i think it completely has to do with the situation. if they are a house pet and they are just never around death. the only/first animal to die before their eyes, then i would say they are no more in the know that a baby who doesnt know either. though once they realize, especially if it is death in the wild where they will explore the body of the dead animal. and usually realize the gravity of the situation. When people who have more than one animal pets, if one of them dies, the other animals usually enter a grieving process that is physically noticeable. meaning i believe they understand what death is, but maybe not so far as to think they go anywhere, just that they are no longer moving and will not be moving anymore. and they sever the emotional ties to the dead animal and retain knowledge of the event. and that is about it. not much thought process. they understand that things die, but not so much the meaning or reason why it happens. Animals who are about to die also usually know, if its natural. dogs will stop eating and moving around as much, pretty much choosing the best place for them to die. its rather sad to witness.