Scent marking in shelter dogs: Effects of body size

•Scent marking allows communication without direct interaction.•Most evidence indicates more frequent marking by more competitive individuals.•We scored scent-marking behavior of 281 shelter dogs that varied in body size.•Small dogs urinated at higher rates and directed more urinations than did large dogs.•We suggest small dogs prefer scent marking because direct interactions are risky. Placing scent marks in the environment allows individuals to transfer information without direct interactions. Given that body size often indicates competitive ability, small individuals may preferentially communicate via scent marking because direct social interactions are potentially costly. However, most evidence indicates a positive relationship between competitive ability and frequency of scent marking. Domestic dogs (Canis lupus familiaris) exhibit extreme morphological variation, which allowed us to examine whether scent-marking behavior varied with body size in shelter dogs. We observed 281 dogs on 20-min walks and recorded total urinations, urinations directed at targets in the environment, and defecations. Some dogs were walked once and others multiple times (total walks, 619). We found that size class influenced rate of urination (P=0.002): small dogs urinated at higher rates (0.36 urinations per min) than both medium (0.26) and large dogs (0.24). There was a tendency for size class to influence percent of directed urinations (P=0.057): small dogs directed more of their urinations at targets in the environment (72%) than did large dogs (60%). Consistent with previous reports, we found that males urinated at higher rates (0.41) than females (0.18; P