Category: Ethology

Does My Ultrasonic Humidifier Hurt My Dog’s Ears?

Does My Ultrasonic Humidifier Hurt My Dog’s Ears?

ultrasonic humidifier with a white base and a clear blue plastic top

No, Your Ultrasonic Humidifier Doesn’t Hurt Your Dog’s Ears

The mechanism in an ultrasonic humidifier has a frequency much too high for dogs to hear. Ultrasonic humidifiers use frequencies ranging from approximately 1,600,000 Hz to 3,000,000 Hz. Dogs can hear up to 45,000 Hz. The sound produced by this very high-frequency device is profoundly out of hearing range for both dogs and humans.

Although sounds outside our hearing range can in some cases damage humans’ ears and possibly dogs’, I’ve seen this documented only for extremely low-frequency sounds (Kugler et al., 2014), not high.

Humans tend to assign a glamour around the fact the dogs can hear in a higher frequency range than we can. Maybe it’s mysterious because we don’t know what’s going on up there? We feel like anything could be happening since we can’t hear it! Whatever the reason, there is a ton of misinformation online about dogs’ responses to high-frequency noises. I’m tackling this myth about ultrasonic humidifiers first.

This post includes a lot of discussion of sound frequency; if you need a review of the concept, check out my post that includes an explanation. Also, you will see me writing out the numerals for frequencies in this piece rather than using the common scientific shorthand. For instance, I will write 1,600,000 Hz instead of 1.6 MHz. I want the magnitude of the numbers to be clear to all readers.

What Is Ultrasound and Can Dogs Hear It?

Ultrasound is defined as sound higher than 20,000 Hz. That base frequency is the approximate upper limit of human hearing.

But the term “ultrasound” has two common usages, and this causes confusion.

One usage is to refer to frequencies in the range immediately above the limit of human hearing. Sometimes an upper limit of this “lower” ultrasound is given as 25,000 Hz or 40,000 Hz. Dogs can hear in this range. I’ve also seen “low-frequency ultrasound” defined as up to 100,000 Hz.

That’s the first usage, and you can see it’s a little fuzzy.

The other usage of “ultrasound” refers to very high-frequency sound in the millions of Herz. These are the frequencies of ultrasound often used in medicine and industry.

These two usages often result in people worrying that dogs can hear up in the millions of Herz range, but they can’t.

Dogs with normal hearing can definitely hear sounds above 20,000 Hz, as in the first usage. Their hearing range tops out at about 40,000–45,000 Hz (Heffner, 1983). They can’t even come close to hearing sound with frequencies of a million Herz.

For a complete comparison of dogs’ hearing with that of humans, check out my blog post on the topic.

What Frequencies Do Ultrasonic Humidifiers Use?

ultrasonic humidifier with a white base and a clear blue plastic top with mist coming out of it

Ultrasonic humidifiers have a vibrating plate that creates ultrasonic waves ranging from 1,600,000 to 3,000,000 Hz (Al-Jumaily & Meshkinzar, 2017; Yao et al., 2019; Yao, 2016).

The function of ultrasound in humidifiers is to atomize the water in the tank into tiny droplets, creating a mist.

In general, the higher the frequency of the ultrasonic vibration, the smaller the droplets produced.

To reiterate, the ultrasound frequencies used by humidifiers are far too high for dogs to hear. A humidifier using a 1,600,000 Hz mechanism is operating at a frequency 36 times the upper limit of dogs’ hearing.

We are not even talking about the same ballpark.

Can Ultrasound Cause Damage to Dogs’ Ears?

It could, in the lower range of ultrasound I’ve discussed. But such a sound would rarely be encountered, and it wouldn’t be coming from a humidifier.

The important factors in causing ear damage are the sound pressure level (SPL) experienced by the individual and the duration—not the frequency. So higher frequencies are not intrinsically worse for dogs’ ears. A noise in the ultrasound range would need to be very loud to cause damage, just as is the case in other ranges of the sound spectrum.

Ultrasound in the lower range can also be damaging if it is focused by a medical or industrial instrument. For example, ultrasound around 25,000 Hz is finely targeted to break up kidney stones in humans. This is a frequency dogs can hear, but what are the odds of a direct, focused exposure to a dog’s ear?

Neither of these cases apply to humidifiers because of the difference in frequency range, and would rarely be encountered by humans or dogs in day-to-day life.

I haven’t found any literature indicating that ultrasound in the millions of Herz would cause ear damage to dogs or humans. I’ll be looking further to make sure, but my guess is that it’s not something to worry about, for two reasons. First, it would be rare to encounter a loud sound source in that frequency range. That takes some extremely specialized equipment. Second, sound waves at ultra-high frequencies dissipate and attenuate (roughly, they scatter and get quieter) very fast as they travel through air.

In a future post, I will review possible sources of psychological irritation from ultrasound. There are indeed sounds in the lower ultrasound range that your dog might hear and find irritating or scary even though they don’t damage his ears. I’ll discuss ways to detect sounds in this range in your home. I’m not including these topics here because they don’t relate to the ultrasound frequencies humidifiers use.

tan dog wearing hearing protection: Mutt Muffs

Consumer Cautions about Ultrasonic Humidifiers

If you read up on the safety of ultrasonic humidifiers, you will find lots of cautions about the fact that they can aerosolize mold, bacteria, and even minerals in water (Environmental Protection Agency, 1991; Yao et al., 2019; Dietrich, Yao, & Gallagher, 2022). These cautions actually made me start cleaning out my humidifier more often and I plan to get distilled water to use instead of tap water.

In the safety instructions, you will not find any cautions about the ultrasounic waves.

Why Did I Even Write about This?

Search the title of this post in Google and you’ll see. There is so much misinformation online about dogs and high frequencies. This post is just a start. Next, I’ll be discussing the erroneous belief that somehow, all sounds with frequencies in the upper half of dogs’ hearing range are intrinsically annoying, painful, or harmful to them.

Copyright 2023 Eileen Anderson

Other Dogs and Sound Posts

Impulse Sounds and the Startle Reflex: Why Some Dogs Fear the Clicker Sound

How to Soundproof a Dog Crate

• Review of Pawnix Sound Cancelling Headphones for Dogs: Unlikely to Work as Described

• My webinar on dogs and sound: Sound Decisions

• How to Tone Down That Plastic Collar Click (and Why)

• How I Taught My Dog to Love the Sound of Velcro

• Using Sound Masking to Protect Your Dog from Loud, Scary Sounds

• Are Dogs Ever Irritated by Sights, Sounds, or Smells?

• 6 Ways to Prepare Your Dog for Fireworks

References

Al-Jumaily, A. M., & Meshkinzar, A. (2017). On the development of focused ultrasound liquid atomizers. Advances in Acoustics and Vibration2017.

Dietrich, A. M., Yao, W., & Gallagher, D. L. (2022). Exposure at the indoor water–air interface: Fill water constituents and the consequent air emissions from ultrasonic humidifiers: A systematic review. Indoor air32(11), e13129.

Environmental Protection Agency. (1991). Indoor Air Facts No. 8: use and care of home humidifiers.

Heffner, H. E. (1983). Hearing in large and small dogs: Absolute thresholds and size of the tympanic membrane. Behavioral Neuroscience97(2), 310.

Kugler, K., Wiegrebe, L., Grothe, B., Kössl, M., Gürkov, R., Krause, E., & Drexl, M. (2014). Low-frequency sound affects active micromechanics in the human inner ear. Royal Society Open Science1(2), 140166.

Yao, Y. (2016). Research and applications of ultrasound in HVAC field: A review. Renewable and Sustainable Energy Reviews58, 52-68.

Yao, W., Gallagher, D. L., Marr, L. C., & Dietrich, A. M. (2019). Emission of iron and aluminum oxide particles from ultrasonic humidifiers and potential for inhalation. Water research164, 114899.

Dog Facial Expressions: Can You See the Stress?

Dog Facial Expressions: Can You See the Stress?

A white dog with brown ears lies on a purple mat in a vet clinic. The muscles in the dog's face are very tight and bunched up.

In February 2013, I published a set of photos of formerly feral Clara at the vet. Trainers worldwide have used those photos, with my permission, as examples of extreme stress in a dog’s facial expressions.

Clara was terribly afraid. She panted, paced, and panicked. We were working on desensitization and counterconditioning to people slowly, in much more controlled situations. But every once in a while she had to go to the vet, and we just had to get through it.

Her fear and panic were obvious.

The photos of 16-month-old Lewis in this post were also taken at the vet. Lewis is friendly and enjoys meeting new people, even at the clinic. But Lewis was stressed as well.

I won’t go into arousal vs. distress vs. eustress here, though the interplay of these is a fascinating topic. That’s a post for another day. Nor do I want to get into “how much stress is OK?” or related philosophical and ethical questions.

My focus here is a simpler one: stressed dogs look and behave in many different ways, and some of them can be harder to spot than others.

We always need to look at the whole dog when reading body language, not just a part. We’ll get there. But this is a tricky case, in that we tend to associate the behaviors Lewis is exhibiting with happiness. I think it’s informative to look at a small part—Lewis’ facial muscles—before going to the big picture.

Photos of Stress Face

Maybe this is overkill (who, me, belabor a point?) but every photo below shows bunched-up muscles on Lewis’ right cheek between his eye and his mouth. And the corner of his mouth itself (commissure) was tight. His pupils were dilated. I took many stills from a one-minute video, and they all showed the same thing. Be sure to zoom in on at least one or two of them.

I’m showing the photos before the video on purpose because it may be challenging to see the stress in the video before you know where to look.

Video of Lewis in the Vet’s Exam Room

This is the video from which I grabbed the image stills. As you’ll see, Lewis was bouncing up and down a little, getting on and off his mat. He was gobbling food, and he was wagging his tail in a fairly happy way. He oriented to me most of the time. He was not calm, but at the time, he didn’t seem upset. But now that I have studied the video and stills, his face shows the stress.

Note: partway through the video, I started to toss treats rather than placing them on the mat. This was not a good idea, since tossing treats can add to excitement, and Lewis was already ramped up. I did it only for that brief period, and that was because it was hard to keep him on the camera screen and put treats on the mat at the same time.

What Was Lewis Not Doing?

You’ve seen Lewis now, and can tell he was excited and tense. How does his behavior compare to Clara’s, or that of another terrified dog? Here are some things he wasn’t doing.

• He wasn’t constantly panting.
• He wasn’t trembling.
• He wasn’t pacing; he just got up and down a few times.
• He wasn’t frantically looking for a way out of the room.
• He wasn’t licking his lips constantly or having trouble swallowing.
• He wasn’t hypervigilant. He oriented to sounds, but didn’t startle.
• He wasn’t flushed or shedding.

If you’d like to see the comparison, this short video includes footage of Clara’s February 2013 visit to the vet where she was so frightened.

Greeting the Vets

Back to Lewis.

It’s always such a bother when you have to drop the camera to participate in real life, isn’t it? When the vets came in, I couldn’t film Lewis’ over-the-top greeting. What you can briefly see is that I grabbed his harness firmly, so he couldn’t cannonball into the vets. Again, having a dog who likes people is awesome. But his greetings verge on frantic, and show he is not entirely comfortable with the situation.

Look at his ear movement before and after the vets entered the room.

In the photo on the left, a vet turned the handle on the door and Lewis was watching and listening, with his ears lifted forward. In the photo on the right, the door was open, and humans were visible. Lewis’ ears dropped, and you can catch briefly on the video that his tail was wagging wildly. As he greeted the vets (not shown), he exhibited puppy-like appeasement behaviors. He crouched low to the ground and flattened his ears as he shot forward. I would approximate what was going on with him as saying both, “Hi, I love you!” and “Please don’t hurt me!”

A Final Look: That “Open Mouth” Thing

This last comparison is fun. Marge Rogers and I, in our book about puppy socialization, talk a lot about looking for an open mouth and relaxed jaw in puppy body language. An open mouth is one of the easiest indicators a pup is relaxed and comfortable in a situation. But there is always nuance.

In the photo on the left, Lewis was sunning himself on the grass in the winter. The weather was cool, and his mouth was shut. But look at his soft eyes and smooth face. He was relaxed, only perhaps a little curious to see what I was up to. Here is the uncropped photo in case you want to see the rest of his relaxed body language.

In the photo on the right from the vet clinic series, Lewis’ mouth is open. But is he relaxed and comfortable? Hell no. There are those bunched muscles and tight mouth. You can even see the tightness in his lower lip. This is the opposite of the relaxed jaw we look for when trying to determine whether a dog is comfortable and happy in a situation.

This is a comparison collage of two pictures. Both show the same white dog with brown ears and ticking. In the photo on the left, the dog's facial muscles are relaxed. In the photo on the right, the dog's mouth is open but his facial muscles are very tense and bunched up.

It’s new for me to live with a dog whose stress can look like happy excitement (or for whom the two commonly combine). Now I know one “tell” to look for. Stay tuned for further adventures!

Related Posts

Dog Facial Expressions: Stress
Shelter Puppy “Smiles” from FEAR after She’s Adopted
Dog Body Language Is Crucial to Puppy Socialization
Is That “Smiling” Dog Happy?
Does a Wagging Tail Mean a Happy Dog?
Dog Body Language Posts and Videos

Copyright 2022 Eileen Anderson

Articles on Puppy Socialization

Articles on Puppy Socialization

We never planned to stop with the book. Marge Rogers and I want more materials on puppy socialization out there, so now we are writing some in bite-sized pieces.

I am pleased to link to the first four (free access) educational articles on the Puppy Socialization Project site!

What is Puppy Socialization?


Dog Body Language Is Crucial to Puppy Socialization


Intensity and Thresholds in Puppy Socialization


6 Dos and 6 Don’ts for Puppy Socialization


Enjoy!

Eileen & Marge

Related Post

Puppy Socialization Book is Now Available

Copyright 2021 Eileen Anderson

Space Invaders: How Humans Pressure Dogs & Other Animals

Space Invaders: How Humans Pressure Dogs & Other Animals

Let’s say you are standing at a party, or in your office, or on your front lawn. Someone you vaguely know walks up to you. He walks up very close, face-to-face like the Seinfeld close-talker. Close enough that you can see up his nose and smell his breath. He starts a conversation. What do you do?

You will probably have a strong urge to step back. You may or may not do it, depending on the social situation and a host of other factors. But when someone we don’t know well enters our personal space bubble, it can be very uncomfortable.

Continue reading “Space Invaders: How Humans Pressure Dogs & Other Animals”
How Does Dogs’ Hearing Compare To Humans’?

How Does Dogs’ Hearing Compare To Humans’?

There is a lot of misunderstanding out there about how well dogs hear. It’s true that their hearing is better than that of humans in a couple ways. They can hear higher-pitched sounds than humans can, and they can hear quieter sounds than we can in some frequency ranges. Because of this, they have a reputation for superb hearing. But their hearing capabilities are not better across the board. Our capabilities are superior to theirs in a few important ways as well.

Continue reading “How Does Dogs’ Hearing Compare To Humans’?”
Shelter Pup “Smiles” from FEAR after She’s Adopted

Shelter Pup “Smiles” from FEAR after She’s Adopted

brown puppy shows a submissive grin

The viral video linked below made the rounds in 2019. Its titles on the major purveyors of cute animal videos were variations on the same theme, such as “This Adorable Puppy Wouldn’t Stop Smiling in Her Shelter Kennel,” “Smiling Dog in an Animal Shelter,” and “Dog in an Animal Shelter Couldn’t Help but Smile after Finding Out She Was getting Adopted.”

But the pup wasn’t “smiling” from happiness. Her facial expression meant something else.

She was scared.

Look at the short video.

Here’s what I see:

  • The puppy huddles at the back of an enclosure.
  • At the beginning of the video, her front legs are braced, pushing her backward.
  • She blinks and squints repeatedly.
  • She looks away and turns her head away several times.
  • Her ears are pulled back.
  • She pulls her mouth back into a “grin” that is associated with appeasement.

All of these behaviors demonstrate stress.

Continue reading “Shelter Pup “Smiles” from FEAR after She’s Adopted”
Local Enhancement and Socially Facilitated Behaviors in Dogs

Local Enhancement and Socially Facilitated Behaviors in Dogs

Three dogs lying on the grass as seen from above. It is local enhancement, imitation, or just that they agree on the best place for sun baths?
 

This post started out as one thing and transformed into another as I went along, as many of mine do. I have been familiar for a while with the term local enhancement for a type of social learning in dogs. I had some videos that I felt were good examples. But while researching this post and putting the clips together into a movie, I learned that the concepts and definitions were a lot less cut and dried than I thought.

This topic is up for lots of interpretation and discussion in the literature and I have found it to be underrepresented in discussions about dog behavior. I felt that at least an introduction to the subject would be helpful. I have gone with the most thorough, most recent, and most cited sources.  I am open to additional information and hope for a good discussion.

Terms and Definitions

There are several different types of socially facilitated behaviors and social learning. These are two separate terms since behaviors can be socially facilitated without subsequent learning (Heyes, 1994, p. 214). Also the types of social facilitation overlap, and more than one can be going on at the same time. Among the types are behavioral contagion, local enhancement, stimulus enhancement, observational conditioning, copying, emulation, and imitation.

I got interested in local enhancement since I was pretty sure I saw it happening with my dogs.  Like most of the other types, it involves animals performing similar behaviors as a result of observation or other perception of another animal. But it is not classified as imitation.

Here are a definition and an example of local enhancement from textbooks:

Local enhancement occurs when, after or during a demonstrator’s presence, or interaction with objects at a particular location, an observer is more likely to visit or interact with objects at that location (Hoppitt, 2013, p. 66).

…When local enhancement is in play, a model simply draws attention to some aspect of the environment by the action he undertakes there (for example, digging for worms). Once the observer is drawn to the area, he learns on his own (Dugatkin, 2004, p. 154-5).

Note that the observer animal doesn’t have to see the demonstrator animal. The observer can happen upon odors the demonstrator left or other signs of its actions in the area.

But if you have more than one dog, I bet you have seen local enhancement now and again.

Socially Facilitated Behaviors Without Learning

One thing that tripped me up is that it turns out local enhancement doesn’t have to involve learning (Thorpe, 1963, p. 154). Sometimes behavior is elicited socially but there is no behavior change in the future. The examples in my movie are probably of this type.

Some researchers say that local enhancement only takes place if the observer animal interacts at the location after the demonstrator has left (Heyes, 1994, p. 215).  That is true in the first of my video examples but is not required by most definitions.

William Hoppitt (2013, p.66), whose definition I included first above, believes that the term local enhancement should be inclusive:

…We suggest that local enhancement be retained to refer to all such location effects, irrespective of whether they result in learning.

He also includes in his definition that the demonstrator animal may be either present or absent. Under that definition, both of the examples in my movie would qualify. When the demonstrator animal is still there, the classification of the observer’s behavior is more difficult. If the observer is interacting at the location at the same time as the demonstrator, we could be seeing general social facilitation. This is the tendency of animals to behave as others in their group are doing (Shettleworth, 2010, p. 467). Consider such contagious behaviors as yawning in humans and barking or fence running in dogs. In one of my examples in the movie, the dogs are attracted to a location but also running around excitedly in a group. Local enhancement and social facilitation are both probably involved.

Thus, local enhancement can end up with two animals doing the same thing at more or less the same place. But it is different from imitation or emulation. These are separate and precisely defined learning methods.

Not Imitation or Emulation

The term imitation has a specific meaning in learning theory.

Imitation: Performing the same action as a demonstrator by virtue of having seen the action performed. The action must be novel… (Shettleworth, 2010, p. 468)

Some definitions stipulate that the observing animal must use the same body parts to perform the behavior they observe. For example, in one study, marmosets watched a demonstrator open a canister. The marmosets that observed a demonstrator using its hands to remove the lids used only their hands. The marmosets that observed a demonstrator using its mouth also used their mouths to remove the lids (Voelkl, 2000). That difference marked their behavior as true imitation.

Emulation means that the observer copies only some of the elements of a complex action (Shettleworth, 2010, p. 468).  The behavior by the observer may be different and may or may not achieve the same end as the demonstrator.

Local enhancement is a much looser concept than both of these. But the more I read about it, the more obvious it seems to me that since animals of the same species would respond similarly to the same stimuli in the same location, it would make sense for them to pay attention to what their conspecifics are doing and where. This could be advantageous and selected for.

When Do We See Local Enhancement?

Almost all studies of local enhancement in the natural environment involve foraging behavior. For instance, one animal will see that another has found a good source of food and will go to that area. Or an animal will happen on the scent of a conspecific and will learn to consume the food in that area or of that type.

Lab experiments follow this model as well. Rather than involving foraging, they generally involve a learned behavior that results in food.

Several domesticated species respond to humans in ways that involve local enhancement. One study shows local enhancement behaviors in horses as a response to the presence of a human near food (Krueger, 2011).  There are several studies with dogs. Some of the human gestural and pointing studies with canids may involve local enhancement.

One of my examples shows two of my dogs investigating a spot in the grass after another dog had appeared to snap at and possibly eat an insect there. The two other dogs waited until the first dog left, then both went to the spot and sniffed for a while. Anthropomorphically speaking, here’s what I imagine going through their heads. “That was interesting. Is it something I need to know more about? Did she maybe leave a piece or is there another one of those? Do they live here?” In the second example, one dog discovers something alive and exciting under a step on my back porch. This is the one where you can see both local enhancement and socially facilitated behavior. After all the dogs arrived, they ran around excitedly and tried to get at the animal (which stayed safe).

Link to the video in case the above embed doesn’t work for you. 

Social Learning Is…Learning

Some dog trainers treat social learning as exempt from learning theory. Nothing could be further from the truth. Depending on the type, social learning includes antecedents, behaviors, consequences, and/or classical associations. It’s just that some of the elements are a little different from what we are used to.

How about your dogs or other animals? Do you see local enhancement? How about between different species?

References

Dugatkin, L. A. (2004). Principles of animal behavior (No. Sirsi) i9780393976595). New York: WW Norton.

Heyes, C. M. (1994). Social learning in animals: categories and mechanisms. Biological Reviews, 69(2), 207-231.

Hoppitt, W., & Laland, K. N. (2013). Social learning: an introduction to mechanisms, methods, and models. Princeton University Press.

Krueger, K., Flauger, B., Farmer, K., & Maros, K. (2011). Horses (Equus caballus) use human local enhancement cues and adjust to human attention.Animal cognition, 14(2), 187-201.

Shettleworth, S. J. (2009). Cognition, evolution, and behavior. Oxford University Press.

Thorpe, W. H. (1956). Learning and instinct in animals.

Voelkl, B., & Huber, L. (2000). True imitation in marmosets. Animal Behaviour, 60(2), 195-202.

Thank you to Yvette Van Veen and Debbie Jacobs for leading me to some good resources on this topic. All conclusions are my own.

Copyright 2016 Eileen Anderson

Ground Scratching: Why Does My Dog Do It?

Ground Scratching: Why Does My Dog Do It?

A sable dog wearing a blue harness is scratching the ground. Her tail is held very high and she have an excited, almost gleeful look.

Why do some dogs scratch with their paws after they eliminate?

I recently read a discussion on Facebook about the meaning of this dog behavior. Some people’s speculations about the reasons for the behavior included:

  • Avoiding something or another behavior (displacement)
  • Expressing anxiety
  • Expressing boredom
  • Relieving stress
  • Expressing frustration
  • Calming oneself
  • Calming another dog
  • Expressing enjoyment of a previous activity
  • Being stressed
  • Expressing high arousal
  • Marking (territorial)
  • Marking by scent
  • Marking visually

Note that all but the last three of these have to do with an emotion or internal state.

I was interested in particular in the conjecture that the behavior was linked to some kind of stress. 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 were scents of another dog’s elimination. Summer also lifts her leg to mark with urine. More on that later.

What Does the Literature Say?

Dirt scratching or scraping has been studied by ethologists. These are mostly observational studies, where numbers of canids were observed performing various elimination, sniffing, and marking behaviors. The behaviors are counted and the surrounding circumstances recorded. Dr. Marc Bekoff points out that it hasn’t been studied all that much in dogs though, compared to the study of other animals.[1]Bekoff, Marc. “The Significance of Ethological Studies: Playing and Peeing.”Domestic Dog Cognition and Behavior. Springer Berlin Heidelberg, 2014. 59-75.  He and others are gradually filling in the blanks, however.

Here are some of the functions for ground scratching that ethologists have proposed:

  • Dispersing scent from the dog’s urine or feces[2]Peters, R.P., Mech, D., 1975. “Scent-marking in wolves.” Am. Sci. 63, 628–637.[3]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.[4]Bekoff, M., Wells, M.C., 1986. “Social ecology and behavior of coyotes.” Adv. Stud. Behav. 16, 251–338.[5]Sprague, Randall H., and Joseph J. Anisko. “Elimination patterns in the laboratory beagle.” Behaviour (1973): 257-267.
  • Dispersing scent from glands in the dog’s paws[6]Peters, R.P., Mech, D., 1975. “Scent-marking in wolves.” Am. Sci. 63, 628–637.[7]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.[8]Bekoff, M., Wells, M.C., 1986. “Social ecology and behavior of coyotes.” Adv. Stud. Behav. 16, 251–338.[9]Sprague, Randall H., and Joseph J. Anisko. “Elimination patterns in the laboratory beagle.” Behaviour (1973): 257-267.[10]Petak, Irena. “Patterns of carnivores’ communication and potential significance for domestic dogs.” Periodicum biologorum 112.2 (2010): 127-132.[11]McClanahan, K., & Rosell, F. (2020). Conspecific recognition of pedal scent in domestic dogs. Scientific Reports10(1), 1-9.
  • A visual demonstration in real time, in the presence of other dogs[12]Kleiman, D., Eisenberg, J.F., 1973. “Comparisons of canid and felid social systems from an evolutionary perspective.” Anim. Behav. 21, 637–659.[13]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.[14]Petak, Irena. “Patterns of carnivores’ communication and potential significance for domestic dogs.” Periodicum biologorum 112.2 (2010): 127-132.
  • A visual demonstration in the form of leaving marks on the ground[15]Kleiman, D., Eisenberg, J.F., 1973. “Comparisons of canid and felid social systems from an evolutionary perspective.” Anim. Behav. 21, 637–659.[16]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.[17]Sprague, Randall H., and Joseph J. Anisko. “Elimination patterns in the laboratory beagle.” Behaviour (1973): 257-267.

Note that none of these hypotheses is linked to an emotion, although one source did note that ground scratching was seen more often “when the individual was aggressively aroused.”[18]Petak, Irena. “Patterns of carnivores’ communication and potential significance for domestic dogs.” Periodicum biologorum 112.2 (2010): 127-132. The main discussion revolves around function, and even then, the conclusions are very circumspect. Dirt scratching may be communication to other dogs, but speculations by ethologists about the content of that communication are still very conservative.

This is a valuable reminder to me that as much as we would love to, we can never know exactly what is going on in our dogs’ minds.

What’s the Smelly Feet Thing About?

One of the hypotheses for the function of the behavior is that glands on the dogs’ paws may give off a scent, and that scratching may deposit and disperse it. What are these glands? Most sources mention sweat glands.

“…paw pads in dogs are one of the few locations that contain eccrine sweat glands. In dogs, apocrine glands are the major type of sweat gland, and the distribution of eccrine sweat glands is limited to the footpads and nose.”  [19]Miller, William Howard, et al. Muller and Kirk’s Small Animal Dermatology 7: Muller and Kirk’s Small Animal Dermatology. Elsevier Health Sciences, 2013.

However, there are other glands that may be involved:

“…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.” [20]Serpell, James, ed. The domestic dog: its evolution, behaviour and interactions with people. Cambridge University Press, 1995.

Since I first published this post, there has been a study of “pedal scent,” scent from the paw glands, in dogs. The researchers found that “dogs recognize scent taken from the pedal glands from other dogs, although the extent to which they use this information to determine the sex of the scent depositor remains unclear.”[21]McClanahan, K., & Rosell, F. (2020). Conspecific recognition of pedal scent in domestic dogs. Scientific Reports10(1), 1-9. They found some differences in how male and female dogs sniff—different right and left nostril preference in females. I think I see Summer switching nostrils when sniffing poop in the movie below at about 1:35.

Male vs. Female Behaviors

A sable dog wearing a blue harness is scratching the ground. Her tail is held very high and her mouth is slightly open. She looks very focused.

Two studies by Marc Bekoff showed that approximately the same percentages of male and female dogs performed ground scratching (about 10%), but also that the males who ground scratched did so much more frequently than the females. [22]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848. [23]Bekoff, Marc. “Scent marking by free-ranging domestic dogs: Olfactory and visual components.” Biology of Behavior, 4, 123-139. Another study showed that among females, those who were spayed were more likely to scratch than those who were intact and not in estrous. (Females in estrous were not included in the study.) [24]Wirant, Sharon Cudd, and Betty McGuire. “Urinary behavior of female domestic dogs (Canis familiaris): influence of reproductive status, location, and age.” Applied Animal Behaviour … Continue reading

The same study also found that females four or more years old directed the majority of their urinations at objects in the environment (marked) and directed more of their urinations when walked off their home area than when walked within their home area. Both of these are true for Summer.

Raised leg urination such as many male dogs perform has also been theorized to have the function of visual display, since it is sometimes performed without urination.[25]Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.[26]Cafazzo, Simona, Eugenia Natoli, and Paola Valsecchi. “Scent‐Marking Behaviour in a Pack of Free‐Ranging Domestic Dogs.” Ethology 118.10 (2012): 955-966. Male dogs have also been observed to raise their legs more frequently to urinate when in the presence of another dog.[27]Bekoff, Marc. “The Significance of Ethological Studies: Playing and Peeing.”Domestic Dog Cognition and Behavior. Springer Berlin Heidelberg, 2014. 59-75. Some female dogs raise their legs as well, including Summer.

So What Does Summer Do?

The movie shows Summer enthusiastically scratching the ground in several different situations:

  1. After squatting to pee;
  2. After raising her leg to pee;
  3. Immediately after entering an area with interesting smells and without eliminating at all; and
  4. After smelling another dog’s droppings (also without eliminating).

If Summer’s behavior is functional, and not some kind of twisted evolutionary leftover, it may support the “dispersing odor from the paws” hypothesis. See what you think.

Link to the movie about ground scratching for email subscribers. 

Function vs. Emotional State

I’m not an ethologist; I’m a pet owner. So while I’m fascinated with the possible function of the behavior of scratching, I’m also interested in my dog’s emotional state when she does it. My observation is that she is enjoying performing a natural doggie activity. The prompts for her behavior seem to be scents, nothing more complex than that.

Summer is a primal sort of dog. Her breeding is so mixed that she resembles a village dog in all but her double coat (she could fit in well as a “northern” village dog, though). She has a strong prey drive and scavenger drive. And although our bond is strong and she loves doing things with me, her natural inclinations are very, very dog-y. In many ways she is more “wild” than my feral-born dog, Clara, who appears to have a wealth of “I like to partner with a human” genes. Go figure.

Clara has never scratched in her life, as far as I’ve seen, but she does lift her leg to mark when she pees. But Summer seems to love scratching the dirt. You could say she gets a real kick out of it.

How about your dogs? Males, females? When do they do it? What is their demeanor when doing so? Do tell!

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Notes

Notes
1, 27 Bekoff, Marc. “The Significance of Ethological Studies: Playing and Peeing.”Domestic Dog Cognition and Behavior. Springer Berlin Heidelberg, 2014. 59-75.
2, 6 Peters, R.P., Mech, D., 1975. “Scent-marking in wolves.” Am. Sci. 63, 628–637.
3, 7, 13, 16, 25 Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.
4, 8 Bekoff, M., Wells, M.C., 1986. “Social ecology and behavior of coyotes.” Adv. Stud. Behav. 16, 251–338.
5, 9, 17 Sprague, Randall H., and Joseph J. Anisko. “Elimination patterns in the laboratory beagle.” Behaviour (1973): 257-267.
10, 14, 18 Petak, Irena. “Patterns of carnivores’ communication and potential significance for domestic dogs.” Periodicum biologorum 112.2 (2010): 127-132.
11, 21 McClanahan, K., & Rosell, F. (2020). Conspecific recognition of pedal scent in domestic dogs. Scientific Reports10(1), 1-9.
12, 15 Kleiman, D., Eisenberg, J.F., 1973. “Comparisons of canid and felid social systems from an evolutionary perspective.” Anim. Behav. 21, 637–659.
19 Miller, William Howard, et al. Muller and Kirk’s Small Animal Dermatology 7: Muller and Kirk’s Small Animal Dermatology. Elsevier Health Sciences, 2013.
20 Serpell, James, ed. The domestic dog: its evolution, behaviour and interactions with people. Cambridge University Press, 1995.
22 Bekoff, Marc. “Ground scratching by male domestic dogs: a composite signal.”Journal of Mammalogy (1979): 847-848.
23 Bekoff, Marc. “Scent marking by free-ranging domestic dogs: Olfactory and visual components.” Biology of Behavior, 4, 123-139.
24 Wirant, Sharon Cudd, and Betty McGuire. “Urinary behavior of female domestic dogs (Canis familiaris): influence of reproductive status, location, and age.” Applied Animal Behaviour Science 85.3 (2004): 335-348.
26 Cafazzo, Simona, Eugenia Natoli, and Paola Valsecchi. “Scent‐Marking Behaviour in a Pack of Free‐Ranging Domestic Dogs.” Ethology 118.10 (2012): 955-966.
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