The short answer

A dog mainly wags their tail in order to communicate with different visual cues. Wagging their tails also allows a dog to communicate via scent by stimulating their anal glands in order to release of their natural odor.

The long answer

Human: "Good morning, buddy!"

Dog: *tail starts wagging*

Human: "Who's a good boy?"

Dog: *wags more*

Human: "You wanna go for a walk?!"

Dog: *wags even more vigorously*

Though the original purpose of dogs' tails was to help them balance, their tails have evolved to be a key part of their communication. Dogs wag their tails in order to send both visual and odor signals to other dogs and animals about how they are feeling.

Fun fact: Over the course of evolution, some dogs' tails have become bushier, lighter on the underside, or developed a contrasting black or white tip to make their tails more visible from a distance.

Like how humans can communicate an incredible variety of emotions with our face, hands, and body, dogs can display a wide array of moods and messages with their tails. Generally as the tail gets higher, the dog is feeling more alert, excited, or aggressive. A tail going lower means the dog is feeling more scared, timid, or submissive.

Here are a few ways dogs communicate with their tails visually:

How wagging communicates via odor

Interestingly, wagging isn't just a visual cue in dog-to-dog communication. It also allows dogs to communicate using odor (we just can't smell it as well).

Each dog has their own unique scent and they use this scent to communicate with other dogs. Wagging their tail allows a dog to stimulate and spread their scent from their anal glands. A dog holding their tail high and wagging fast will release more of their scent than a dog with their tail low and between their legs.

Check out some other curious questions:

Sources

Coren, S. (2011, December 5). What a Wagging Dog Tail Really Means: New Scientific Data. Psychology Today. https://www.psychologytoday.com/intl/blog/canine-corner/201112/what-wagging-dog-tail-really-means-new-scientific-data

LLC, A. (n.d.). Why Do Dogs Wag Their Tails?. Doghealth.com. https://www.doghealth.com/behavior/how-and-why/1905-why-dog-wag-their-tails

Llera, R., & Buzhardt, L. (n.d.). Interpreting Tail Wags in Dogs. VCA Animal Hospitals. https://vcahospitals.com/know-your-pet/interpreting-tail-wags-in-dogs

Wild Earth. (n.d.). Why Do Dogs Wag Their Tails? The Science Behind Tail Wagging. Wild Earth. https://wildearth.com/blogs/dog-knowledge/why-do-dogs-wag-their-tails/

YouTube. (2020). Interpret Dog Tail Wags: How to Understand Dog’s Body Language. Jaw-Dropping Facts. Retrieved December 31, 2023, from https://www.youtube.com/watch?v=j9yc0PPXAFg.