a secret ingredient in vanilla ice cream ...

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

Vruyr Gabrielian , UK

Answers
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, California, US



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 flavouring 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 , By email



As the mother of both a 9-month-old baby and 3-year-old child who has recently been toilet-trained, I've noted that faeces smell differently according to what the child has eaten. A fully breastfed baby's faeces are usually a mustard-yellow-brown colour and, while this is a shock for the new mother, it is not very smelly at all. When the child starts solids, the texture, colour and odour change ? and it smells stronger. Once the child starts eating meat, it becomes basically the same as adult excrement. It's about this time that most of us wish our children still drank breast milk.

Christina Martin , Hornsby Heights, New South Wales, Australia



Ecologically, excrement is so dangerous, so valuable and so versatile that it pervades interaction within and between species. 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

[colmurph]

I had a cocker spaniel who used to eat his own droppings. The vet told me that dogs will do this to get all the minerals out of it. He said that the food goes through them too fast for the minerals to be absorbed the first time. I said "Fine....you sit on my sofa when he wants to lick someone's face"! I got some mineral supplements that I started feeding him and he stopped scarfing his own poop.