How do dogs recognize other dogs?

How do dogs recognize other dogs?

I'm not asking where they sniff first. I'm asking how does a Great Dane know that a teacup Yorkshire terrier is as much a dog as he is, but that a Shetland pony, which I remind you is roughly the Great Dane's size, is not? How does an Irish wolfhound recognize a papillon as a fellow canine and not as a snack in the wolfhound's food dish?

That dogs do make this distinction was never more clear than when my daughters and I took part in a fundraiser for a dog shelter.

The event was a 5K run to which participants were encouraged to bring their dogs. Many people, as you may have noticed, need very little encouragement to bring their dogs anywhere. They bring them to shopping malls and parades and fireworks displays and weddings and fine restaurants. Now here was an occasion designed for dogs, and people brought them by the zillions. The shelter brought dogs too, each of them wearing an "Adopt Me!" bandana. "Adopt Me!" dogs could be borrowed by dogless runners, and many of them, caught up in the spirit of the thing, were clutching the leashes of animals they had never met before.

It was during this pre-race melee that I noticed how diverse -- how extremely diverse -- the dog kingdom is. Dogs are tall and gangly and enormous and wooly and massive and drooling; they're mid-sized and curly and sleek and rough; they're tiny and lean and roly-poly and fluffy and bug-eyed and wiry.

And that's not even considering their personalities: their confidence in a crowd, their ability to take a joke, their natural inclination to sniff or be sniffed.

"How do dogs know?" we wondered, looking here at a 3-pound terrier with ears the size of dinner plates; there at a statesmanlike German shepherd who seemed to be channeling Franklin Roosevelt; here at a Saint Bernard hogging a plastic wading pool; there at a haughty Jack Russell; and here again at a yellow Labrador retriever who was lolling across her owner's lap, tongue dangling and eyes glazed while she enjoyed a belly rub.

That dogs not only recognize other dogs but also have no biases in a state of nature was evident to anyone who looked at the many mixed breeds on hand. Although some were clearly the offspring of two different but pleasantly compatible breeds, others suggested wildly improbable match-ups: beagles and mastiffs; cocker spaniels and Doberman pinschers; chihuahuas and Rottweilers. Although the various mental images raised plenty of questions having to do with simple logistics, that nature triumphed was indisputable. The evidence was before us.

How then, did those dogs know that they were canoodling with another dog and not with an entirely different breed of animal?

I suspect the answer comes back to dogs' noses and what they do with them. I can't say I'm surprised.

Margo Bartlett is a ThisWeek staff writer: E-mail mbartlett@