"Dr Friederike Range, of the University of Vienna's neurobiology department, has shown that dogs feel intense jealously when they spot that they are unfairly treated compared with other dogs. 'Dogs show a strong aversion to inequity,' she said."
While I love any research that sheds new light on animal self-awareness, I have to wonder if anyone living with more than one dog needed a study to be convinced that animals feel envy. (Paws up to pup-loving science-writer David Williams  for the alert on this research.)
Listen to NPR's more detailed story  about fair play research.