Apparently many animals have the ability to smell death. The smell of certain fatty acids (presumably decomposing) is enough to warn some animals instinctively that something is amiss. This mechanism likely evolved over 400 million years ago as a means of protecting animals from contagious illnesses and localized predators, according to this rather edifying BBC News article which lays out the history of this discovery.

Various mammals, including deer, groundhogs and rabbits will avoid the smell of blood. They also avoid the smell of various soaps, which are largely comprised of fatty acids

Rats will seal up burrow entrances that have been treated with cat litter, but it is not known what actual smell is the deterrent, or whether it is specific to cats or faeces

Mice can tell if another has been exposed to radiation or is carrying a virus

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