Scent marking is a common form of communication across a wide range of mammals. Although dogs can scent mark in various ways, they most often use urine, which is obvious to anyone who has watched dogs pee here, there and everywhere out on walks or during play time.
Urination, and other forms of scent marking, allow animals to convey a large amount of information in an indirect manner. That means that they can communicate without direct interactions. That has the advantage of avoiding the costs of social interactions, which can include stress, the energetic costs of interacting and potential injury. In many species, body size is closely correlated with competitive ability, which is why scent marking may be especially important to smaller individuals, who may be unlikely to fare well in direct encounters.
Dogs have an enormous size range for a single species, but only recently has the effect of size on frequency of scent marking been investigated. Researchers wondered whether smaller dogs take advantage of the indirect nature of scent marking through urine to be more competitive with larger dogs.
In the recent study, “Scent marking in shelter dogs: Effects of body size”, researchers walked 281 shelter dogs (mostly mixed breeds) that they categorized by size. Small dogs measured 33 cm or less at the withers, large dogs measured 50 cm or more, and medium dogs were above 33 cm but less than 50 cm. They recorded urinations during the first 20 minutes of each walk, noting whether they were directed at a target or not. (Targeted urinations were those that occurred after sniffing a spot on the ground or on some other surface, and those that involved urinating somewhere other than the ground even without sniffing it first.) The study found that smaller dogs marked more often than medium or large dogs and that they were more likely to direct their urine at targets compared to large dogs. Though smaller bladder capacities of smaller dogs could explain increased frequency of urination, that cannot account for the increased frequency of urinating on targets.
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Small Dogs, Small Bladders
The higher rates by small dogs could reﬂect the known relationship between bladder capacity and body size: bladder capacity increases with body mass in mammals (Yang et al., 2014). Thus, the higher rate of urination by small dogs could be due to small dogs having smaller bladders than medium and large dogs. However, factors other than bladder capacity must inﬂuence rate of urination as evidenced by the consistent sex difference within each size class in our study: on average, rates of urination for males were at least twice those of females. Additionally, bladder capacity cannot explain why, when compared with large dogs, small dogs also directed more of their urinations at targets in the environment. – Betty McGuirea and Katherine E. Bemis
As expected, males also marked more frequently and directed their urine at targets more often than female dogs did. The length of time that dogs had spent in the shelter was positively associated with frequency of directed urinations, but not with total number of urinations. Size had no effect on the frequency of defecations on walks, but dogs who had been at the shelter longer were a little bit more likely to defecate on walks.
The study notes, “Our ﬁndings that rate of urination and percentage of directed urinations during walks were higher in small dogs than large dogs are consistent with those of McGreevy (2013) who surveyed dog owners about problematic in-home behaviors associated with urination. These authors reported that urination when left alone, urine marking, and emotional urination were more common in smaller dogs (height used as a measure of body size).”
The authors concluded that smaller dogs use scent marking in the form of urination more frequently that medium or large dogs. It is possible that they are using scent marks in order to avoid direct interactions.