Putrefaction and decomposition gas
After death, bacteria that live without oxygen break down tissue and release gases that make a body swell, and sometimes burst.
Putrefaction is the stage of decay in which microbes, mostly bacteria from the gut, break down an animal's tissues once the oxygen in them has been used up. These anaerobic bacteria feed on proteins, fats and sugars and give off gases: mainly hydrogen sulphide, carbon dioxide, methane and nitrogen.
The gases collect in the body cavity, so the body swells. This is called the bloat stage. As the pressure rises and the skin weakens, fluids are pushed out and the body can split open.
An everyday example is a sealed tin or jar of food that bulges: microbes inside are making gas with nowhere to go. The bigger the animal, the more gas there is to trap. In January 2004 a 56-foot sperm whale burst open on a lorry in Tainan, Taiwan, from the gas built up inside it.
A common misconception is that decay needs air. The early stages of putrefaction happen precisely because the air, and its oxygen, has run out.
