Dogs Mourn

From Phys-Org:

Grief is one of the basic emotions dogs experience, just like people, said Dr. Sophia Yin, a San Francisco-based veterinarian and applied animal behaviorist. Dogs also feel fear, happiness, sadness, anger, as well as possessiveness. 

Dogs who mourn may show similar signs to when they're separated for long periods of time from the individual they're bonded to, she said. Of those signs, depression is the most common, in which dogs usually sleep more than normal, move slower, eat less and don't play as much. 

The beginnings of such a strong inter-species bond between humans and dogs dates back some 15,000 years, when early man and the ancestor of today's dog roamed the Earth together.