They may have the largest physical variety among all animal species on Earth, but dogs can still recognise one of their own over any other animal based on simple images of their faces.

Since their domestication somewhere between 15,000 and 100,000 years ago, dogs have been learning to use facial cues as an important part of their social communication. When interacting with us, dogs can read and use our facial expressions to gauge where our attention lies and sometimes what we're feeling. Studies have also shown that they're better than wolves and some primates at understanding the hints we give them about an object's whereabouts - particularly if it's a treat - by reading our eyes and head movements. Dogs also display a range of facial expressions themselves, which researchers believe are used for communicating with other dogs, whether it's to impart hostility, friendliness, fear, and so on.

Along with the ability to make and read facial expressions, another important part of being a social animal is recognising the face of your fellow species member, or conspecific, even when the other senses are masked. A number of studies have demonstrated the ability of certain animals to recognise their conspecifics based exclusively on visual cues, including Rhesus macaques, sheep, cows, and some birds and invertebrates.

Dogs also fall into this category, which is pretty impressive, considering the huge variety of breed combinations and morphological differences within this species. Breeds range in size from the biggest 100-kg Mastiffs to the smallest 1-kg Chihuahuas. Tails can be straight or curly, as can their coats, which can come in many colours, lengths and hair types. Ears can be floppy or pointy. Eyes can be obscured by fur or excess skin and mouths can be droopy or taut. There are now 400-500 recognised dog breeds around the world, and because dogs breed quickly, there's no sign that this morphological variety will slow down any time soon.

Knowing that dogs can identify a picture of another dog, two independent teams of researchers, one led by Anais Racca from he Department of Biological Sciences at the University of Lincoln in 2010 and the other by Sanni Somppi from the Faculty of Veterinary Medicine at the University of Helsinki in 2012, tested if dogs could recognise pictures of other dogs when also faced with pictures of humans and inanimate objects. While both confirmed that this was the case, the teams restricted their experiments to 24 pictures per category, and this, says Dominique Autier-Dérian from LEEC and National Veterinary School in Lyon, France, in a new Animal Cognition paper, is not enough. “Because of the small number of stimuli used, these studies could not take into account morphological species-specific diversity. In fact, there is more morphological diversity among breeds in domestic species compared to wild species."

So Autier-Dérian and his team put together a more comprehensive experiment to test whether dogs can recognise each other as a separate group, away from other animals, despite their incredible physical diversity. Nine dogs were selected (see profiles above) and routinely exposed to a large variety of dog, human and other non-dog species' faces, with the aim of better understanding how well they know their own kind.

The nine dogs were exposed to a pool of 3,000 pictures of different breeds, both pure and mixed, on one of two computer screens. On the other screen, a selection of 3,000 faces from 40 wild and domestic “non-dog species” including cows, cats, rabbits, birds, reptiles, wild felines and humans appeared*. The computer each would appear on was randomised, and the faces were scaled to take up 70% of the canvas for uniformity. They could be front-on, side-on, or turned slightly towards the viewer. Each dog ended up viewing more than 144 pairs of pictures randomly selected from the two pools, and never saw the same picture twice.

The dogs were trained to wait in a windowless “sitting area” where it would view the two computer screens. When two images appeared on the screens, the dog would be told “image!” from an instructor behind, prompting it to move towards one of the screens and place its paw on a tablet in front of it. The instructor would then say ‘‘place!/here!’, and the ’dog would retake its original sitting position to await the next pair of images. If the dog choose the face of another dog, it would receive a treat, indicating that the instructor wanted it to choose a dog’s face every time. If the dog made an incorrect choice, it would return to its sitting area empty-handed.

The researchers found that despite the huge diversity of dog breeds and non-dog species shown to these nine dogs, each one managed to successfully recognise which faces belonged to dogs and which faces didn't. Whether it was the face of a big, shaggy dog or a tiny, sleek one, the dogs managed over a number of different trials to lump them all into the same category, away from any of the other species.

“The fact that dogs are able to recognise their own species visually and that they have great olfactory discriminative capacities insures that social behavior and mating between highly morphologically different breeds is still potentially possible and therefore that, although humans have stretched Canis familiaris to its morphological limits, its biological entity has been preserved,” the researchers concluded.

What’s left to do now is to figure out what physical characteristics the dogs were using to distinguish the pictured dogs from the other species.

*The researchers note that this time, no faces of wolves or foxes were included.

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