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Your cat is inspecting his surroundings
If your cat encounters a new smell, he will try to inspect it, first by sniffing it then you may see him raise his upper lip while holding his mouth partially open and sticking his tongue out.
This is a phenomenon called “Blepping” (Have no clue where this word came from!).
Many scientists believe that the act of protruding the tongue is just the way cats exhibit the Flehmen response.
Also observed in many species, this behavior only lasts a 1 minute or so, during which the tongue will perform a series of in and out movements to carry the sent molecules (both airborne or fluid-borne) to the Vomeronasal Organ (auxiliary smelling organ) through the ducts located just behind your cat’s incisor teeth.
Your cat is just feeling relaxed or sleepy
You may notice that your cat sticks its tongue out when you caress him, it just means that he is feeling good and happy.
Indeed, your companion can associate your caresses with his grooming, which can trigger a feeling of well-being, appeasement and affection.
Sometimes, your cat sticks his tongue out while sleeping, so don’t worry neither! It’s just that he feels good and his muscles are relaxed.
This case is quite common in older cats, but without any health repercussions.
This occurs, usually, during the paradoxical sleep also known as REM phase of sleep, which is a distinct stage of sleep with intense brain activity in the forebrain and midbrain. It is characterized by dreaming and the absence of motor function with the exception of the eye muscles and the diaphragm. 
Blepping may be a precursor of an underlying health issue
If your cat has been exposed to extreme heat during a sunny day of summer, he may expose his tongue in an effort to cool down, you’ll probably say that cats don’t pant! Well they usually don’t but if their body is overheated, this can be a way of cooling down.
However, if the panting continues for a long period of time you need to take it seriously especially if other symptoms add up such as:
- Excessive drooling,
- Difficulty to breath,
- Swelling of the tongue,
- Pale gums.
Those symptoms refer to a potential heat stroke, if that’s the case you need to move your cat to a cool place immediately and after a while, bath him using cold water. And prepare for a visit to the vet.
However, never immerse him in cool water right away, as this will cause a thermal shock that can be fatal.
If you notice that your cat is sticking out his tongue without any other particular symptoms and regularly throughout the day or for periods of more than 10 minutes, it may be a sign of respiratory distress.
Thus, he may have a problem with his breathing system, or maybe there is a foreign body blocking his airways, either way breathing becomes difficult.
In this case, it is a veterinary emergency!
So don’t waste time and get to your Vet to avoid asphyxia and choking.
The hanging tongue syndrome
This syndrome is essentially the result of severe nerve damage.
Usually caused by mandibular trauma (e.g. due to a fall, a cat fight or a dog attack) or a severe dental disease.
As a result the cat has little to no control over his tongue which makes it continuously protruding.
If your pet sticks out his tongue while sleeping or when you cuddle him, there is nothing wrong with that, he is simply relaxed and soothed.
On the other hand, if he starts sticking his tongue out, spontaneously, several times a day or for long periods of time, or if this gesture is accompanied by symptoms such as coughing or breathing distress, there might be an underlying problem.
So in front of a dyspneic cat, just try to stay calm as he will seek, on his own, to adopt a position that relieves him.
And if your cat is showing signs of heatstroke, try cooling him by moistening his pads and transferring him in a cooler room.
Yet, if he bleps repeatedly throughout the day or for periods longer than 10 minutes, or if his tongue turns blue, take him to the vet immediately!
 The Domestic Cat: The Biology of its Behaviour (3rd edition) 2014 – Communication in the domestic cat: within- and between-species.
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