Rigor mortis in cats lasts about 12 hours on average. The onset of this process typically begins within 3 to 4 hours after the death of the cat and is dependent upon numerous variables.
The environment, physical condition, and the actual cause of death are all factors that determines the rate at which rigor mortis sets in. The size of the animal is also an important determining factor. The larger the animal, the longer the process takes to complete and this is when the cells and contractile proteins start to break down, returning the body to a softer state. It is at this point that the decomposition of the body begins.