We can speculate about ‘depth’ or ‘severity’ of mourning (how do you measure these things, anyway) but the fact that dogs suffer from separation is beyond doubt:
‘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.
Case in point: a grainy video posted on YouTube that captured the image of a scruffy terrier running onto a busy highway in Chile to rescue another dog, hit moments earlier, by a car. As vehicles whiz by the terrier, he instinctively wraps his paws around the injured dog, dragging him off the road to safety.’
I have witnessed dogs put themselves in danger to defend other dogs and people. If you think about it, that’s a profoundly ‘counter evolutionary’ thing to do. Yet we can all understand the depth of emotion like that.