Monday, February 28, 2005

Questions You Were Afraid to Ask

NEW SCIENTIST - It's an unfortunate question I know, but why does human excrement smell so badly? I realize cows eat different foods, but their excrement is far less offensive. Why is ours so awful?

Why Your Offal Smells Awful (abridged)

Your body ejects poisons and things it won't need,
And these are the things on which you mustn't feed.
If your offal smelt tasty, like fresh cherry pie,
Then you might eat it, and then you would die.

If your offal smells so bad that you kick it off of your plate,
You then may survive and may procreate.
If ever an animal found its waste in good taste,
Evolution has cured that condition post-haste.

- Mark Gilkey Palo Alto CA

The smelly substance in excrement is skatole (3-methylindole), and it is the substance to which the human nose is most sensitive on a per molecule basis. No doubt it is present in faeces because it is a breakdown product from haemoglobin that enters the gut via bile. We have evolved to be repelled by it because excrement transmits disease and we should steer clear.

However, skatole doesn't always trigger the disgust reaction in humans, nor is the reaction shared by the whole animal kingdom. The substance is used in small amounts as flavoring in food, notably in vanilla ice cream. Dung flies are attracted to it, and the phallus-like spadix of the arum lily (Zantedeschia aethiopica) synthesises skatole to attract flies that pollinate the flowers. The civet cat also has glands that produce skatole, presumably to mark territory and repel others. This gland is used in perfumery. - Bill Rathmell

Even though animals avoid their own dung, many relish the dung of other species: wild dogs covet the droppings of jackals, while hyenas eat the dung of both. Many a cute domestic dog loves the ordure of humans and cats. Tortoises eat weathered carnivore dung for its minerals. Chicken manure is valuable cattle feed and, in a shared enclosure, rabbits will gobble the droppings of cats before the flies get a look-in. Autocoprophagy also happens. Rabbits and many rodents eat certain types of their own droppings as a necessary part of their digestive process, much as cattle chew cud. Baby elephants eat elephant dung, thereby charging their guts with the digestive microbes they need. Other animals anoint themselves or scatter or accumulate their own excreta, either as protection, or to convey a wealth of territorial or social signals.- Jon Richfield Somerset West, South Africa

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