Nothing is more of a telltale sign of happiness than, well, a dog's wagging tail.
It's a main method of communication for man's best friend—but "what about dogs without tails?" reader Leanne Kirby asked in comment on a recent story exploring how dogs observe left versus right tail wags.
So for Weird Animal Question of the Week, we asked dog expert Monique Udell if a pet who is tailless due to breed, accident, or cosmetic procedure has trouble communicating with other dogs or humans. (Take National Geographic's dog quiz.)
"It definitely does impact their communication," says Udell, founder of the Canine Cognition and Behavior Lab at the University of Florida in Gainesville.
For instance, dogs with cropped tails or breeds with short, curly tails, like pugs, don't have the range of expression that a dog with a long tail has, Udell noted.
But luckily, canines can compensate with other body parts to show their emotions.
The Tail, Decoded
In general, a dog tucks its tail under its body to convey submissiveness, an important signal to other dogs.
It also "can be used to cover the genital area, so it's communicative and functional," she said.
Dogs cover their genital area with their tails if they're not ready to trust another dog. That's because dogs gain information about another dog by sniffing scent glands in their anal area—so allowing that area to be exposed is a measure of trust.
"Tucking the tail [in that situation] is a way to say, 'I'm nervous here ... I don't really want to have this interaction'-like a human a crossing their arms and shifting away." (See "Dogs Get Jealous, Too.")
Then there are wags. "Not all wags are created equal," Udell says, and they don't all mean friendliness.
A relaxed tail with an enthusiastic or circular wag is friendly or playful; a slow, controlled wag where the body is more tense isn't friendly and signals "back off" to humans or other dogs.
The Truth About Hackles
Luckily, tail-impaired dogs have a fallback strategy in hackles—stiff hairs that go from a dog's neck all the way to its tail. Udell says they're extremely informative.
Raised hackles are thought to indicate aggression, but actually they convey various kinds of arousal, including aggression, fear, or excitement, she said.
"When my dog was a puppy, her hackles would go up when we got her a new toy because she was so excited," Udell says.
Generations of being selectively bred for physical appearance has hampered the hackles of some dogs, though.
Poodles and Afghan hounds, bred for their dramatically beautiful coats, can still raise their hackles, but they may be obscured by the heaviness of their coats. It's common in domestic dogs and in breeds that have been modified that "sometimes their signals don't match their behavior," and even people trained to work with dogs can be confused.
Facial expressions are also a dog-to-dog signal, and different breeds control those expressions differently. For instance, bared teeth can signal aggression.
Again, though, some breeds send mixed signals—bulldogs are famously bred to have underbites, which means their teeth are showing pretty much all the time. (See dog pictures submitted to National Geographic.)
"Those dogs tend to look angrier, and it may be because their lips can't cover their teeth," which may influence how other dogs perceive them, Udell says.
But there's another way to signal anger—pulling back the ears, she added.
Know Your Breed
Because dogs are so diverse, familiarizing yourself with the communication styles of your dog's breed can tell you volumes.
Dogs "watch their humans all day," and their observations make them much more able to be responsive to us as individuals," Udell says. (Related: "Can Dogs Feel Our Emotions? Yawn Study Suggests Yes.")
"If we did the same thing with our dogs, we would learn a lot of interesting things."