I'm pretty sure that if that human died in the animal's presence it would realize that they are dead. In a case where they were not together when the human being died, I could not say.
We hear of dogs waiting patiently for years for their master to appear again, so in the latter case maybe the dog's cannot understand death. But in the former, dog's have been known to set start howling as if in grief the instant their master dies.
Then there is a report of a cat that KNOWS when a patient in the hospital where it lives is going to die and goes in and spend the night in the same room. The cat senses whether or not that person is going to die despite the diagnoses and opinions of the medical staff. It goes into then leaves the room of the person who will survive the night, but stays in with those who do die. And apparently this has happened far more times than to be sheer chance.
I suspect we still know very little about what animals think and feel, but what we do know is that, as domestic pets, they can have a very close bond to us humans.
That sounds awfully cynical, Cecelia. Bit like a trainer at a guide-dog training facility I went to a few years ago who said that dogs only love you because they know you feed them. Sorry, I think there to it is more than just keeping warm.