Nothing quite ruins a country walk as when you gag at the foul odour emanating from corpse of a dead sheep or other animal.
Or worse, you might encounter a similar repellent stench in the loft or coming from behind a heavy appliance, indicating that a creature, probably a mouse, rat or a bird, has died there and is rotting.
The sweet, sickly, nausea-inducing smell is emitted as the animal decomposes and usually provokes a strong reaction in humans. Most will beat a hasty retreat and it is not uncommon for people to be physically sick. The smell of rotting fish may be even worse. And it is not just animals. Rotting plants, notably seaweed, cabbage or even nettles, can produce smells that bring out a sense of deep disgust in us.
So why do dead things smell so bad to us? And what produces the smell?
When an organism dies, its cells are no longer being kept alive by oxygen and the immune system stops working so it no longer keeps bacteria at bay. The body's own enzymes begin to digest the body's tissues. Microbes from within the body and the wider environment continue the process of breaking down the organic matter.
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During the decomposition of proteins, amino acids (lysine and ornithine) break down producing two particular molecules: cadaverine and putrescine.
As their names suggest, they are associated with death and decay and they are the chief cause of that unpleasant smell of dead and rotting creatures. And more are produced as the decomposition progresses. Things just get worse.
When fish die – particularly marine species - there is an extra element that adds to the foul reek. Sea-going fish have particularly large quantities of an organic compound known as trimethylamine oxide (TMAO) in their blood and cells.
This enables them to survive in the sea by balancing their internal fluids with the external saltwater. But when a fish dies, enzymes and bacteria break down the TMAO into trimethylamine (TMA), which is a foul-smelling gas. Add this to cadaverine and putrescine, which are also produced by rotting fish, and you have a truly heady cocktail to turn the strongest of stomachs.
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Rotting smells also seem to hang around for a long time, even after the source has decayed completely or has been removed. For instance, the 'dead mouse smell' in a loft or garage might linger for days or weeks after all corpses have been removed.
This is because the cadaverine and putrescine molecules are volatile organic compounds. The particles are heavy and oily, meaning they stick to surfaces or penetrate porous materials such as carpets, curtains and wood furniture.
They are continually released and so the smell lingers. It will eventually dissipate – with patience. Plus, our noses can get used to bad smells over time. Prolonged exposure to a strong smell leads to noseblindness or olfactory fatigue. It's a coping mechanism to help us live our daily lives without overloading our nervous system. Once the bad odour goes away, our sense of smell will rectify itself.
Human revulsion at the smell of cadaverine, putrescine and TMA is something that has evolved over time to protect us. Rotting flesh and even plant matter will contain a huge number of potentially dangerous bacteria, lethal molds and deadly parasites. So our sensitive noses are doing us a big favour. When these molecules are picked up in our nasal receptors, a big 'disgust' warning is sent to the brain and we avoid.
Most animals have evolved to react in a similar fashion. But there are some creatures that are actively attracted by these molecules. Carrion flies, rats, burying beetles and vultures all feed on dead and rotting flesh.
Rats and vultures, for instance, have evolved immunity to certain pathogens found in rotting flesh. They also have extremely strong stomach acid to kill off harmful bacteria and parasites before they can cause an infection. And it is a good thing, too. Without animals that clear up the dead, the Earth would be inundated with rotting organisms, bad smells and diseases.
So next time you recoil at the sight of a maggot or a vulture, try to remember that they are performing an important waste management service. Plus, for them it's a free meal.
