Why are small dogs so fierce?

With their large, muscular bodies, dogs such as rottweilers and pit bulls, especially those that are trained to act menacingly, may seem like the scariest of their species. But evidence shows that smaller dogs can actually be more aggressive than many of their outsized counterparts.

So why are little dogs so fierce?

First, it's important to define what counts as tiny for a dog. "Small dogs in the less-than-20-pound [9 kilograms] range tend to be more reactive," said James Serpell, director of the Center for the Interaction of Animals and Society at the University of Pennsylvania School of Veterinary Medicine. For example, dachshunds, Chihuahuas and Jack Russell terriers are the most likely to attempt to bite, according to a 2008 study in the journal Applied Animal Behaviour Science that surveyed the owners of more than 30 dog breeds. Other aggressive behaviors seen in little dogs include growling, snarling and lunging. There are a few theories for why this is the case.

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Small size may make dogs more fearful, and they may lash out in self-defense, Serpell said. Tiny breeds may be more likely to react defensively because they have more to be afraid of.

Alternatively, small dogs' reactivity could be a result of their owners' behavior. "Owners tend to infantilize small dog breeds and think of them as being like helpless infants," Serpell told Live Science. Pet owners may be overly protective of small breeds, so they may not socialize them or teach them how to properly react to stressful situations.

Aggression could also be baked into the pups' evolution. "If you're attacked by a Chihuahua, obviously the consequences are much less significant than if you're attacked by a Great Dane or a Siberian husky," Serpell said. In other words, over the millenia, humans may not have bothered to breed aggressive behaviors out of little dogs because the consequences weren't as dire as they were for being attacked by sizable hounds.

That last theory is supported by studies that have found a link between aggressive behavior and the growth factor gene that makes small dogs small. The association could be coincidental, but research has shown that small dogs aren't just more extreme in their aggression-related behaviors; compared with large dogs, small dogs tend to have more severe separation anxiety, usually bark more and are more likely to urinate in the house, Serpell said. This gives credence to the theory that genetics plays a role in little dogs' extreme behavior.

Scientists aren't sure which of the theories are correct because they haven't studied the root of little dogs' fierce behavior yet, Serpell said. All of the theories could potentially play a role.

Some tiny breeds are an exception to the rule. One example is the Coton de Tulear, also known as the Royal Dog of Madagascar. This dog is small, white and fluffy, similar to a bichon frise. 

"For some reason, that breed seems to have quite subdued behavior across the board, but it also has all kinds of medical problems," Serpell said. "It's unclear whether that's because they maybe lack some critical genetic factor. Or it could be related to the fact that they're just more unhealthy and less physically able to react strongly." 

Some aggressive behaviors can be addressed safely while living with the dog, but a professional dog trainer should be the one to work on correcting the behavior, according to VCA Animal Hospitals, a system of animal hospitals.

Body size may not be the only attribute of small dogs that is associated with aggression. Lower height alone is associated with undesirable traits in some breeds, according to a 2013 study in the journal PLOS One. For example, dog breeds with shorter legs are more likely to be fearful of unfamiliar dogs and to have owner-directed aggression. It's unclear why this is the case, but it may be something to keep in mind when you're looking for your next pet.

Editor's Note: Updated on June 8, 2021 at 3:33 p.m. EDT to fix the name of VCA Animal Hospitals. It is not Veterinary Centers of America, as previously stated.

Originally published on Live Science.