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Ground scratching as part of a composite signal
I'm often asked why dogs occasionally scratch the ground after they pee or poop. I've also been asked why a dog sometimes will scratch the ground without having previously peed or pooped. A few weeks ago a woman at a local dog park asked me why her mutt sometimes will scratch the ground after peeing, and today I received an email from Salima Brown that began, "I have an intact 5 year old male Labrador retriever who scrapes enthusiastically, often making the most hilarious deep-chest rumbling growls that sound, nevertheless, ecstatic. And the expression on his face looks like laughter. I have often wondered whether the scraping is really to spread scent, but not the smell of the poop or pee, but of the sweat on the paw pads."
Ms. Brown's note got me rethinking about ground scratching, because the reason(s) dogs do this remain somewhat mysterious. In Unleashing Your Dog: A Field Guide to Giving Your Canine Companion the Best Life Possible, Jessica Pierce and I note that dogs also often scratch the ground after peeing or pooping. Dogs have scent glands in their paws, and when they scratch, they might be trying to send an olfactory message to other dogs by spreading the scent from their paws or by sharing the odor of the pee or poop they deposited. Scratching also leaves a visual mark on the ground. Ground scratching could be yet another form of social communication, and taken together, peeing, pooping, and ground scratching are a good example of how dogs may use what ethologists call composite signals to enhance their messages to other dogs, by using both olfactory and visual components. And, as Ms. Brown points out, there might be an auditory component as well.
Salima Brown's note motivated me to revisit a research paper I published in 1979 called "Ground Scratching by Male Domestic Dogs: A Composite Signal," most of which is available for free online. The part that is missing is cited here as my summary of the research in italics, and the references for this essay can be seen in Note 1 below. My students and I learned:
"GS [ground scratching] by male dogs after RLU marking [RLU = Raised Leg Urination during which a dog raises a leg and urinates], RLD = Raised Leg Display during which a dog raises a leg and does not deposit any noticeable urine, often called dry marking], or simple urination was influenced by the presence of other dogs. Because GS did not invariably follow these behaviors even in the presence of other dogs, it does not seem to be essential to convey the message 'urine was just deposited.' However, GS most likely conveys this information, because it rarely occurred before, or in the absence of, these behaviors. GS after the RLD might communicate an erroneous message, since no urine was deposited. However, scent from interdigital glands also might have been deposited and a visible slash produced." (See "When Small Dogs Pee, Are They Saying It's Really Not Me?" for more detailed discussion of different patterns of urination.)
I concluded:
"Ground scratching may be viewed as a composite signal (Smith, 1977; Wickler, 1978) combining chemical and visual components that carry information over different distances at different speeds. For example, GS may serve to communicate immediately, over long distances, that urine was deposited, thereby extending the more limited range of the urine scent, the visible slash on the substrate, or scent from interdigital glands. GS may also be an intimidation display (Ewer, 1968). Visual aspects of scent deposition must be studied as well as the olfactory effects of various chemicals. In many species, selection for conspicuous marking behaviors that were not required for deposition of a particular scent (e.g., male canids urinate without performing the raised leg posture) is obvious (Hediger, 1949)."
Renowned ethologist Dr. R. F. Ewer's observations that ground scratching might be an intimidation display are very interesting. My friends Tyla and John Talley told me that their dog Rigby never ground scratched before another dog, Bodie, joined their household, after which Rigby began doing it regularly, even if Bodie wasn't around. (See Canine Confidential: Why Dogs Do What They Do and "Pissing Matches in Dogs: Territorial, Lots of Fun, or Both?")
In the essay, "Ground Scratching: Why Does My Dog Do It?" Eileen Anderson nicely summarizes my study and others, provides some useful references, and discusses the behavior of her dog, Summer. She writes, "My dog Summer is a 'scratcher' and she does it with what I observe to be exuberance and satisfaction. [You’ll see in the movie.] Interestingly, she doesn’t scratch only after eliminating. She will also scratch where there are scents of another dog’s elimination. Summer also lifts her leg to mark with urine. More on that later." She also writes, "…It has been suggested that the scratching action itself may leave scent in the environment produced by either interdigital glands, sweat glands on the foot pads, or sebaceous glands in the fur between the toes.”
If a dog is a ground scratcher, let them do it
My own and others' observations, including Ms. Brown's and Ms. Anderson's, show that in addition to any messages that might be sent when dogs engage in ground scratching, dogs often enjoy doing it and that it's important to let them do it. Ms. Anderson uses the words "exuberance and satisfaction" and Ms. Brown writes that her dog makes "the most hilarious deep-chest rumbling growls that sound, nevertheless, ecstatic." Being allowed to ground scratch seems to be very important to dogs who do it so, when they're leashed or unleashed, let them do it. One of the dogs with whom I shared my home scratched so vigorously he often fell over as he did it. He appeared to be having the time of his life as he was lost in the moment. In Unleashing Your Dog: A Field Guide to Giving Your Canine Companion the Best Life Possible Jessica Pierce and I write, "...let your dog finish their message—give them time to scratch after they have peed or pooped—before continuing your walk."
The best explanation for ground scratching by male dogs, and perhaps females as well, is that it likely has different functions depending on who is doing the ground scratching, who else is around, and what they're trying to communicate, if anything, to others. There are visual, olfactory, and possibly auditory components that might serve as a composite signal and some dogs might simply like to do it. Composite signals contain information from different sensory modalities and may contain more information than signals in a single sensory modality. (See "Why Dogs Growl" for more.)
We still don't know much about ground scratching by females, however, Ms. Anderson's observations of Summer's behavior offer some insights. Along these lines, in a study of free-ranging dogs, my students and I found that males ground scratched significantly more than females after marking and males did it significantly more when other dogs could see them do it. (See Bekoff, Marc. “Scent-Marking by Free Ranging Domestic Dogs: Olfactory and Visual Components.” Biology of Behavior 4, 123–39, 1979).
Stand by for more discussions of why dogs ground scratch. There still is much to learn and it's clear that there is much more going on than meets our eye.
Literature cited in "Ground Scratching by Male Domestic Dogs: A Composite Signal"
BEKOFF, M. 1979. Scent marking by free ranging domestic dogs: olfactory and visual components. Biol. Behav., 4:123-139. BIRCH, M. 1974. Pheromones. Elsevier, New York, 495 pp. EISENBERG, J. F., AND D. G. KLEIMAN. 1972. Olfactory communication in mammals. Ann. Rev. Ecol. Syst., 3:1-32. EWER, R. F. 1968. Ethology of mammals. Plenum Press, New York, 418 pp.; 1973. The carnivores. Cornell Univ. Press, Ithaca, New York, 494 pp. HEDIGER, H. 1949. Saugetierterritorien und ihre Markierungen. Bilds. tot de Dierdke, 28:172-184. JOHNSON, R. P. 1973. Scent marking in mammals. Anim. Behav., 21:521-535. KLEIMAN, D. G. 1966. Scent marking in the Canidae. Symp. Zool. Soc. Lond., 18:167- 177. MÜLLER-SCHWARZE, D., AND M. M. MOZELL. 1977. Chemical signals in vertebrates. Plenum Press, New York, 609 pp. MYKYTOWYCZ, R. 1972. The behavioural role of the mammalian skin glands. Die Naturwiss., 59:133-139. PETERS, R. P. 1974. Wolf sign: scents and space in a wide-ranging predator. Unpubl. Ph.D. dissert., Univ. Michigan, Ann Arbor, 227 pp. PETERS, R. P., AND L. D. MECH. 1975. Scent-marking in wolves. Amer. Sci., 63:628-637. SCOTT, J. P. 1967. The evolution of social behavior in dogs and wolves. Amer. Zool., 7:373-381. SEIDENSTICKER,J. C. IV, M. G. HORNOCKER, W. V. WILES, AND J. P. MESSICK. 1973. Mountain lion social organization in the Idaho primitive area. Wildl. Monogr., 35:1-60. SMITH, W. J. 1977. The behavior of communicating. Harvard Univ. Press, Cambridge, Massachusetts, 545 pp. SNEDECOR, G. W. 1956. Statistical methods. Iowa State Univ. Press, Ames, 534 pp. SOKAL, R. R., AND F. J. ROHLF. 1969. Biometry: the principles and practice of statistics in biological research. W. H. Freeman, San Francisco, California, 776 pp. SPRAGUE, R. H., AND J. J. ANISKO. 1973. Elimination patterns in the laboratory beagle. Behaviour, 47:257-267. VON UEXKÜLL, J., AND G. E. SARRIS. 1931. Das dutfeld des Hundes. Z. Hundesforschung, 1:55-68. WICKLER, W. 1978. A special constraint on the evolution of composite signals. Z. Tierpsychol., 48:345-348.
