Rigor mortis is the stiffening of the body after death, and it is caused by the breakdown of ATP, which is the energy currency of cells. The larger the animal, the more ATP it has, and therefore the longer it takes for rigor mortis to set in.
For example, in humans, rigor mortis typically sets in within 2-4 hours after death, and it lasts for about 24 hours. In larger animals, such as cows and horses, rigor mortis may not set in for up to 12 hours, and it can last for several days.
The size of the animal also affects the rate at which rigor mortis progresses. In smaller animals, rigor mortis may set in quickly and progress rapidly, while in larger animals, it may set in more slowly and progress more gradually.
There are a number of factors that can affect the time frame for rigor mortis, including the temperature of the environment, the humidity, and the cause of death. However, the size of the animal is one of the most important factors that determines how long rigor mortis will last.