Anything with a fear of death knows it is mortal (can be killed). I think even the most basic creatures understand this.

That seems a fairly circular argument - after all, can you be afraid of something happening to you without knowing that it can happen to you? Anything with a fear of asteroids or boogey monsters 'knows' that those exist.

So I disagree that "even the most basic creatures understand this", even for very broad definitions where understand can be interpreted as 'act, instinctively or not, as they would should they have consciousness'.

I suspect many understand pain and have a fear of pain (again, where very loosely pain is 'bad things' they have previously experienced). Death is often a corollary of that pain, so they will act similarly, but I don't believe creatures at a most basic level understand death.

Having said that, this was a fascinating article on a topic that has interested me for years, so I'd love to see more research supporting either your thoughts or mine.

For loose enough definitions of 'understand'.

For example, when I hold my breath, the urge to breath increases steadily until I am forced to start breathing again. Is this because my body explicitly understands that not breathing eventually results in death, or is it more appropriate to say the response is automatic?

It's a case of both. The diaphragm muscle in your abdomen is both controlled manually by yourself, and automatically by your brain.

The very core of self-preservation, which indeed is one of the most basic insticts, shared by even the simplest of the vertebrate, and quite possibly also by many other animals of "lower rank" taxonomy.

This has been observed with other seemingly less intelligent species as well, for as long as man has kept pets. Anyone who has ever had f.e. dogs or cats at home as part of the family, coupled with one pet in the "litter" dying suddenly or accidentally, knows how notably different the other animals in the home will react to this compared to when one dies from drawn-out illness or old age. It is like that other side of the coin of missing someone, something most of us know is a characteristic that animals share with us. Animals being in mourning during sudden deaths of their kinsmen and companions is something the pet keepers among the population knows very well - there just haven't been much of formal or scientific studies in the topic.

People expect their pets to be happy based on human features (shape of the mouth and eyebrows), which don't correspond to the actual mood of the pet.

So Long, and Thanks for All the Fish!