It turns out that dogs are not very good at recognizing themselves in a mirror. A story in National Geographic quotes Liz Stelow, an animal behavior clinician at University of California, Davis, who says,
"Dogs are very intelligent and adaptable creatures who, like countless others, lack the cognitive development necessary to self-recognize visually, whether in a mirror, on a video, or in a photo." (Though they do seem to know when they see another dog or human.) So which animals trump the dog? s Studies show that elephants, dolphins, magpies, and some great apes species can tell that they're looking at themselves in the mirror. We shouldn’t be too hard on our canine buddies for leading the pack: Dogs use their incredible scenting ability as a recognition took, so the visual recognition just didn’t evolve in them as much as it did for other species.
Diana Reiss, an animal psychologist at Hunter College in New York and author of The Dolphin in the Mirror, notes that self-recognition is not as easy as it may seem: "It takes a good deal of sophisticated integration of information about yourself and your own movements and what you're seeing in front of you in that glass," she tells the magazine.
Even human babies need practice—it takes 18 to 24 months for them to understand what they're seeing in the mirror.