Why Don't Penguins' Feet Freeze?: And 114 Other Questions - Mick O'Hare (2009)

Chapter 3. Plants and animals

8986 Birdzzz

Why do birds never fall off their perches when sleeping? Do they, in fact, sleep?

Graeme Forbes

Kilmarnock, Ayrshire, UK

Birds have a nifty tendon arrangement in their legs. The flexor tendon from the muscle in the thigh reaches down over the knee, continues down the leg, round the ankle and then under the toes. This arrangement means that, at rest, the bird’s body weight causes the bird to bend its knee and pull the tendon tight, so closing the claws.

Apparently this mechanism is so effective that dead birds have been found grasping their perches long after they have died.

Anne Bruce

Girvan, Ayrshire, UK

Yes, birds do sleep. Not only that, but some do it standing on one leg. And even more surprising, may be hypnotised into sleep at will. My myna bird was.

If you wish to try it, bring your eyes close to the cage, and use the hypnotist’s ‘your eyes are getting heavier’ principles (not spoken) on your own eyes. Act as if you are gradually falling asleep and the bird will follow you, finally holding one leg up under its belly, tucking its head under its wing and falling into a deep sleep.

What’s more, most pet bird owners know that all you need to do to make your pet fall asleep is to cover the cage with a blanket to simulate night.

David Leckie

Haddington, East Lothian, UK

Birds do sleep, usually in a series of short ‘power naps’. Swifts are famous for sleeping on the wing. Since most birds rely on vision, bedtime is usually at night, apart from nocturnal species, of course.

The sleeping habits of waders, however, are ruled by the tides rather than the sun.

Some other species are easily fooled by artificial light. Brightly lit city areas can give songbirds insomnia. A floodlit racetrack near my home gives an all-night dawn effect on the horizon, causing robins and blackbirds to sing continuously from 2 am onwards. Unfortunately, I don’t know whether it tires them out as much as it does me . . .

Andrew Scales

Dublin, Ireland

8986 Tank madness

Following a recent bereavement we would like to know why fish jump out of small aquariums.

Rowan White and Vicky

University of East Anglia, Norwich, UK

Fish jumping out of small tanks is quite a common problem for enthusiasts, and is the reason why some owners choose to have a glass cover on the top of their aquarium.

There are several theories as to why fish might jump from a small aquarium. It has been suggested that one reason fish leap from the water is that in the wild they use this method to attempt to rid themselves of ectoparasites.

Although the questioners did not mention the gender and species mix of the fish in their aquarium, it is possible that their fish could have been leaping to avoid predators or unpleasant interactions with other creatures, or even to show off to their conspecific fish, in some previously unknown courtship or territorial ritual.

In the meantime, my sincere condolences for your loss.

R. Rosenberg

Stockholm, Sweden

To captive fish, the air on the other side of the aquarium glass looks like water. And in fish lore, the water is always cleaner on the other side.

John Chapman

North Perth, Western Australia

8986 Baaa-rmy

Why do sheep always run in a straight line in front of a car and not to the side?

Aled Wynne-Jones

Cambridge, UK

Sheep and other animals run ahead of cars because they do not realise that cars cannot climb grassy banks. Ancestral sheep were pursued by wolves and big cats. If an animal tries to turn aside some yards from the hunter, the pursuing animal sees what is happening, makes an easy change of course and intercepts the victim, which is presenting its vulnerable flank.

If, however, the prey dodges at the last minute, the outcome is different. The hare is the master of this strategy: as the greyhound reaches out with its jaws, the hare jinks to one side and the dog overshoots or, with luck, tumbles head over heels.

The instinctive response of a sheep or a hare to an approaching car is at least not as maladaptive as that of the hedgehog.

Christine Warman

Clitheroe, Lancashire, UK

Herbivores are killed by predators who normally grab them by the throat while running alongside, so the prey will always do its best to keep a potential threat behind its tail, swerving as the predator attempts to overtake. That’s why a kangaroo, seeing a car drawing alongside, will jump onto the road right ahead in order to keep the car directly behind it, and often get run over in the process. As long as a car proceeds in a straight line behind a sheep, the sheep will try to outrun it in a straight line.

G. Carsaniga

Sydney, Australia

Sheep are much underrated. They don’t merely run in a straight line – they run straight for a while, then dive to the side. This is not woolly thinking, it’s perfectly logical. Sheep loose in the road are usually confined to country areas, where roads are bounded by steep verges, cliffs, hedges, fences and ditches. The sheep recognises that if it cannot beat the car on the flat, it stands no chance whatsoever up a bank, so it attempts to outrun the vehicle down the road.

What happens then is that the vehicle slows, and when it reaches a speed that is slow enough for the sheep to think it might beat the car over the obstructions at the side of the road, it swerves. And since most of the time this action is proved correct (most vehicles don’t follow sheep off the road), the sheep carries on behaving in this way. QED, by sheep logic.

Clearly, this is a much more successful approach to road safety than that shown by humans. Humans rarely try to outpace the oncoming car. They tend to dive to the side of the road. Since more people are run over than sheep, one can conclude we have much to learn from sheep logic.

William Pope

Towcester, Northamptonshire, UK

Sheep, being clever animals with an instinctive grasp of psychology, know that most drivers, though enjoying an occasional kill as long as they can use the ‘it jumped in front of me, there was nothing I could do’ excuse, are not so depraved as to deliberately run something down. Thus running in a straight line has a distinct advantage over veering to the side.

Erik Decker

National Institute of Animal Husbandry Department of Cattle and Sheep

Tjele, Denmark

8986 Fried fish

My young neighbour asked me what happens when lightning strikes water. Do all the fish die and what happens to the occupants of metal-hulled boats?

Chris Cooper

Kempston, Bedfordshire, UK

When a bolt of electricity, such as a lightning bolt, hits a watery surface, the electricity can run to earth in a myriad of directions.

Because of this, electricity is conducted away over a hemispheroid shape which rapidly diffuses any frying power possessed by the original bolt. Obviously, if a fish was directly hit by lightning, or close to the impact spot, it could be killed or injured.

However, a bolt has a temperature of several thousand degrees and could easily vaporise the water surrounding the impact point. This would create a subsurface shock wave that could rearrange the anatomy of a fish or deafen human divers over a far wider range – tens of metres.

If someone in a metal-hulled boat was close enough to feel the first effect they would be severely buffeted by the second. Besides which, metal hulls conduct electricity far better than water, so a lightning bolt would travel through the ship in preference to the water.

Andrew Healy

Ashford, Middlesex, UK

When lightning strikes, the best place to be is inside a conductor, such as a metal-hulled boat, or under the sea (assuming you are a fish).

Last century, the physicist Michael Faraday showed that there is no electric field within a conductor. He demonstrated this by climbing into a mesh cage and then striking artificial lightning all over it. Everybody except Faraday was surprised when he climbed out of the cage unhurt.

Eric Gillies

University of Glasgow, UK

8986 Blowfish

Fish don’t fart, why is this?

Christine Kaliwoski

Brentwood, California, US

The writer probably thinks that fish don’t fart because she has not seen a string of bubbles issuing from a fish’s vent.

However, fish do develop gas in the gut, and this is expelled through the vent, much like that of most animals. The difference is in the packaging.

Fish package their excreta into a thin gelatinous tube before disposal. This includes any gas that has formed or been carried through digestion. The net result is a faecal tube that either sinks or floats, but as many fish practise coprophagia, these tubes tend not to hang around for too long.

Derek Smith

Long Sutton, Lincolnshire, UK

I have on several occasions witnessed my cichlids passing wind to the displeasure of my eel.

This seems to be a result of them taking in too much air while wolfing down flaked foods floating on the surface of the water. If the air was not expelled it would seriously affect their balance.

Peter Henson

University of London, UK

Most sharks rely on the high-density lipid squalene to provide them with buoyancy, but the sand tiger shark, Eugomphodus taurus, has mastered the technique of farting as an extra buoyancy device. The shark swims to the surface and gulps air, swallowing it into its stomach. It can then fart out the required amount of air to maintain its position at a certain depth.

Alexandra Osman

London, UK

8986 Cold feet

Why do Antarctic penguins’ feet not freeze in winter when they are in constant contact with the ice and snow? Years ago I heard on the radio that scientists had discovered that penguins had collateral circulation in their feet that prevented them from freezing but I have seen no further information or explanation of this. Despite asking scientists studying penguins about this, none could give an answer.

Susan Pate

Enoggera, Queensland, Australia

Penguins, like other birds that live in a cold climate, have adaptations to avoid losing too much heat and to preserve a central body temperature of about 40 °C. The feet pose particular problems since they cannot be covered with insulation in the form of feathers or blubber, yet have a big surface area (similar considerations apply to cold-climate mammals such as polar bears).

Two mechanisms are at work. First, the penguin can control the rate of blood flow to the feet by varying the diameter of arterial vessels supplying the blood. In cold conditions the flow is reduced, when it is warm the flow increases. Humans can do this too, which is why our hands and feet become white when we are cold and pink when warm. Control is very sophisticated and involves the hypothalamus and various nervous and hormonal systems.

However, penguins also have ‘counter-current heat exchangers’ at the top of the legs. Arteries supplying warm blood to the feet break up into many small vessels that are closely allied to similar numbers of venous vessels bringing cold blood back from the feet. Heat flows from the warm blood to the cold blood, so little of it is carried down the feet.

In the winter, penguin feet are held a degree or two above freezing – to minimise heat loss, whilst avoiding frostbite. Ducks and geese have similar arrangements in their feet, but if they are held indoors for weeks in warm conditions, and then released onto snow and ice, their feet may freeze to the ground, because their physiology has adapted to the warmth and this causes the blood flow to feet to be virtually cut off and their foot temperature falls below freezing.

John Davenport

University Marine Biological Station

Millport, Isle of Cumbrae, UK

I cannot comment on the presence or absence of collateral circulation, but part of the answer to the penguin’s cold feet problem has an intriguing biochemical explanation.

The binding of oxygen to haemoglobin is normally a strongly exothermic reaction: an amount of heat (DH) is released when a haeomoglobin molecule attaches itself to oxygen. Usually the same amount of heat is absorbed in the reverse reaction, when the oxygen is released by the haemoglobin. However, as oxygenation and deoxygenation occur in different parts of the organism, changes in the molecular environment (acidity, for example) can result in overall heat loss or gain in this process.

The actual value of DH varies from species to species. In Antarctic penguins things are arranged so that in the cold peripheral tissues, including the feet, DH is much smaller than in humans. This has two beneficial effects. Firstly, less heat is absorbed by the birds’ haemoglobin when it is deoxygenated, so the feet have less chance of freezing.

The second advantage is a consequence of the laws of thermodynamics. In any reversible reaction, including the absorption and release of oxygen by haemoglobin, a low temperature encourages the reaction in the exothermic direction, and discourages it in the opposite direction. So at low temperatures, oxygen is absorbed more strongly by most species’ haemoglobin, and released less easily. Having a relatively modest DH means that in cold tissue the oxygen affinity of haemoglobin does not become so high that the oxygen cannot dissociate from it.

This variation in DH between species has other intriguing consequences. In some Antarctic fish, heat is actually released when oxygen is removed. This is taken to an extreme in the tuna, which releases so much heat when oxygen separates from haemoglobin that it can keep its body temperature up to 17 °C above that of its environment. Not so cold-blooded after all!

The reverse happens in animals that need to reduce heat due to an overactive metabolism. The migratory water-hen has a much larger DH of haemoglobin oxygenation than the humble pigeon. Thus the water-hen can fly for longer distances without overheating.

Finally, foetuses need to lose heat somehow, and their only connection with the outside world is the mother’s blood supply. A decreased DH of oxygenation by the foetal haemoglobin when compared to maternal haemoglobin results in more heat being absorbed when oxygen leaves the mother’s blood than is released when oxygen binds to foetal haemoglobin. Thus heat is transferred into the maternal blood supply and is carried away from the foetus.

Chris Cooper and Mike Wilson

University of Essex, Colchester, UK

8986 Flying fins

Why do flying fish fly? Is it to escape predators, or to catch flying insects, or as a more efficient means of getting around than swimming? Is there some other entirely different reason?

Julyan Cartwright

Palma de Mallorca, Spain

The usual explanation for flight in flying fish is as a way to escape predation, particularly from fast-swimming dolphin-fish. They do not fly to catch insects; flying fish are largely oceanic and flying insects are rare over the open sea.

It has been suggested that their flights (which are actually glides, because flying fish do not flap their ‘wings’) are energy-saving, but this is very unlikely as the vigorous takeoffs are produced by white, anaerobic muscle beating the tail at a rate of 50 to 70 beats per second, and this must be very expensive in terms of energy use.

Flying fish have corneas with flat facets, so they can see in both air and water. There is some evidence to suggest that they can choose landing sites. This might allow them to fly from food-poor to food-rich areas, but convincing evidence of this is lacking.

There seems to be little doubt that escape from predators is the major purpose of their flight, and this is why so many fly away from ships and boats, which they perceive to be threatening.

John Davenport

University Marine Biological Station

Millport, Strathclyde, UK

Strictly speaking, the flying fish does not fly, it indulges in a form of powered gliding, using its tail fins to propel it clear of the water. It sustains its leap with high-speed flapping of its oversized pectoral fins for distances of up to 100 metres. The sole purpose of this activity seems to be to escape predators. If one can manage to tear one’s eyes away from the magic of the unexpected and iridescent appearance of a flying fish, a somewhat more substantial fish can often be seen following its flight path just below the surface.

Tim Hart

La Gomera, Canary Islands, Spain

I have seen whole schools of flying fish become airborne as they try to escape tuna which are hunting them, and minutes later have seen the school of tuna attempt similar aerobatics as dolphins move in for their supper of tuna steaks.

A morning stroll around the decks of an ocean-going yacht will often provide a frying-pan full of flying fish for breakfast. Presumably they are instinctively trying to leap over a predator (in this case the boat) but as they don’t seem to be able to see too well at night they land on the deck. They rarely land on board during the day. Most alarmingly they will land in the cockpit, and even hit the stargazing helmsman on the side of the head.

Don Smith

Cambridge, UK

8986 Not mush room

Near where I live there are toadstools growing through the pavement, the surface of which they have displaced in fairly large chunks. What mechanism allows toadstools – essentially very soft and squashy items – to push through two inches of asphalt?

John Franklin

London, UK

The toadstools forcing their way up through asphalt are probably ink-cap mushrooms (Coprinus) growing on buried plant debris. They are pushing upwards because their stalks function as vertical hydraulic rams.

The upward pressure comes from the turgor pressure of the individual cells making up the wall of the hollow stalk of the mushroom. Each individual cell grows as a vertical column by inserting new cell wall material uniformly along its length.

The major structural component of the cells is a shallow helical arrangement of fibres of chitin winding round the axis of the cell. These chitin fibres are embedded in matrix materials, making the wall material like a carbon fibre composite. Chitin is an exceptionally strong biopolymer (also used by insects for their exoskeletons) and gives immense lateral strength to the fungal cell wall, so that internal pressure is confined as a vertical column. Water enters the cell by osmosis, and the resulting turgor pressure provides the vertical force that allows the mushroom to push up through the asphalt.

This phenomenon was first investigated 75 years ago by Reginald Buller, who measured the lifting power by loading weights onto a mushroom that was elongating inside a glass tube. He calculated an upwards pressure of about two-thirds of an atmosphere.

The cells have a gravity-sensing mechanism that keeps the mushroom exactly vertical. A mushroom that is put on its side will rapidly reorient to grow vertically again.

Graham Gooday

University of Aberdeen, UK

Two inches of asphalt is nothing to the muscular mushroom. One large shaggy ink-cap (Coprinus comatus) discovered in Basingstoke lifted a 75 by 60 centimetre paving stone 4 centimetres above the level of the pavement in about 48 hours.

Historically, mushrooms often sprang up in foundries, supposedly from horse manure used in preparing loam for casting, and were often reported as having lifted heavy iron castings. Presumably these would have been some type of field mushroom such as Agaricus campestris. Whatever the species, the mechanism by which the force was exerted is likely to be the same, namely hydraulic pressure.

As Buller found, the exquisite and fragile Coprinus ster-quilinus exerts an upward pressure of nearly 250 grams with a stem 5 millimetres thick, so it is not surprising that more robust species can tear the tarmac.

Richard Scrase

Mushroom Makers

Bath, Somerset, UK

8986 Them!

I have been amazed to see ants emerge seemingly unharmed after being zapped in the microwave, usually after hitching a ride on my coffee cup. They seem to run around quite happily while the microwave is in operation. How can they survive this onslaught?

Judith Kelly

Darwin, Northern Territory, Australia

The answer is quite simple. In a conventional microwave, the waves are spaced a certain distance apart, because that is all that is needed to cook the food properly. The ants are so minute that they can dodge the rays and so survive the ordeal.

Li Yan

Norwich, Norfolk, UK

The phenomenon that the ants take advantage of is that microwaves form standing waves within the oven cavity.

So in some places in the oven space, the energy density is very high, whereas in others it is very low. This is why most ovens have turntables to ensure that cooking food is heated evenly throughout.

This standing wave pattern can be observed by putting a static tray of marshmallows in the microwave, and heating for a while. The result will be a pattern of cooked and uncooked marshmallows. The standing wave pattern, however, varies according to the properties and position of any material within the oven, such as a cup of water.

The ant will experience this pattern as hot or cold regions within the oven and can thus locate the low-energy volumes. If the ant is in a high-field region, its high surface-area-to-volume ratio allows it to cool down more quickly than a larger object while it searches for a cold spot.

It is a common myth that microwaves are too big to heat small objects. The fallacy of this has been demonstrated by chemists such as myself who employ microwave heating in their work. Certain types of catalyst consist of microwave-absorbing particles – typically of submicron size – dispersed throughout an inert support material. There is convincing evidence that microwaves are capable of heating only the tiny catalyst particles.

A. G. Whitaker

Heriot, Borders, UK

There is very little microwave energy near the metallic floor or walls of the oven. The electromagnetic fields of microwaves are ‘shorted’ by the conducting metal, just as the amplitudes of waves in a skipping rope, swung by a child at one end but tied to a post at the other, are reduced to nothing at the post. An ant crawling on the rope could ride out the motion near the post, but might be thrown off nearer the middle.

For a quick demonstration of this, place two pats of butter in a microwave in two polystyrene coffee cup bottoms, one resting on the floor, the other raised on an inverted glass tumbler. Be sure to place a cup of water in the microwave as well. On heating, the raised butter will melt long before the butter on the floor.

Charles Sawyer

Camptonville, California, US

8986 How about gnat?

How is it possible for gnats to fly in heavy rain without being knocked out of the air by raindrops?

L. Pell

Uffington, Oxfordshire, UK

A falling drop of rain creates a tiny pressure wave ahead (below the raindrop). This wave pushes the gnat sideways and the drop misses it. Fly swatters are made from mesh or have holes on their surface to reduce this pressure wave, otherwise flies would escape most swats.

Alan Lee

Aylesbury, Buckinghamshire, UK

The world of the gnat is not like our own. Because of the difference in scale, we can regard a collision between a raindrop and a gnat as similar to that between a car moving at the same speed as the raindrop (speed does not scale) and a person having only one thousandth the usual density – for example, that of a thin rubber balloon of the same size and shape. A balloon is easily bounced out of the way, and would burst only if it was crushed up against a wall.

Tom Nash

Sherborne, Dorset, UK

8986 Beeline

My girlfriend tells me it is impossible to explain how the bumblebee flies. Apparently it defies the laws of physics. Is this true?

Torbjørn Solbakken


The infamous case of the flightless bumblebee is a classic example of carelessness with approximations. It stems from someone trying to apply a basic equation from aeronautics to the flight of the bee. The equation relates the thrust required for an object to fly to its mass and the surface area of its wings. In the case of the bee, this gives an extremely high value – a rate of work impossible for such a small animal. So the equation apparently ‘proves’ bees cannot fly.

However, the equation assumes stationary rather than flapping wings, making its use in this case misleading. Of course if equations fail in physics there is always empirical observation – if a bee looks as if it is flying, it most probably is.

Simon Scarle

London, UK

8986 Conquering conkers

I was once advised by a friend that the way to strengthen a conker before a conker fight is to bake it. From my childhood I remember being told that the way to improve your conkers was to pickle them in vinegar. Which method produces winning conkers and why?

By email, no name or address supplied

The simplest and best way to harden conkers is to put them away in a drawer until the following year. However, if they were not attached to a string when new and soft they will have to be drilled.

Both my children and grandchildren played with my leftover conkers, some of which were 50 years old. They have never been defeated.

F. Grisley

Barry, South Glamorgan, UK

I have been in conker fights for about 50 years and I always soak them in vinegar. This hardens them into champion conkers. I was content with this method until a few years ago when I was beaten by someone who had smeared his conker in Oil of Olay. Apparently, this made the conker more malleable, allowing it to absorb the impact of my prize pickled nut.

Michael Dutton

Gloucester, UK

Neither baking nor pickling in vinegar is an effective way to strengthen conkers. Baking makes chestnuts brittle, which means they can be knocked off their string with a single blow. Pickling rots the inside. Varnishing, another technique used, is also ineffective (and readily detectable).

In fact, no intervention is necessary to toughen a conker. Simply avoid using chestnuts from the current season (conkers is obviously an autumn pursuit) and use old ones instead. The older they are, the harder they are. Such conkers are readily identified – instead of having a glossy chestnut brown skin they will look dull and dark, perhaps even black. And finally, make the hole for threading the string as narrow as possible.

Nick Aitchison

Longlevens, Gloucestershire, UK

The best way to make a conker invincible is either to leave it for a year and use it in the season after you found it or, to speed up the process, bake it. Put all your conkers in the oven at gas mark 1 (120 °C) for about two hours.

Do not leave them any longer otherwise the flesh inside the conker will become charred and weak. Even if the heat breaks the shell of your conker, the flesh will be rock hard.

Do not put your conkers in vinegar. Although vinegar hardens the shells, the flesh will soften up if there is any gap in the shell, making the conker useless.

Patrick Wigg

London, UK

My brother was disqualified from the school conker championships for having a conker that was vacuum-impregnated with epoxy resin.

J. McIntyre

Balsham, Cambridgeshire, UK

Every conkerer disagrees with every other on how best to produce an invincible nut. As such disputes are an essential part of the sport, we leave the question with the totally contradictory answers given above – Ed.

I was intrigued by all the dialogue about conkers, and I gather that they are chestnuts, somehow attached on a string. What is it you British do with the chestnuts? I suspect it doesn’t involve consumption.

Jennifer Holtzman

North Hollywood, California, US

What kind of game is conkers? Is it like a pillow fight with rock-hard chestnuts? Sometimes we yokels in the former colonies need a little edification . . .

Jay Kangel

By email, no address supplied

Conker fighting appears to be a mainly British pursuit so we realise we have to enlighten our international readership. Conkers are the hard fruit of the horse chestnut tree. These are collected in autumn, removed from their spiky casing and left to mature. A hole is then drilled in the conker, and a string threaded through. The full rules of genuine competition are complex but the game as played by schoolchildren (and overgrown schoolchildren) is between two opponents each with one conker. One player dangles their conker by the string, holding it steady, while the opponent swings their conker on its string and attempts to strike the hanging conker. Players take it in turns to do this until one conker is so damaged that it is dislodged from its string. The winner is obviously the player with the intact conker. Naturally, the stronger and harder the conker, the more chance of success.

This is perhaps further proof, if it were needed, of the British obsession for devising eccentric and meaningless methods of competition – Ed.

The Australian game is called ‘bullies’ and is played in a manner similar to the way the English play conkers.

I know of it being played at least between 1900 and 1970 in western New South Wales and South Australia and before the advent of the TRS-80 and Commodore 64.

As most fair-dinkum Aussies with a bush background will know, the bully is the seed of the quandong tree. Also known as the wild peach, this widely distributed bush tree requires a host tree to survive and fruits annually, producing a tart, bright-red fruit, up to 40 millimetres in diameter.

This fruit has been an Australian delicacy in pies and jams for many years and is only now becoming more commercially available. The stone from the fruit is perfectly round and dimpled like a golf ball. It is usually about 20 millimetres in diameter, with the requisite internal nut and as hard as a stone. Drilling was a difficult task, with good bullies causing the demise of many a parent’s drill bit.

Local rules governed the length of the string and the size of the playing circle. Ownership of losing bullies was not an issue, as my recollection is that losers shattered.

No heat treatment was needed or desired – fire and heat are necessary for the germination of most Australian bush seeds, so heating would certainly weaken the bully. I am unaware of an international challenge in this enthralling sport, but I did consider proposing it for the Sydney Olympics in 2000. My money would have been on the bullies from the colonies.

Jim Bills

By email, proudly from Australia

8986 Eggsactly

Why are most eggs egg-shaped?

Max Wirth

Bowness-on-Windermere, Cumbria, UK

Eggs are egg-shaped for several reasons. First, it enables them to fit more snugly together in the nest, with smaller air spaces between them. This reduces heat loss and allows best use of the nest space. Second, if the egg rolls, it will roll in a circular path around the pointed end. This means that on a flat (or flattish surface) there should be no danger of the egg rolling off, or out of the nest. Third, an egg shape is more comfortable for the bird while it is laying (assuming that the rounded end emerges first), rather than a sphere or a cylinder.

Finally, the most important reason is that hens’ eggs are the ideal shape for fitting into egg cups and the egg holders on the fridge door. No other shape would do.

Alison Woodhouse

Bromley, Kent, UK

Most eggs are egg-shaped (ovoid) because an egg with corners or edges would be structurally weaker, besides being distinctly uncomfortable to lay. The strongest shape would be a sphere, but spherical eggs will roll away and this would be unfortunate, especially for birds that nest on cliffs. Most eggs will roll in a curved path, coming to rest with the sharper end pointing uphill. There is in fact a noticeable tendency for the eggs of cliff-nesting birds to deviate more from the spherical, and thus roll in a tighter arc.

John Ewan

Wargrave, Berkshire, UK

Eggs are egg-shaped as a consequence of the egg-laying process in birds. The egg is passed along the oviduct by peristalsis – the muscles of the oviduct, which are arranged as a series of rings, alternately relax in front of the egg and contract behind it.

At the start of its passage down the oviduct, the egg is soft-shelled and spherical. The forces of contraction on the rear part of the egg, with the rings of muscle becoming progressively smaller, deform that end from a hemisphere into a cone shape, whereas the relaxing muscles maintain the near hemispherical shape of the front part. As the shell calcifies, the shape becomes fixed, in contrast to the soft-shelled eggs of reptiles which can resume their spherical shape after emerging.

Advantages in terms of packing in the nest and in the limitation of rolling might play a role in selecting individuals which lay more extremely ovoid eggs (assuming the tendency is inherited) but the shape is an inevitable consequence of the egg-laying process rather than evolutionary selection pressure.

A. MacDiarmid-Gordon

Sale, Cheshire, UK

8986 Lucky mark

Local birds tend to eat little black insects. So how come they void themselves on me from a great height with a white and annoyingly conspicuous product?

M. Rogers

Great Hockham, Norfolk, UK

It is a common misconception that the white droppings produced by birds are faeces.

In fact, they are urine. Birds excrete uric acid rather than urea because it is an insoluble solid. This way they avoid wasting water when urinating – just one of their adaptations for a good power-to-weight ratio.

Guy Cox

University of Sydney, Australia

The white material that comprises the droppings of birds, and indeed many reptiles, is their urine.

The more primitive vertebrates excrete toxic nitrogenous waste relatively directly, having masses of water at their disposal with which they can dilute substances such as ammonia.

However, birds and reptiles – at least lizards and snakes, with whose droppings I’m very familiar – are different. It would appear that the conversion of their toxic nitrogenous waste products into a relatively insoluble one that can then be formed into a paste was an evolutionary adaptation. This enabled them to lead a terrestrial rather than aquatic life, and even to live in ecological niches where water is scarce.

In such niches it is particularly important not to have to find extra water with which to dilute toxic waste products and flush them from the system, so birds and lizards solved this by evolving to produce a paste of insoluble and relatively non-toxic uric acid.

Interestingly, birds that consume a lot of roughage with their diets, such as the heather-eating grouse and ptarmigan, produce droppings that are very similar to guinea-pig faeces. Only here and there among the droppings is it possible to make out the telltale white patches of their urine, so copious is their production of faeces.

Philip Goddard

By email, no address supplied

Your previous correspondents omit one fact, oviparity. The evolution of insoluble excreta has nothing to do with a ‘good power-to-weight ratio’ or the ability to ‘live in ecological niches where water is scarce’.

It evolved because all birds and many reptiles begin their life inside an egg. Even heavy egg-laying amniotes that live in water as adults, such as penguins and crocodiles, must survive this early phase without poisoning their shelled enclosure with any water-soluble metabolites.

Örnóflur Thorlacius

Reykjavik, Iceland

They do so from a great height because from a lower height it’s just too easy to hit the target – no challenge at all. The deposit needs to be white so that, from said great height, they can see where it lands and who it hits.

S. B. Taylor

Canterbury, Kent, UK

8986 Red or white?

Why is red meat red and white meat white? What is the difference between the various animals that makes their flesh differently coloured?

Tom Whiteley

Bath, Somerset, UK

Red meat is red because the muscle fibres which make up the bulk of the meat contain a high content of myoglobin and mitochondria, which are coloured red. Myoglobin, a protein similar to haemoglobin in red blood cells, acts as a store for oxygen within the muscle fibres.

Mitochondria are organelles within cells which use oxygen to manufacture the compound ATP which supplies the energy for muscle contraction. The muscle fibres of white meat, by contrast, have a low content of myoglobin and mitochondria.

The difference in colour between the flesh of various animals is determined by the relative proportions of these two basic muscle fibre types. The fibres in red muscle fatigue slowly, whereas the fibres in white muscle fatigue rapidly. An active, fast-swimming fish such as a tuna has a high proportion of fatigue-resistant red muscle in its flesh, whereas a much less active fish such as the plaice has mostly white muscle.

Trevor Lea

Oxford, UK

The colour of meat is governed by the concentration of myoglobin in the muscle tissue which produces the brown colouring during cooking.

Chickens and turkeys are always assumed to have white meat, but free-range meat from these species (especially that from the legs) is brown. This is because birds reared in the open will exercise and become fitter than poultry grown in restrictive cages. The fitter the bird, the greater the ease of muscular respiration, and hence the increased myoglobin levels in the muscle tissue, making the meat browner.

All beef is brown because cattle are allowed to run around in fields all day, but pork is whiter because pigs are lazy.

T. Filtness

Winchester, Hampshire, UK

8986 Job swap

If polar bears were transferred to Antarctica could they survive?

And would penguins survive in the Arctic?

Richard Davies

Swansea, West Glamorgan, UK

Polar bears would probably survive in the Antarctic, and the Southern Ocean around it, but they could devastate the native wildlife. In the Arctic polar bears feed mainly on seals, especially young pups born on ice floes or beaches. Many of the differences in breeding habits between Arctic and Antarctic seals can be interpreted as adaptations to evading predation by bears.

Polar bears would find plenty of fish-eating mammals and birds around Antarctica. Penguins would be particularly vulnerable because they are flightless and breed on open ground, with larger species taking months to raise a single chick. Bears can only run in short bursts, but they could catch a fat, sassy penguin chick or grab an egg from an incubating parent.

In the Arctic polar bears hunt mainly on the edge of the sea ice, where it is thick enough to support their weight but thin enough for seals to make breathing holes. The numerous islands off the north coast of Canada, Alaska and north-west Europe provide plenty of suitable habitats. The Antarctic continent is colder, with only a few offshore islands, so bears would probably thrive at lower latitudes in the Southern Ocean than in the Arctic.

We can only hope that nobody ever tries what the questioner suggests. Artificially introduced predators often devastate indigenous wildlife, as it is not accustomed to dealing with them. This occurred with stoats in New Zealand, foxes and cats in Australia, and rats on many isolated islands.

Large, heavy animals would also trample the slow-growing, mechanically weak plants and lichens of the Antarctic. For instance, Norwegian reindeer have decimated many native plants in South Georgia, an island in the South Atlantic Ocean, since they were introduced 80 years ago.

C. M. Pond

Department of Biological Sciences

The Open University

Milton Keynes, Buckinghamshire, UK

While, as far as I know, no one has ever been stupid enough to introduce polar bears into the Antarctic, there have been at least two practical attempts to transplant penguins to the Arctic.

The original ‘penguin’ was in fact the late great auk (Pinguinus impennis), once found in vast numbers around northern shores of the Atlantic. Although no relation to southern hemisphere penguins, it was very similar in appearance, and filled much the same ecological niche as penguins, particularly the king penguins of the subantarctic region.

With any attempt to introduce an alien species, there must actually exist an appropriate ecological niche for it to fill, and it must be vacant. For the most part, the ecological niches occupied by penguins in the south are filled by the auk family to the north. But the demise of the great auk in the mid-19th century at the hands of hungry whalers created not only a vacancy that one of the larger penguins might neatly slot into, but also a potential economic demand for the penguin’s fatty meat and protein-rich eggs.

It was perhaps the possible economic opportunities that prompted two separate bids to introduce penguins into Norwegian waters in the late 1930s. The first, by Carl Schoyen of the Norwegian Nature Protection Society, released groups of nine king penguins at Røst, Lofoten, Gjesvaer and Finnmark in October 1936. Two years later, the National Federation for the Protection of Nature, in an equally bizarre operation, released several macaroni and jackass penguins in the same areas, even though these smaller birds would clearly find themselves competing directly with auks or other native seabirds.

The outcome was unhappy for the experimenters and, most particularly, for the penguins. Among those whose fate is known, one king was quickly despatched by a local woman who thought it was some kind of demon, while a macaroni died on a fishing line in 1944, although from its condition it had apparently thrived during its six years in alien waters.

And it soon became obvious that the real reason why any attempt to fill the ecological gap left by the great auk was destined to fail was the very reason that the niche was vacant in the first place – such large seabirds could not happily coexist with a large and predatory human population. Of course, it is the steadily increasing human presence in the far south that is now threatening penguins in their native habitat.

Hadrian Jeffs

Norwich, Norfolk, UK

8986 How does he smell?

Why are dogs’ noses black?

Rachel Colin (aged 11)

Eudlo, Queensland, Australia

While a majority of dogs have black noses, not all do. The noses of dogs such as vizslas and weimaraners match their coat colours – red and silver, respectively – and it is not unusual for puppies of any breed to start out with pink noses that then darken as the animal matures. I had a Shetland sheepdog that retained pink on the insides of her nostrils for the whole of her life.

Dogs have most likely developed black noses as a protection against sunburn. While the rest of the dog’s body is protected by fur, light-coloured noses are exposed to the full force of the sun’s rays. Pink-nosed dogs, hairless breeds and dogs with very thin hair on their ears need to be protected with sunscreen when they go out of doors, just as humans sometimes do, or they risk the same sorts of cancers and burns.

In addition, dog breeders have long singled out a black nose as the only acceptable colour for many breeds. Though this is based on nothing more than an aesthetic preference, it still serves as a selective influence for people breeding pedigree dogs. This adds a bit of human-directed evolution to what was already a natural tendency towards black noses.

Julia Ecklar

Trafford, Pennsylvania, US

Black nose leather contains the skin pigment melanin, specifically in its dark brown or black eumelanin forms. Melano-cytes, the cells that produce the raw material, secrete it into the skin cells, and the sun then darkens it further. Melanin in skin cells protects the DNA in cells from mutations caused by ultraviolet radiation from the sun.

Jon Richfield

Somerset West, South Africa