Can dog identify other dogs only by sight?

Humans can identify each other easily, because most of humans have similar head shape, body structure, and proportion of body and limbs. For example, an Asian can identify the human stand in front of he or she even that human comes from Africa. That is to say, humans can identify each other only by sight regardless the races in most of condition. However, there's a doubt that if dogs can identify each other by sight. As far as we know, dogs have lots of species, and there are big differences in sizes, fur color, and head shape between some species. Therefore, there are possibilities that dogs can't identify each other by sight. They may identify each other by olfactory investigation. 

To find out if dogs can identify other dogs by sight, researchers set up a simple experiment. In the experiment, there are two screens with a divider between them. In each trial, two images would show simultaneously on both screens, and dogs were signaled to approach one of the screen and rewarded if they approach the "correct" one. To find out if dogs can identify other dogs based on appearance alone, researchers create a common language with their dog subjects with the help of  training sessions. In the training sessions, dog subjects would be shown two images. One is a dog picture, and the other one is a non-dog picture. When dogs approach the screen which showed the image of dog, they will be rewarded. Importantly, same dog picture was used throughout the training sessions. Before move on to the formal tests, dog subjects must approach the dog image 10 out of 12 times in consecutive sessions. 

In the formal tests, dogs were shown never-before-seen non-dog images and never-before-seen god images. Same with the training sessions, dogs should approach the dog images to get treat. However, it was no longer a easy task because those images captured dogs' morphological diversity in shape, size, color, and ear position. In the meanwhile, there was also a wide range of non-dog faces including humans, cats, cows, and rabbits shown to dogs. If dogs can still approach the images of dog, it means that dogs can identify other dogs by sight alone.

In the end of the test, the dogs prevailed. All nine dog subjects identified dog faces from non-dog faces. Some dogs Babel, Bag, Cyane and Vodka, were able to do so quite quickly, taking few sessions to approach the required 10-out-of-12 dog images. Other dogs, like Bahia and Cusco, were slower on the pickup and took more sessions to identify “dog” from “non-dog” (dog subjects needed anywhere from 2 to 13 sessions to meet criteria), but it doesn't mean that Bahia and Cusco are not able to identify other dogs by sight. researchers indicated that the personality and behavior of dogs can also affect the result of this test. Even though there are some factors that affect the identification of dogs, dogs seem to be able to tell who falls in the category of dogs and who doesn't.