Why do dogs show their pleasure by wagging their tails? asks Jeff Ewener, of Toronto, Canada.Does your dog erupt in a frenzy of frantic tail-wagging when you arrive home from work or school? Scientists say that dogs' wild ancestors evolved tail-wagging as part of a suite of ways to communicate. In fact, our dogs' closest wild relatives, wolves, are also waggers.
Dogs wag their tails when they're around other animals (including us). A dog may wag her tail happily when she sees a rabbit in the yard. But like a human saying hello to a pile of black clothes (instead of a sleeping cat), a dog's tail falls silent when she realizes she's waving at a pile of leaves.
And studies show that the wag's direction tells a tale. If a dog wags to his right, he's feeling positive about what's happening, and wants to come closer. But a dog wagging to his left is feeling uncertain, worried or afraid; his tendency is to back off.
In one 2013 study, researchers found that dogs wagged most enthusiastically (and more toward the right) when they saw their human companions. With a person they'd never met before, they wagged to the right, but with less eagerness -- a shy "Hi." When a cat showed up, the rightward wags became slow and hesitant.("Is this going to be fun, or painful?")
The dogs also watched dog videos. Shown an aggressive dog, they reacted with leftward wagging, increased heart rates and anxious looks. Like a person noting someone's body language, dogs notice what another's tail is doing, as a signal of how to react. Dogs became stressed when they saw a dog with a left-wagging tail, but acted relaxed and calm when they saw rightward wags.
Beyond the loose, friendly wag, a tail held high, waving rigidly above a tensed body, signals dominance. When a dog's tail looks like it's virtually vibrating, he may be about to take off after that clueless squirrel (or another dog). And, of course, a tail turned under and held between the legs, tip twitching wildly, sends a message of submission, and, often, guilt. "I didn't mean to eat your shoes. No, really!"
Why left or right? The left brain controls the body's right side, and vice versa. Scientists say the left hemisphere may be more active during positive experiences, the right brain during negative ones.
Like wearing their hearts on their (nonexistent) sleeves, many animals use their tails to express emotions and telegraph intentions.
A cat whipping her tail is in a very bad mood. Likewise, an annoyed horse will swish his tail with great force. And agitated Eastern gray squirrels fast-flick their tails, warning fellow squirrels about lurking predators.