Not smell? At Scientific American blogs, animal behaviour researcher Julie Hecht asks,
Does a dog know, merely by sight, that an approaching being is a fellow dog? Before you answer, remember this: Canis familiaris is the least uniform species on the planet. Members of this species come in a wide range of body shapes and sizes from itty bitty teeny weeny to absolutely ginormos. Adult members of this species appear as tight little packages, huge weightlifters, lean ballerinas, elongated hotdogs and everything in between.
Of course the obvious response is, “How do humans manage it?”
The old canard about the gullible couple buying a chihuahua that turns out to be a yappy rat* is funny precisely because it is not likely to happen. If that were a likely fraud, it wouldn’t be a joke at all.
Hecht describes a test:
To find out if dogs could ID other dogs based on appearance alone, the researchers first had to create a common language with their dog subjects. They did this with the help of three training sessions where dogs received a treat only when they approached the screen that had a picture of a dog’s face. Importantly, the same dog picture was used throughout the training sessions. During the training phase, the other screen was either all black, all blue, or had a picture of a cow’s face. The dog subjects were not rewarded if they approached any of the other non-dog pictures. This created a common language: “You are rewarded for approaching this ‘dog’ image, nothing else.”
We hope the treat was porterhouse steak because, with varying degrees of success …
The dogs prevailed! The nine subjects successfully identified “dog” from “non-dog” faces. More.
The researchers hope next to discover how dogs know each other only by sight.
Note: Snopes traces the story to the US, early 1980s and draws moral lessons about immigration. But I recall the legend clearly from Canada in the 1960s. At the time it seemed to descend from legends about people buying a piglet unseen in a sack—and it turns out to be a tomcat who runs away and climbs a tree.
It is likely a millennial (possibly perennial) joke, and maybe that aspect of the situation should be considered too: How do people really know, despite massive differences in canine appearance?
Is there a Platonic form “dog,” that survives the schemes of breeders?
See also: Furry, feathery, and finny animals speak their minds
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