I don't think any of us who are observers of behavior by profession, and dog lovers by nature, could ever question whether dogs feel and express loss.
They grieve over the loss of a bonded companion pet ( which may be a hamster, cat, turtle or other dog- companion is companion), as well as their humans.
I volunteer with a breed rescue group. The breed is known for its extreme bonding to both people and other animals. On Petfinder.com, there are many bonded pairs of dogs for adoption, some dog/ cat bonded pairs, and I was told of a rabbit/ dog bonded pair.
Bonded pair means that the two are as close as a mother dog/ puppy and are extremely distressed when separated for even short periods of time.
We also take in dogs whose owners have died. Some of them never stop looking out the window, or listening at the door.
I remember reading about " Greyfriars Bobby" when I was a little girl.
Greyfriars Bobby was a
Skye Terrier who became known in 19th-century
Edinburgh,
Scotland, after reportedly spending fourteen years guarding his owner's grave, until his own death on 14 January 1872.
Here's the Wikipedia entry about him. It is extremely touching and I have always believed that G.B. did spend most of his time at his master's grave.
http://en.wikipedia.org/wiki/Greyfriars_Bobby