may be particularly important during periods of food shortage. The hypothesis is tested in three different experiments, using free-ranging red foxes as subjects. Experiment I establishes that fox do urine mark food remnants. Experiment II shows that foxes investigate for a significantly shorter period of time (P<0.001) food remnants exhibiting both the odor of food and the odor of urine as compared to remnants exhibiting just the odor of food. Experiment III suggests that there a hierarchy of stimuli which determines different responses in the fox's scavenging behavior. The experiments also suggest that there is a degree of social behavior in the scavenging activities of red foxes. Foxes appear to use each other's urine marks to increase the efficiency of their scavenging behavior. Thus this study definitely support LEYHAUSEN'S (1965) statement that the social life of solitary animals is frequently more complex than we realize. Solitary species probably show many ingeniously adapted mechanisms for occupying niches where highly social species could not be maintained. The social evolution and ecological advantages of solitary species deserve to be the focus of future research.