Recognizing one's self in the mirror requires sophisticated cognitive function and a sense of self awareness. Children do not acquire the ability to recognize themselves in the mirror until 18 - 24 months of age. This was demonstrated in experiments involving putting red dots on their faces. Younger babies showed little interest in the facial decorating, however the older kids began touching their own faces indicating that they understood it was their own reflection they were looking at. Similar experiments have been carried out with other primates and even dolphins and magpies showing some degree of self awareness. That is they understood that this is "MY" face, it is part of "ME".
When dogs first encounter a mirror they respond as if it is another dog. You may have seen this yourself when a young dog attempts to play with the reflection as in this video, or take himself as a threat and start growling. Eventually most dogs become habituated to the reflection and just ignore it. So does this mean dogs haven no sense of self awareness and therefore lack consciousness?
These studies all centred on animals whose primary sensory input is vision. Dogs, as we all know, are olfactory centred creatures. University of Colorado biologist Marc Bekoff tried an experiment involving one of dogs' favourite pastimes, sniffing pee. Over the period of five winters, he relocated yellow snow to different locations, including those of his own dog whom he had earlier walked. On the next walk each time as expected his dog would stop and sniff the yellow snow and in most cases pee over top of it. But when he encountered his own relocated yellow snow, would briefly sniff then ignore. Bekoff concluded that we can say that dogs do have some of the same aspects of self-awareness that humans have. Mirror reflections, having no smell, just aren't interesting enough to demonstrate it.
Other experiments have been done to see if dogs understand how mirrors work. Tiffani Howell and Pauleen Bennett of the Anthrozoology Research Group set up an experiment in which one dog owner would direct the dogs attention to the mirror, while another owner holding a toy or treat could only be seen in the mirror. Out of 40 participants only 7 turned to look at the second person. As I recall high school science class, the angle of incidence is equal to the angle of reflection. This probably means as little to Twist as it now does to me, but he never forgets how to sniff out a treat.
References: What do Dogs See in Mirrors, Julie Hecht, Scientific American,
21 August 2017
Does My Dog Recognize Himself in a Mirror?, Psychology Today, Stanley Coren, July 7, 2011
What Do Animals See in the Mirror? Liz Langley, National Geographic,
February 14, 2015