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Head trauma

How much does a human head weigh? Obviously I can measure the volume of my head by simple water displacement, but I can’t tell its density, nor can I work out the weight and density of its various components. Can any of your readers help me out?

• Measuring the weight of your head involves effectively isolating it from the rest of your body. Decapitation has the obvious disadvantage of you not being alive to see the results. However, there is a solution. Your neck vertebrae are responsible for holding your head’s weight. If you hang upside down from your feet the vertebrae in your neck move apart slightly because of the weight of your head pulling on them.

To weigh your head you must simply lower yourself slowly onto a scale while hanging upside down. You continually observe the distance between the top vertebra of your neck and your skull, using, say, an ultrasound scanner, and the instant the vertebra starts moving toward the skull you stop and read the scales. Because your neck is not imparting any force onto your head this isolates your head from your neck thus giving an accurate measure of your head’s weight.

Andy Phelps

Burnham-on-Sea, Somerset, UK

• As a canoeist and kayaker, I remember when learning to do an Eskimo roll that my instructor told me to make sure that however much I needed a breath, the last thing to leave the water as my body emerged should be my head. He said the average human head weighs around 4.5 kilograms. Unfortunately, I found that to be a lot of extra weight to lift clear of the water using only the blade of a paddle!

Andy Wells

Grantown-on-Spey, Highlands, UK

• Andy Wells seems to be about right in his recollection of a head’s weight. We weren’t able to measure the weight of a head directly but we did measure its volume and guess its density on the assumption that the brain, like the rest of the body, is mostly water and we know the density of water at 0 °C.

To measure the volume of the head a suitable, virtually bald, volunteer from New ÐÓ°ÉÔ­´´ lowered his head into a bucket of water filled to the brim. The water was as near to 0 °C as the volunteer could bear and his head was lowered vertically and crown downwards until the water reached the base of the chin. The water that spilled over the sides of the bucket collected in a larger bowl in which the bucket was standing and its volume measured. This was repeated five times. The average volume of water displaced was 4.25 litres, giving an estimate of the weight of a human head at 4.25 kilograms – Ed

Vicious fruit

Why did pineapples evolve a fearsome array of spiny leaves that make the large, sweet and juicy fruit almost impregnable? Surely the usual purpose of a sweet fruit is to encourage seed-dispersing animals to eat it. So what disperses the pineapple in its native range?

• The short answer is that pineapples aren’t eaten in the state that we normally see them but after they have ripened much further and fallen to the forest floor.

The pineapple, Ananas comosus, was originally found in South Brazil and Paraguay and indigenous peoples spread it throughout South and Central America and to the West Indies.

The plant is a herbaceous perennial and grows up to 1.5 metres high and 1 metre wide. It has a rosette of long pointed leaves around a terminal bud. This bud produces the flowering stem which turns out an inflorescence of reddish purple flowers, each attached to the rest of the plant by a leaf-like structure called a pointed bract. In the wild these flowers may be pollinated by humming birds and will produce small, hard seeds in the fruit.

As everyone who eats pineapples knows, commercially grown fruit have no seeds. That is because pineapples, like bananas, will still develop fruit even if they are not pollinated and fertilised. Like many other plants, pineapples are unable to self-pollinate

The pineapple fruit is created by the fusion of between 100 and 200 individual fruitlets that are embedded in a fleshy edible stem (see photograph, above). The ovary of each flower becomes a berry, and all the berries coalesce into one solid structure. This is referred to as a multiple fruit or sorosis. The tough, waxy impregnable skin still contains the pointed bracts and the remains of the flower.

Although the pineapple plant can grow from seed, it also spreads very efficiently by a variety of vegetative means: from slips that arise from the stalk below the fruit, suckers that originate at the leaves, crowns that grow from the top of the fruits and ratoons that come out from the underground portions of the stems.

The pineapple we buy today in the supermarket is very different from its natural relatives in South America. The wild pineapple is much smaller. By the time it has dropped off its stem, hits the ground from quite a height and lain on the forest floor for a few days in the hot sun, it is very ripe and very soft. So when eaten, it is likely to be mushy and to split open easily, revealing the sweet and juicy fruit inside.

Humans tend to eat commercial pineapples and bananas before they are truly ripe. However, a soft, mushy pineapple lying on the ground is likely to be attractive to many animals including monkeys and small mammals, which will help spread the seeds.

Thanks to Philip Griffiths, Royal Botanical Gardens, Kew, London, UK for his help in compiling this answer

This week’s question

Frost-free freezing

I’ve just bought a brand new freezer that never needs defrosting. I understand how, in old freezers, condensation introduced each time the door is opened leads to surfaces frosting up. But I can’t work out how this can be avoided in the new non-frosting models. So how do they make my life so much easier?

Martin Ryan

Harrow, Middlesex, UK