Size Differences Between Dogs & Why It Matters

A while back I read an article from the San Francisco SPCA on size differences between dogs. Their adoption policy, which I'd never heard of in other rescues, is to use a "50% rule" when adopting a dog into a home with existing pets.  This 50% rule states that the smaller of the two dogs must be at least half the weight of the larger dog. Common sense tells us that smaller dogs are more fragile and so playing, living and socializing with larger dogs does put them at greater risk for injury. But the specifics as to why the SF SPCA chooses to make this policy is something interesting, informative and important for all dog owners and rescues to be aware of.  It's called "Predatory Drift"

According to trainer Kelley Filson, CTC, CPDT:
Predatory Drift is a sudden, and drastic change in a dog's demeanor that is characterized by behaviors associated with hunting small prey. The term is most often used to describe a medium to large dog who has suddenly and uncharacteristically targeted a smaller dog as prey.  Predatory Drift is NOT Aggression, but it can mean injury or death for small dogs. Predatory Drift happens when the larger dog's instinct to hunt are triggered. These instincts can be triggered when play escalates or gets too much like the real thing (an out of control chase game).  It can happen when a small dog gets scared or injured and squeals or wriggles in a way that makes them look like prey, Predatory Drift can happen just because the size difference says, "You are comparatively bite-sized, or move like something that is bite-sized, and I am a canine predator." The most alarming fact about Predatory Drift is that it can happen even with well-behaved, well-socialized, playful dogs who play well and often with no aggression, and no fights. Dogs who are triggered into predatory drift, may or may not have ever been in a dog fight, and may or may not be generally well-behaved and obedient. 


This information is another example of why we must always remember that while our pets are also family members, they are not human. They are animals with instinct and sometimes that instinct kicks in. This isn't to say that small and large dogs can't get along, but it is important to always supervise interactions between dogs, understand your dogs body language and behavior and to safely and effectively separate dogs when they cannot be supervised.

I think the authors on this topic have said it best and so I have referred to an excerpt from an article by trainer Gail T. Fisher on the subject below. Read on to learn, be informed and understand an important aspect of dog behavior...


There are six specific behaviors that may be included in dog play: stare, stalk, chase, nip, grab and shake.


Not all dog play involves each of these. Your dog may race around and wrestle with another dog (chase, grab and shake/wrestle). Or grab a toy and shake the heck out of it. And depending on genetics, your dog may stare intensely at a buddy, stealthily approach, then suddenly break into a great game of chase -- all normal, harmless, fun behaviors.


Play behaviors are programmed in the dogs’ DNA, but not as play. These are all hunting behaviors related to survival: finding, approaching, catching, and killing dinner. 


The domestic dog shares many of the same genes as its relatives, members of the genus Canis (the domestic dog is Canis lupus familiaris). This genus includes wolves, jackals, coyotes and dingoes. Even the sweetest dog in the world shares DNA with his predatory ancestors -- hunters and scavengers. Play between wolf cubs looks just like dog play, but the wolf cubs are honing their skills for hunting as adults.


Consider the behavior chain you’ve likely seen in a film of a wolf pack on the hunt. The wolves eye their prey, staring intently. Then they stalk stealthily closer, sneaking as close as possible before being noticed. Sensing danger, the animal takes off, and the chase begins. When the wolf catches up, it bites, grabs, shakes and kills.


With the exception of “kill,” each element of the hunt is what we see in normal, non-violent play between dogs: stare, stalk, chase, nip, grab, and wrestle. Put another way: Dogs play behaviors are the vestiges of predatory behavior. Which brings me to the topic for this week’s column: “predatory drift.”


Predatory drift happens when normal play suddenly “drifts” over the line and the dog’s predatory instincts kick in and take over. At that moment, if something doesn’t happen to stop the behavior, it can turn ugly. 


Predatory drift can happen suddenly and unpredictably. It can even occur between two dogs who have played together for years – or live together and have always gotten along. Suddenly something triggers a fight.


The trigger for predatory drift can be pain, over-arousal (play that gets out of hand), or a yelp or scream. It can happen when a dog is hurt or frightened by another dog in play and squeals. The squeal triggers a predatory reaction, which can ripple through the entire pack in an instant. Unless people are right there to stop it, the result is what happened to Jasper the Old English Sheepdog I wrote about, who yelped and was attacked by 13 dogs.


The attacking dogs are not bad dogs. They are simply dogs. It isn’t a conscious reaction; it’s visceral and instinctive. 


It isn’t just dogs that behave this way. Consider what happens to a mob of fans at a soccer game, or a group of normally-civilized people standing on the sidewalk yelling “Jump!” to the poor, troubled person out on the ledge. Or consider how differently teenagers may behave in a group, or gang members when they’re together in a “pack.” 


It isn’t that two differently-sized dogs might not play very well together, but if predatory drift occurs between dogs of disparate sizes, the “shake” can kill the small dog quickly, before anyone can even begin to intervene.


My purpose in writing about these topics is not to alarm or upset readers, it is to educate so you’ll be your dog’s best advocate and protector. Dogs greatly enjoy and benefit from playing with other dogs in a multi-dog setting such as a playgroup, dog park or daycare. It would be sad to deprive them of this terrific outlet and enjoyment.


The caveat for owners is to be aware of the possibilities and take all due care. Don’t put your dog in an unsupervised or poorly supervised play environment, and at a dog park, don’t put your small dog in a group with larger dogs, and stop overly exuberant play before it might go over the top. 


Stopping play is a simple as stepping between two dogs, and redirecting their play, or holding them quietly on leash for a minute or so to give them a chance to settle down. If the highly aroused level of play continues, the dogs are likely overly stimulated, and it’s time to go home. 

I hope that this information provides some insight into dog behavior. My goal is to help dogs be understood as animals and for humans to understand their dogs' behavior. Far too often we anthropomorphize, project, misinterpret or even take personally our dogs behavior when we see them through human eyes. It is important to remember that though these amazing and wonderful creatures have managed to become human-like in our eyes, they still retain their animal instincts. They still remain animals whose behavior, communication and learning abilities are magnificent, intriguing and quite interesting to learn about when we take a moment to step out of our world and take a glimpse into theirs.