Cats are enigmatic little creatures. It's hard to get a read on them. Does your cat love you, or would it gladly stab you in your sleep, if only it had thumbs and a slightly larger brain? The cat never tells - it thrives on inscrutability. But it can't help betraying certain signs of its inner life: It's hard to play things totally cool when you have a large, ungainly tail sticking out of your back, swishing this way and that for no immediately clear reason.
Do these movements actually mean anything? Or is this just the species' way of distracting us from whatever it is they're really feeling?
For this week's Giz Asks, we reached out to a number of cat specialists to get to the bottom of what it means, if anything, when a cat wags its tail. As it turns out, a cat's tail is a highly expressive instrument - not as expressive as a dog's, say, but still a decent barometer of what's going on inside a given cat.
Professor Emeritus, Clinical Sciences, Cummings School of Veterinary Medicine at Tufts University
It's strange question in a way, because you can ask: Do cats even wag their tails? Most people would say: No, dogs wag their tails, cats do not. But cats do move their tails, and their tails can be very expressive, and give you some indication as to what exactly is going on in a cat's mind.
Actual dog-like wagging is usually a sign of extreme agitation. I once worked on an extremely aggressive cat. It would walk forward slowly with one foot in front of the other, towards a person, eyes riveted on them, its tail wagging like a dog's from side to side, almost stalking them - and then at the last minute it would spring and attack them.
There are other forms though. A cat who's happy with you might gently swish its tail from side to side as it walks towards with you - but it's sort of barely perceptible - I wouldn't really call it wagging. It's not even really swishing - that's too powerful a word.
When a cat's goal is thwarted - if, say, a cat is divided from a bird or a bush outside by a window or screen - they will sort of switch and flick their tail from side to side in frustration. It looks agitated, and I think it is agitated.
When they're marking their territory by spraying - or in a neutered cat, phantom spraying (when no urine comes out) - they will point their tail bolt upright in the air, 90 degrees to the spine, and the tip of it will flick from side to side. At that point, they brace their legs and either nothing comes out or they will squirt out a stream of urine. They are kind of being territorial.
When dogs wag their tails, it's a sort of energy indicator, like a rev-counter in a car. But just because a dog's wagging its tail doesn't mean it's happy - it just means it's in some kind of animated state, and not necessarily a pleasant one. When they wag their tails with a bias to the left, they're not happy about a situation; when they wag their tails with a bias to the right, that's a sign of affection and happiness.
That same has not been shown in cats, and my guess it that it does not actually apply to cats - though the rev-counter/energy-meter indication is applicable. Whatever motion their tail is making, it suggests a change of energy level in the cat that is causing this appendage on her rear-end to move in certain ways, and in a way it means something is going on.
It means that they're thinking of something, and usually the something isn't particularly good - for the most part, it's along the lines of agitation, frustration, mild annoyance, thwarted opportunity and so on, except for the gentle side-to-side wag.
The direction and speed with which cats move and swish their tails conveys how they're feeling - their tails act like emotional barometers. When they're thrashing their tails quickly back and forth, it means that they're unhappy and you should leave them alone. When you see tails that move slowly, left to right, that can indicate the cats are a little bit annoyed - they will also sometimes move their tails that way when they are hunting and about to pounce in play.
Sometimes - and my cats will do this - when they're really happy to see me they will face me, their tails are up in the air with a slight curve and the base of their tails are slightly puffed out while they quiver. This indicates that they're happy to see me. People often become a little alarmed at first when their cats do this because it looks similar to spraying.
Other times when cats will quiver and move their tails are when they are getting ready to stalking and about to pounce on prey. There are a few theories about this; one of them is that there is a flood of neurotransmitters and adrenaline into the cat's system, which is over-stimulating and causes their tails to move.
Some cats may flick their tails a little bit when they're sleeping and are disturbed. They are letting you know that they want to be left alone - but they also want you to know they hear you. It's not quite ignoring you: They just want to continue taking their nap.
It's not just tail language alone you should pay attention to: You also want to take into account the rest of the cat's body language including vocalisations, ear positions, if the eyes are relaxed or the pupils are dilated or constricted. These, along with the tail movements and positions successfully communicate cats emotions and intentions.
Animal Behaviorist, founder of Pet Problems Solved
Cats tails are quite expressive. Those movements need to be taken in context, however, and the cat's entire body language needs to be taken into account to tell how the cat is truly feeling.
Cats tails can swish, indicating that they are about to pounce; this may be accompanied by placing the back legs firmly on the ground, and sometimes the rear end swaying too, giving them leverage to jump.
They can twitch - often when cats are intently watching a toy move or a bird outside the window, you will see the twitchy tail in action.
They can flick - which indicates displeasure and will often be used just before attacking you.
They can curl - cats curl their tail around you as they walk past, generally as a sign of affection, which transfers their scent on to you (i.e. they own you!).
There's the question mark tail, where the tail is held high in the air, often with a little kink at the end, when the cat is greeting you.
They fluff up their tails, often when they're alarmed. It makes them look larger and probably confuses the enemy.
The cat tail is used in communication, but also in balance. It helps them jump onto high surfaces and walk across narrow platforms and can be curled around them, possibly to keep them warm.
Certified Feline Behaviour and Training Consultant, Feline Behaviour Solutions
Cats are great communicators, if you know how to speak their language. A cat's tail position can act as a barometer for her mood: Generally speaking, the higher up a cat's tail is, the more confident and outgoing she is. When a cat's tail sinks lower towards the ground, she may be feeling less sure of her circumstances, or even afraid. But context is important, because there are exceptions to these general guidelines.
When a cat wags her tail, it's not the same as when a dog wags his tail - it's generally not a sign of unabashed excitement. As you might imagine, context is (again) very important. Frequently, a cat will wag her tail back and forth to indicate irritation or annoyance - slight twitching at the end of the tail suggests minor annoyance, and a widely swishing or slapping tail is a sure sign that the cat wants you to stop what you're doing.
A cat who is hunting or playing may crouch down and wag her tail back and forth - this happens just before the cat pounces towards her prey or a toy. And a cat who is happy to see a friend - human or feline - may gently wave her straight-up tail side to side (kind of like we might wave our hand to a friend), or wrap it around your leg as she snakes her way between your feet.
Other people report that cats who are sick or feeling ill may wave their tails gently when they are lying down. As always, though, what the cat's tail is doing should always be viewed within the context of other body language indicators.
Bruce Gordon Komreich
Associate Director, Cornell Feline Health Center
We're used to dogs wagging their tails when they're happy or excited, but cats are a bit subtler in their body language.
When they're annoyed or nervous, and want you to back off, they might quickly swish their tail back and forth.
When they're happy, or greeting you, their tail might go upright, and quiver a bit. Some cats might wrap their tail around your leg for the same reason.
Sometimes cats will wag their tails when they're stalking something - it's a sort of twitch, which indicates that the cat is interested in or concentrating on something.
If a cat's tail starts to look puffy - like a pipe cleaner - it generally means the cat's responding to a threat, and trying to make itself look bigger, especially if it's ears are down and its hair is standing up on its back.
As to why cats have evolved this way: Dogs have evolved in packs, and might use their tails to greet pack members, or to communicate while hunting, whereas cats are more solitary - but that's all of course speculation.
Of course, the tail is only one feature of the body language you have to look at - you also need to pay attention to their head position, to what their hair is doing, to their ears and eyes.