ÐÓ°ÉÔ­´´

No bull

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?

• 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, no address supplied

• 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

Ice Art

On some cold mornings the frost on windows and cars makes patterns that look just like leaves, ferns and branches. How does this happen?

• Waking up to frosty bedroom windows is becoming a thing of the past thanks to the insulating properties of double glazing and cosy central heating. But if you are still stuck with single glazing, on winter mornings your view will be obscured by fern-like patterns of frost.

Panes of glass lose heat quickly on cold nights, cooling the water vapour molecules in the indoor air nearest the glass. The temperature of the water molecules in the air can fall below 0° C without them actually freezing. But as soon as this supercooled water vapour touches the cold glass, it turns directly to ice without first becoming water.

Tiny scratches on the surface of the glass can collect enough molecules to form a seeding crystal from which intricate patterns then grow. Up close, the crystal surface is rough with lots of dangling chemical bonds. Water vapour molecules latch onto these rough surfaces and crystals can grow quickly. The structure of the elaborate branching depends on both the temperature and humidity of the air, as well as how smooth and clean the glass is. When the air is dry, the water molecules condense slowly out of the air and cluster together in stable hexagons. The six straight sides of these crystals are relatively smooth with very few dangling bonds, giving water vapour molecules little to hang onto.

Feather-like patterns are more likely to form on clean windows and when the air is heavy with water molecules. Under these conditions, lots of water vapour molecules bombard the seed crystal and there is no time for the stable hexagons to form. Instead, the molecules latch onto the dangling bonds that stick out of any bumps in the crystal, which means the bumps grow even faster. These bumps eventually grow into large branches, and in turn the bumps on the branches become lacy fronds (see New ÐÓ°ÉÔ­´´ 23/30 December 2000, p 26) – Ed

This week’s question

Blubber bullets

How fat would you have to be to be bulletproof, so that your fat layer would prevent a bullet fired from an ordinary handgun from reaching your vital organs? I recently read it was about 500 kilograms, but find this hard to believe.

Ward van Nostram

By email, no address supplied