杏吧原创

Towering above

Is there any truth in the oft-heard statement that male offspring, when they become adults, will always be taller than their mothers? This question was triggered by a link on the BBC website. Intuitively, wouldn鈥檛 it make more sense to say male offspring will always be taller than the shorter of two parents, this usually being the mother? If this condition is true, does it apply to other species?

鈥� Not every son will grow taller than his mother. Figures quoted from the UK Child and Family Nursing Service (see ) show that a fully grown adult鈥檚 height is the average of the height of the parents plus 7 centimetres for sons or minus 7 centimetres for daughters. So a mother 180 centimetres tall and a father 150 centimetres tall should expect sons of 172 centimetres, significantly shorter than mum. If the formula holds, sons will obviously be taller than the shorter parent and daughters shorter than the taller parent.

In reality the formula is for likeliest height, not every person鈥檚 actual height: the calculations give only the peaks of the normal distribution curves for heights. They tell you what to bet on, but you could get daughters taller than anyone else in the family and sons shorter.

What鈥檚 more, apart from normal variations, particular genes and medical conditions often make nonsense of reasonable expectations. Some genes can cause various forms of height abnormalities in otherwise normal families. In achondroplasia, for example, the long bones fail to elongate, giving people with the condition unusually short limbs. Pituitary malfunctions may produce offspring who are unusually tall, or well-proportioned and exceptionally short.

Jon Richfield

South Africa

Floundering about

How do certain animals, such as flounder, change their colour to match their background?

鈥� Many fish in the teleost group, such as the minnow, change colour in response to the overall reflectivity of their background. Light reaching their retina from above is compared in the brain to that reflected from the background below.

The interpretation is transmitted to the skin pigment cells via adrenergic nerves, which control pigment movement. Teleost skin contains pigment cells of different colours: melanophores (black), erythrophores (red), xanthophores (yellow) and iridiophores (iridescent). Pigment granules disperse through the cell from the centre. The area covered by the pigment at any time determines that cell鈥檚 contribution to the skin tone.

Many flatfish, including flounder, go further than overall reflectivity and develop skin patterns according to the light and dark divisions of their background (as in the questioner鈥檚 photograph, New 杏吧原创, 27 July). This seems to involve a more discriminating visual interpretation and produces distinct areas of skin with predominantly, but not exclusively, one type of pigment cell. For example, black patches contain mainly melanophores and light patches mainly iridiophores, which can produce the chequerboard appearance seen in the picture.

Since these responses are visual, blindfolding the fish would result in all the components of the chromatic system being stimulated equally. The fish would adopt an intermediate dark or grey skin tone similar to that on a dark night. Over time, hormonal responses via direct light stimulation of the pineal gland through the skull also affect the amount of pigment and number of cells, hence the 鈥渂lack鈥� plaice sometimes sold in Britain, which have come from the sea around the dark volcanic seabed off Iceland.

Cliff Collis

London

鈥� Many animals change the shade or even colour of their skin in response to certain stimuli. In cephalopods such as the cuttlefish, pigment-filled sacs can become extended (flattened) by the action of radially arranged muscle fibres that are controlled by the nervous system. Colour change in these animals is both rapid and spectacular.

In crustaceans and many fish, amphibians and reptiles, specialised dermal pigment-storing cells called chromatophores relocate the pigment internally. The pigment in these chromatophores is either concentrated in the centre of the cell, or dispersed when the pigment fills the cell to the edge.

Imagine a white floor with a small pot of black paint standing in the middle. From above, the floor will look very light, despite a substantial amount of pigment seen as a small black spot in the middle. When the same paint is spread over the floor, the floor looks black. The beautiful trick of the black chromatophores (known as melanophores) is that they can reverse the process, concentrating the pigment in a small area.

Flatfish, such as plaice, flounder and others, are expert at imitating not only the general shade of the surface on which they rest, but also patterns of dark and light material. Not surprisingly, perhaps, their eyes are used to perceive the shade and patterns. Light hitting the retina from above affects the ventral or lower area of the retina, while light reflected from the bottom strikes the dorsal or upper retinal surface.

If the light intensities from the two areas are similar, a signal causes the pigment of the melanophores to be concentrated in the centre of the cell, so the fish turns pale. On the other hand, when the bottom is dark the two areas of the retina receive very different light intensities, and the reverse of the signal causes pigment dispersion and a dark fish. The masters of disguise, the flatfish, can also discern patterns in the bottom surface and imitate them by regulating nerve activity to groups of melanophores.

Stefan Nilsson

Gothenburg University, Sweden

This week鈥檚 questions

Double chin

My son-in-law discovered a strange property of the remote control that switches his car alarm on and off. By pressing it firmly to his chin, he can increase considerably the range at which it works. Not believing him, we experimented with other chins. It works, and in some cases the range is doubled depending on the type of car alarm. How does this effect occur?

Mike Welch

Stockton-on-Tees, Cleveland

Vodka on the rocks

When I placed a bottle of vodka in the freezer compartment of my refrigerator, I noticed that after a few hours it contained what appeared to be rhombic ice crystals about 8 millimetres long each side. Strangely, they were all floating vertically. Why?

Ian Cameron

Ontario, Canada

See hear

I watch cricket and football matches on television with the sound turned down so I can listen to the commentary on the radio. Can anyone explain why there is about a half-second delay between what I hear and what I see?

Jenni Thornley

Brigg, North Lincolnshire