Why Do Dogs Stick Out Their Tongues and Pant?
― A Body That Can't Sweat Evaporates Water with Its Breath
On a hot day, a dog sticks out its tongue and breathes fast, over and over. It looks like hard work, but it is the dog's own kind of "sweating". People cool down through the skin. Dogs do it along the airway.
On a summer evening, a dog comes home from a walk and stands in the entryway with its tongue hanging out. Its breath is so fast it seems it can't stop: huh-huh-huh-huh.
The person who walked with it has sweat on the back of their neck. But if you stroke the dog's back, its fur is hardly damp at all.
Same road, same distance. The person sweats; the dog pants. The two bodies simply cool down in different ways.
There are just two reasons
In dogs, the places that release cooling sweat are almost limited to the paw pads. Across the fur-covered body surface, almost no heat-shedding sweat comes out.
When air keeps passing over the wet tongue and the inside of the nose, water evaporates and takes heat with it. Fast breathing is a way to swap that air more often.
The basic idea, that water takes heat away as it evaporates, is the same as in human sweat. What differs is where the evaporation happens. Let's go through it step by step.
People cool the skin; dogs cool the airway
When water evaporates, it draws a lot of heat from its surroundings. That is why you feel cold if you stay wet after a bath. People spread sweat over the skin of the whole body and let it evaporate over that large area to cool down.
A dog's body is covered in fur. The air inside the fur barely moves, so even if sweat reaches the skin, it is slow to dry. So the dog uses the airway, rather than the skin, as its place for cooling. Take a look at Figure 1.
Studies have confirmed that a hot dog mainly breathes in through the nose and out through the mouth. The inside of the nose is intricately shaped, and its surface is always wet. The air picked up water there as it came in, and the outgoing breath passes over the long, extended tongue. Because the air flows in one direction, fresh dry air keeps passing over the wet surfaces, so evaporation does not easily stall.
Why such shallow, fast breaths?
A dog at rest breathes a few dozen times a minute. When it pants in the heat, the rate is said to reach around 300 breaths a minute. The hallmark is not deep breaths but many shallow ones in a row.
Shallow breaths have a clear advantage. The air they move mostly goes back and forth through the nose, throat and windpipe, and barely reaches deep into the lungs. Since the aim is to cool the wet airway, that is enough. If the dog took deep breaths at that speed, it would blow off too much carbon dioxide from its blood and feel unwell. Shallow, fast breathing cools the dog while keeping the make-up of its breath in balance.
Another trick is rhythm. The chest and lungs tend to swing at a steady tempo when shaken. A dog breathes in time with that tempo, so it is thought to keep up fast breathing with little muscle effort. It is the same principle as pushing a swing in time with its motion.
Horses sweat heavily over the whole body, as people do. Cats, like dogs, sweat only through their paw pads, and they cool down by evaporating saliva from licked fur. Pigs, which can hardly sweat, soak in mud or puddles and lose heat from their wet bodies. Birds have no sweat glands, and many kinds cool down by rapidly fluttering their throats. Elephants shed heat by flapping their large, blood-vessel-rich ears.
Cooling by evaporation works less well when the surrounding air is humid. A dog has far less area to cool with than a person, so it feels this effect more strongly. Short-nosed breeds tend to have less wet surface inside the nose, too. On hot, humid days, walk your dog at cooler times, and never leave it in a closed car, even for a short while. If your dog is limp and slow to respond when you call, cool it down and contact a vet right away.
Summary
Dogs can't sweat over the whole body the way people do. Instead, they breathe in through the nose and out through the mouth, evaporating water from the wet nose and tongue to cool down. Shallow, fast breathing boosts the cooling while keeping the make-up of the breath steady.
People sweat through the skin.
Dogs sweat through their breath.
How evaporation takes heat away is explained in more detail in why the wind from an electric fan feels cool, and how body size relates to heat loss in why smaller animals keep eating. For how hot a car gets, see the heat inside a car parked in the sun; for a bird's trick of not losing heat through its feet, see why ducks' feet don't freeze. And for the opposite trick, flinging off water so as not to get too cold when wet, see why wet dogs shake their bodies.
- If you have a dog, count its breaths for one minute while it rests in a cool room. Count one rise and fall of the chest or belly as one breath.
- Count again while it is panting, for example after a walk. If it is too fast to count, count for 15 seconds and multiply by 4.
- If you don't have a dog, wet the back of just one hand and blow on both hands with the same strength. Only the wet hand should feel cool.
The wet hand cools because your breath makes the water evaporate and take heat away. The same thing happens on a dog's tongue hundreds of times a minute. Please don't strain a dog's body just to test this.
For those who want to know more ― terms, formulas, and links to textbooksEach part is labelled by level, from junior-high science to university-level courses
- Junior highCovered in junior-high science
- High schoolCovered in high-school "Basic Physics" and "Basic Biology"
- High school+Advanced high-school material, or textbook-sidebar content
- UniversityNot taught in high school; university-level courses (animal physiology, thermodynamics)
- ResearchNot yet taught as settled fact even at university; topics researchers are studying now
Junior highTerms: this phenomenon has names
- Panting: a way of breathing in the heat, with shallow, fast breaths that shed body heat.
- Heat of vaporisation (evaporation heat): the heat a liquid takes from its surroundings as it turns into gas.
- Eccrine sweat glands: glands that release watery sweat to lower body temperature. In people they cover the whole body; in dogs they are found mainly in the paw pads.
- Dead space: the part of the airway, such as from the nose to the windpipe, that air passes through but where no gas exchange takes place deep in the lungs.
Junior highHigh schoolChecking with a formula: how much water must evaporate to lower body temperature by 1 ℃?
Taking a 10 kg dog as an example, we estimate the heat that must be removed to lower its body temperature by 1 ℃, and the amount of water needed to carry that heat away. Heat is in joules and water is in grams. The symbols mean the following.
| Symbol | Meaning and unit |
| m | The dog's body mass (grams) |
| c | Specific heat of the body: heat needed to warm 1 gram of body by 1 ℃ (joules) |
| ΔT | The drop in body temperature we want (℃) |
| L | Heat taken away when 1 gram of water evaporates = heat of vaporisation (joules) |
| w | Mass of water evaporated (grams) |
| In symbols | m × c × ΔT ＝ w × L (so w ＝ m × c × ΔT ÷ L) |
| In words | Heat lost by the body (mass × specific heat × temperature drop) ＝ heat carried off by the evaporated water (mass of water × heat of vaporisation) |
| Where it comes from | Conservation of energy. The heat leaving the body does not vanish; it is a heat balance assuming all of it goes into turning water into gas |
| Dog's body mass | 10 kilograms (10000 grams) |
| Heat needed to warm 1 gram of body by 1 ℃ (specific heat of the body) | said to be about 3.5 joules |
| Heat taken when 1 gram of water evaporates near body temperature | about 2400 joules |
| Heat to shed to lower body temperature by 1 ℃ | 10000 × 3.5 ＝ 35000 joules |
| Amount of water needed to carry it off | 35000 ÷ 2400 ≒ 14.6 grams |
| If the same heat were shed with water at body temperature (ratio to evaporation) | 2400 ÷ 3.5 ≒ 686 times |
By this calculation, evaporating only about 15 grams of water, roughly a tablespoon, can cool a 10 kg body by 1 ℃. In reality, though, heat keeps coming in from exercise and sunlight. And not all the water in the breath comes from the tongue and nose, so treat this as a figure close to the upper limit.
High schoolHigh school+Shallow breaths and the easy-swing tempo
High schoolOf the air moving in and out with each breath, the part that stays between the nose and the windpipe is not used for gas exchange in the lungs. With shallow breaths, this airway air makes up a larger share of the movement. So even when the breathing rate rises a lot, carbon dioxide in the blood is not easily blown off in excess.
High school+The chest and lungs, like a spring with a weight, tend to swing at a set tempo. Breathing in time with that tempo moves a lot of air with little force. A study of dogs in the 1960s reported that the panting rate is close to this tempo. It is the same idea as resonance in a swing.
UniversityOne-way airflow and heat recovery
As the heat grows, a dog is said to increase the share of breaths drawn in through the nose and out through the mouth. The complex bony folds inside the nose widen the wet mucous surface. In ordinary breathing, the outgoing air recovers some moisture and heat as it passes through the nose, which prevents drying out. When hot, sending the outgoing air through the mouth stops this recovery, and is thought to switch the system toward dumping heat and water outside. This arrangement, in which incoming and outgoing air exchange heat along the same passage, is treated as a kind of counter-current heat exchange. Glands on the outer part of the nose release fluid and are reported to support the evaporation of the incoming air.
📖 Derivations and going further: Heat of vaporisation (Wikipedia)
ResearchWhat is still not clear
- Differences in cooling power between breeds. Short-nosed breeds have been reported to tolerate heat poorly, but how much nose shape, body build and coat each contribute is still being sorted out.
- Ways of protecting the brain. One idea is that blood cooled in the nose in turn cools blood on its way to the brain. How far this works in dogs depends on which study you read.
- Getting used to heat. People's sweating changes as they adapt to heat. How adaptation changes the efficiency of panting in dogs is not well understood.
In other words, what this article says is "an explanation within what is known today". Cooling by evaporation is the firm basic idea, but the details of which parts of a dog's body contribute how much are still being studied.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Junior high | Science: changes of state and heat; animal respiration | Water takes heat as it evaporates |
| High school | Basic Physics: quantity of heat and specific heat / Basic Biology: homeostasis | The calculation for a 1 ℃ drop; how shallow breathing works |
| High school+ | Physics: simple harmonic motion and resonance | Breathing in time with the easy-swing tempo |
| University | Animal physiology: temperature regulation and respiration | One-way airflow; heat and water recovery in the nose |
| Research | Comparative physiology and veterinary science | Breed differences, brain cooling, adapting to heat |
| ― | Links to daily life | Walk times on hot days; never leaving a dog in a car |
- Schmidt-Nielsen, K., Bretz, W. L., Taylor, C. R. (1970) Panting in dogs: unidirectional air flow over evaporative surfaces. Science, 169, 1102–1104.
- Crawford, E. C. (1962) Mechanical aspects of panting in dogs. Journal of Applied Physiology, 17, 249–251.
- Schmidt-Nielsen, K. Animal Physiology: Adaptation and Environment (Japanese edition, シュミット＝ニールセン『動物生理学 ― 環境への適応』, University of Tokyo Press (東京大学出版会))
- National Astronomical Observatory of Japan (国立天文台), ed., Chronological Scientific Tables (理科年表, Maruzen Publishing (丸善出版)) ― heat of vaporisation of water
※This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand the mechanism. For any judgment about your dog's health, please consult your regular vet.