Do pets know when they're terminally ill or dying?

The question of what dogs and other animals know when they're sick or dying is very much up in the air. I have numerous stories from people, especially about dogs, sometimes with cats and other companion animals, who say they know their dog who was dying knew that s/he was dying. I'm not sure how we could know that, but I want to be really careful to say that just because I don't know it, and I'm not sure anybody really knows it, that doesn't mean animals don't know when they're dying. It's just really hard to know for sure.

The question also rises, whether they know that another individual is dying. Once again, we just don't know. One of the things that's really important is that when an animal disappears, you can't conclude they've died. For example, I've done a lot of field work and my students and I did an eight-and-a-half-year project on wild coyotes living in the Grand Teton National Park. 

One instance that really comes to mind is when a female, a mother, disappeared. She was a mother and a wife of her mate. She disappeared and we have no idea what happened to her; nobody reported finding a corpse. It's possible she passed away. It's also possible she left the study area. We don't know, but the behavior of her family pack changed. They began taking forays out, clearly looking for her. They stopped playing. They stopped hunting; they had enough food around to last for a while. Their whole demeanor changed. I feel comfortable saying they were saying in their head, "Where's Mom? Mom is missing". But once again, I don't think they were going, "Mom isn't here anymore, and she died". It's really hard to know what the grieving animals know about the fate of that individual. 

There have been some very interesting and moving photographs of chimpanzees at a sanctuary gathering around the fence when an individual had died. Depending on who's writing the story, the interpretation is that they've come to awaken the one who died or they've come to mourn the loss of that particular chimpanzee. Once again, I would stress that what they know about themselves and what they know about others is up in the air. 

Please don't interpret that as me saying they don't know they're dying, or they don't know that others have died. We just don't know.

When my cat Cougar was ill (he was initially misdiagnosed), my other cat Sapphire would go and lay next to Cougar. I knew something was terribly wrong because those two were always mean to eachother. But at this time Sapphire showed Cougar extreme love. He knew. Cougar, it turns out had pancreatic cancer. Although it was so sad, it was also very beautiful to see this end of life love extended to his "enemy".