Why Can't Elephants Jump?: And 101 Other Tantalising Science Questions - New Scientist (2010)

Chapter 4. Plants and animals

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 Jumping jumbos

Is it true that elephants are the only quadrupeds that cannot jump?

Tad and Lydia Forty (aged 13 and 8)

Bath, Avon, UK

Thanks to Colin Watters and others for pointing out this splendid YouTube video of an elephant on a trampoline at bit.ly/1LWsJ – Ed.

This is a fun question, but it is not true even if we include only four-legged animals that routinely walk on land.

Elephants cannot jump, from level ground anyway. This is true even when they are babies, as far as we know, but they are not alone. Probably all turtles cannot truly jump. It is also likely to be true for some salamanders and large crocodiles, some chameleons and other lizards. In fact, the statement is almost certainly not true even if restricted to mammals. Hippos probably cannot or do not jump, along with moles and other burrowing mammals, sloths, slow loris and other climbing specialists.

However, the truth is that no researchers have looked at this question in a rigorous way. We don’t even know specifically why – in terms of detailed anatomical mechanisms and physics – any of these animals cannot jump. There are just scattered anecdotes and folklore, like the tired myth that elephants have four knees, which I still encounter again and again from the public. Elephants actually have two knees like all other mammals because their anatomy is essentially the same.

So the question is certainly worth addressing. But there are a lot of species out there, so as a general rule it’s probably best to assume there is unlikely to be any species that is alone in being unable to do some seemingly common activity.

John R. Hutchinson

Reader in evolutionary biomechanics

Royal Veterinary College

University of London, UK

Racehorses weighing about half a tonne are among the largest quadrupeds that can make impressive jumps. In horse racing, the Chair, the highest fence on the Grand National course, is 1.8 metres high.

The largest wild animal I have seen making an impressive jump was an eland, one of a group that I saw galloping in Kenya. Its jump was high enough to have cleared the back of another eland, roughly 1.4 metres from the ground. The animal probably weighed about the same as a racehorse.

Large male African elephants weigh around 5 tonnes, and Asian elephants only a little less. After them, the heaviest quadrupeds are the hippopotamus (about 3 tonnes) and the white and Indian rhinos (about 2 tonnes). Whether these and other large animals can jump depends on what you count as jumping. A film I took of a white rhino galloping at 7.5 metres per second showed that, at one stage of its stride, all four feet were off the ground. I do not think of that as jumping, but I cannot think of any clear-cut definition of jumping that would exclude it.

Big jumps require strong leg bones and muscles. The vertical component of the force the feet exert on the ground, averaged over a complete stride or jump, must equal the animal’s weight. In a substantial jump, the animal is off the ground for longer than it would be in a running stride, so its legs will be subject to larger forces at take-off and landing.

Simple physics tells us that if big animals were precisely scaled-up versions of smaller ones, their weights would be proportional to the cubes of their linear dimensions. The cross-sectional areas of bones and muscles, however, would be proportional only to the squares. An animal with double the linear dimensions of another would be eight times as heavy, but its legs would be only four times as strong, and so less able to jump.

Of course, even closely related animals of different sizes are not scale models of each other. For example, a 500-kilogram eland has relatively thicker, straighter legs than a 5-kilogram dik-dik – but the differences are not sufficient to eliminate the disadvantage for large jumpers.

Other than size, a quadruped’s anatomy or physiology may be unsuitable for jumping. Some desert lizards that burrow in loose sand have greatly reduced limbs, tortoises have very slow muscles and the limbs of moles are highly modified for digging. I have never seen any of those quadrupeds jump, and do not expect to.

R. McNeill Alexander

Emeritus Professor of Zoology

University of Leeds

West Yorkshire, UK

Elephants are not the only quadrupeds that cannot jump. Some of the quadruped dinosaurs could not jump, including apatosaurus and diplodocus.

Edward Rivers (aged 7)

Angmering, West Sussex, UK

Really heavy animals like rhinos and hippos can hardly jump or land without injury. After reaching terminal velocity, mice would bounce after hitting the ground whereas elephants would break, or, according to urban legend, ‘splash’.

Even so, don’t jump to optimistic conclusions if a large animal chases you over a ditch. Does it still count as ‘being able to jump’ if the jump causes the animal injury? If so, then you are in trouble because, yes, Indian elephants can jump. J. H. Williams in his book Elephant Bill relates how a stampeding female jumped a ditch handily, though she went lame in both forefeet soon after.

Jon Richfield

Somerset West, South Africa

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 Water, water, everywhere

One of your previous books has explained how fish drink. But what about water-dwelling mammals such as dolphins and whales. Do they get thirsty? And if they do, how do they drink?

Daniel Gough

Glasgow, UK

Dolphins and whales do not drink. Just as we humans cannot use salt water as our source of water, neither can marine mammals. This is because they would need to ingest more fresh water than the seawater they consume in order to excrete the salt it contains.

Much of their water comes from fish and squid, which can contain more than 80 per cent water by mass. They can also obtain water through metabolising fat. In order to reduce their water loss they have similar internal designs to those of desert-dwelling mammals, including a long loop of Henle in the kidney nephron.

As well as internal adaptations, marine mammals did away with sweat glands to stop any water loss through sweating. Instead, they use their surroundings to cool down.

Matthew Tranter

Newcastle-under-Lyme, Staffordshire, UK

Marine mammals certainly are less prone to thirst than land-dwelling mammals; for one thing, they have no need to sweat. They do not swallow any more salt water than they can help, though. Unlike seabirds and turtles, they lack special salt-excreting glands, so every bit of salt they swallow exacts a penalty.

However, whales eat animals, and sirenians (manatees and dugongs) eat plants. In such foods, salt is as little as one-fifth as concentrated as in seawater because the food target has expended energy to excrete salt.

You might say that marine mammals rely on their food to desalinate their water. Even mammalian prey can contribute to this process as they eat low-salt organisms.

Interestingly, as they are unable to sweat or increase their water intake dramatically when thirsty, whales and seals are vulnerable to changes, especially increases in water temperature. In particular, many species have great difficulty crossing the equator, while most are comfortable as close to the poles as foraging will take them.

Jon Richfield

Somerset West, South Africa

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 Foil attack

I get two bottles of milk delivered to my house each day. One, containing whole milk, has a silver foil top, whereas the other, containing semi-skimmed milk, has a silver top overprinted with red stripes. Based on observation over several years, the local magpie population will often try to peck at and remove the striped top but hardly ever attack the plain silver foil top. Have other readers observed magpies or other birds being so discerning, and is there a scientific explanation for it?

Barry Chambers

University of Sheffield, UK

There are two potential explanations. Firstly, the magpies in your garden may be showing some form of aversion to the silver bottle-tops. Birds often show unlearned aversions to food of certain colours, but these tend to be colours that are associated with toxic insects, such as the black and yellow stripes of wasps or the red and black spots of ladybirds. While this could be the case with your magpies, I doubt it because the red-striped tops would appear to be more reminiscent of the colour patterns of toxic insects than the silver tops.

The theory I favour is that your birds know what’s good for them! Insectivores such as magpies need a diet rich in protein with lower levels of both carbohydrates and fat. Just like in humans, high-fat diets can cause magpies to suffer from high levels of cholesterol and all the medical problems associated with that. Birds are excellent judges of the toxin and nutrient content of the food they eat and by choosing to drink the semi-skimmed milk over the whole milk they get the benefit of a high-protein food source without the costs associated with eating fatty foods.

Maybe magpies could teach us all a thing or two about healthy eating.

John Skelhorn

The Institute of Neuroscience

Newcastle University, UK

In my youth, we used to have ordinary milk, with a red top, and creamy Guernsey milk, with a silver top, delivered to our home. The blue tits always attacked the creamier Guernsey bottles. After a couple of years, the dairy changed the cap colours to silver and gold, respectively. The birds learned the new colour code in about two weeks.

Your magpies are either stupid or fashionable – assuming, of course, there is a difference.

Alan Calverd,

Bishop’s Stortford, Hertfordshire, UK

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 Tomato attack

Every time I collect tomatoes in the garden, my hands end up covered with an invisible substance with a pungent smell. It seems to come from the tomato leaves and branches. When I wash my hands with soap, the substance becomes very bright yellow-green – almost fluorescent – and it stains my soap, towel and wash basin. However, if I don’t use soap to wash the substance off, it remains invisible. What is it?

Alex Saragosa

Terranuova, Italy

Leaves of plants in the family Solanaceae (including tobacco, tomato, potato and capsicum) all have minute hairs on their surface which exude drops of a sticky fluid.

The function of this sticky substance is not altogether clear, but it could ward off attacks from aphids and other sapsucking insects. The flavonoids and possibly other pigments in the fluid react with soap, which is alkaline, and change colour accordingly.

David Whitehead

Cape Town, South Africa

I remember being impressed long ago by the bright green colour that developed when washing my hands after helping my father tend his tomatoes. Some time later, as a botany student, I examined the tomato leaf epidermis under a stereomicroscope, with interesting results.

The tomato epidermis carries two types of multicellular hair – long ones of several millimetres readily seen with the naked eye, and much shorter hairs with four glandular sacs like short sausages at the apex. These are filled with khakicoloured contents and have very thin, fragile cell walls. I found that prodding one of these sacs with a needle released sticky contents that could be drawn out as a thin thread which set rigid within about 2 seconds, leaving a deposit on the needle.

Later still, I found some photos of a mite wearing what can only be described as concrete boots. These were apparently made up of accumulated secretions from a tomato plant, collected as it rambled over the plant’s surface. The deposit clearly encased the mite’s legs, preventing it from hanging onto the leaf, thereby acting as a defence mechanism against this and other small, walking herbivores.

Casual brushing against a tomato leaf will transfer only a little of the substance. This reserves the secretion for organisms that ramble among the hairs – or anything grasping the plant strongly enough to bend the longer hairs.

The deposit’s colour is lost against the skin of the average gardener, but it can certainly be detected by smell and, I suspect, by texture.

Jim Kent

Minehead, Somerset, UK

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 Quackers

I was watching a duck and her eight chicks walking in a line across the grass. All of a sudden a couple of other chicks entered the group. The mother duck immediately weeded out the stranger chicks and sent them on their way. To us they looked identical, so just how did the mother duck achieve her feat? Is it just that animals are exquisitely sensitive to visual differences between members of their own species? Or was the mother duck relying on non-visual information as well and, if so, what?

Byung O Ho

San Jose, California, US

Chasing away non-descendant young is called ‘kin discrimination’ and is often considered less efficient in birds than in other animals. However, eider ducks have been reported to discriminate against ducks that are not part of their family unit. Coots have also been seen to do the same thing, but neither species seems to use appearance as the way to recognise their young.

Many birds use acoustic recognition and can identify each other’s voices. Swallows, finches, budgies, gulls, flamingos, terns, penguins and other birds that live in larger flocks do this. Odours can also play a role in determining how some birds recognise each other.

In ducks, sound seems to be the principal method of recognition: they have been fooled into returning to the wrong nest, only to be greeted by a portable cassette player rather than their ducklings.

The ability to recognise their own young saves colonyliving birds from expending energy in raising someone else’s offspring. It also stops ducklings running the risk of aggression from adults if they beg food from the wrong ones. Natural selection favours individuals who know who they are talking to.

Waterfowl have long been thought to be unable to keep track of their own young. They have been seen to lose their own ducklings to another parent, or to mistakenly accept and care for non-descendant ducklings. This has been put down to the fact that birds do not generally have a central family unit.

Ducks do behave in a different way towards their own ducklings, though. Parents sometimes favour their own offspring over non-descendant young, as with the duck in this question, or they may tolerate or encourage the ducklings to mix. Consequently, some provide what is called alloparental care, a form of adoption. This is seen when a duck is able to increase the chances of survival of her own offspring by accepting non-descendant ducklings into her entourage. Her own ducklings might be better off because the risk of any individual being eaten by a predator is lower if it is part of a bigger group. To improve the advantage even more, the non-descendant ducklings may be positioned at the edge of the brood, further away from parents. This has been seen in Canada geese; the adopted goslings were noted to generally potter further away from their adoptive parents than the biological offspring, and therefore not as many survived.

Jo Burgess

Department of Biological Sciences

Rhodes University, Grahamstown

South Africa

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 Worm baiting

While sitting on a bench beside a local green, I noticed a gull performing an excellent version of Riverdance. Then it stopped and scrutinised the grass around its feet. This sequence was repeated for about 15 minutes. I assume the gull was trying to attract worms to the surface with its rhythmic dance. Was it? If so, how does the strategy work?

Danny Hunter

Dublin, Ireland

Yes, like many species of birds, some gulls have learned the earthworm-raising trick. Earthworms stay underground during the day unless flooded out by rainwater or alarmed by ground vibrations that suggest the approach of a mole. Just jab a garden fork into earth well populated with earthworms and some will pop out to avoid the little creature in black velvet.

Different birds have different techniques. Blacksmith plovers, rather than hunting earthworms, flush out grasshoppers, caterpillars and moths by tickling short grass with a trembling foot held forward.

Gulls that have learned the trick stamp for earthworms. Similarly, I have seen thrushes stamp by hopping hard with stiff legs. Once I was startled to see a red-winged starling watching an olive thrush’s technique attentively, then having a go itself. It did get a worm or two, but its technique was faulty, with long, loping leaps instead of jerky thumps, so it did not scare enough worms and soon gave up. Or maybe it just didn’t like the flavour of those earthworms it had caught.

Antony David

London, UK

The gull was indeed trying to get worms to surface. Underground, the rhythm of the gull’s feet sounds much like rain.

Earthworms like to surface during rain because it enables them to move around overground without drying out – this is impossible when it is dry. By tricking the earthworms, the gulls get an easy meal. The gulls may have learned this trick from watching other gulls, or may have inherited the behaviour.

Laura Still

Devon, UK

I was sitting on Henley Beach in South Australia recently, watching a gull ‘puddling’ the sand at the water’s edge before inspecting the water for any food items it might have disturbed. It seemed to be doing quite well for itself.

It appears that the gull seen by the questioner was applying successful food-gathering behaviour that evolved in one environment to another. This does make sense when one considers that gulls originate not in marine environments, as is frequently supposed, but in moorland ones. Presumably the behaviour evolved in environments that contained bogs, where the puddling behaviour would work successfully in damp ground some way from the water’s edge.

The vital question, though, is whether the gull was successful in drawing up worms, or anything else edible, to the surface.

Graham Houghton

Aldgate, South Australia

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 The hole truth

I have always been fascinated by evolution, and while I can usually see why and how certain characteristics evolved in different species, I’m confused by whales and dolphins. How did their breathing holes evolve, bearing in mind their ancestors were land mammals?

Joe Bilsborough

Tarbock, Merseyside, UK

Blowholes are paired nostrils that evolution has shortened and redirected towards the most convenient spot for snorkelling – the top of the head. They do not pass through the brain, though.

As in most swimming, air-breathing vertebrates – such as frogs, crocodiles, capybaras or hippos – whales’ nasal openings, or nares, are placed high up so they can breathe with as little raising of the head or snout as possible. They also have protective valves to keep water out.

However, most of the creatures in that list are oriented largely towards the world above: they periodically leave the water for terrestrial activities and they float with nostrils and eyes just above the surface, watching for food and threats.

In contrast, the terrestrial ancestors of ichthyosaurs, cetaceans and sirenians (manatees and dugongs) evolved into creatures with their attention directed towards the underwater world. Their ears and eyes did not migrate upwards, only their nostrils shortened and the nares migrated towards the highest part of the head because although food and threats no longer came from above, the air they needed to breathe still did. In sirenians that migration is incomplete, so watch this space for another 10 million years.

Antony David

London, UK

The benefit of the location of cetaceans’ blowholes is clear, but it’s not so clear what factors motivated the initial steps in the migration of the nostrils from the nose to the top of the head. Natural selection certainly does not seem to have made significant progress until the whale’s distant ancestors had irreversibly abandoned the land and shifted to a marine lifestyle.

The earliest identified precursor of modern cetaceans is Pakicetus, which lived during the early Eocene, about 53 million years ago. It resembled a hyena with hooves, was quite definitely a terrestrial animal and had nostrils at the extreme front of its long snout. It was not until the late Eocene – about 20 million years later – that the first ‘true’ whale appears in the fossil record. Named Basilosaurus, it featured nostrils that had migrated up its snout to a point just in front of its eyes. Basilosaurus had nostrils not only shifted backwards in comparison with its forebears, but also converging towards a location on top of the skull, in a clear move towards the modern arrangement. Because Basilosaurus was fully aquatic, it seems clear that it was the benefits of the modern set-up that were the driving force behind this particular aspect of its evolution.

Certainly, by the mid-Miocene, some 15 million years ago, the first modern whales and early dolphins all sported blowholes precisely where they are found on present-day species, although even today evolution has not arrived at a definitive form for a whale’s nostrils. Baleen whales, such as the humpback and the blue, have two, while toothed species such as the sperm whale have just one.

Mystery still surrounds the reptilian predecessors in the cetacean’s ecological niche. The dolphin-like ichthyosaurs were, as their classical name ‘fish-lizard’ suggests, particularly well developed for a marine lifestyle. They survived for 140 million years, almost three times as long as whales and dolphins have had to evolve from their terrestrial ancestors.

Yet even the very largest of ichthyosaurs retained two conventional nostrils set just in front of their eyes, very similar to Basilosaurus, despite this configuration requiring them to lift most of their heads out of the water to breathe, and exposing them to attack by predators.

So perhaps the real question is not why whales and dolphins have evolved their manifestly beneficial breathing arrangement, but why their reptilian analogues – and other marine mammals such as the dugong – did not do likewise.

Hadrian Jeffs

Norwich, Norfolk, UK

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 The birds

In New Zealand one of our radio stations broadcasts native birdsong each morning. It is obvious that seabirds have a much harsher screeching sound than the more melodious bush and landbased birds. In fact, I can usually tell a bird’s habitat simply by the sound it makes. Why is there such a difference, and is it the same throughout the world?

John Finlayson

Maungaturoto, New Zealand

Birdsong indeed varies by habitat type because the habitat has a profound effect on how these long-distance signals are transmitted. To minimise habitat-induced degradation, the acoustic adaptation hypothesis predicts that birds living in dense forests will have slower and more tonal calls, while those living in more open habitats will have faster-paced and buzzier calls.

The effect is most pronounced when comparing contrasting habitat types, such as very open and very closed ones. Other factors, including the songs of species competing for acoustic space and the songs produced by closely related species, can also play a role.

Daniel T. Blumstein

Department of Ecology and Evolutionary Biology

University of California

Los Angeles, US

The subject is more complex than the question suggests. The South African bush hosts croaking corvids, harmonising antiphonal shrikes, shrieking parrots, raucous francolin, swizzling weavers and tweeting wagtails.

Calls seem to be adapted to distance, noise, obstacles, habit and competition. The most elaborate singers inhabit open bush, where their song can convey complex information over long distances. In thick bush, only deep ventriloqual notes such as those of the ground hornbill carry for any distance. White-eyes foraging among dense leaves cheep softly, keeping flocks together at short range.

Even the apparently unsophisticated croaks, screams and yarps of seabirds vary in complexity and carrying power according to their habits and individual circumstances. When calling through wave noise over long distances they tend to screech shrilly, whereas when they are intimate they are quieter.

Details vary, but the fundamental principles of auditory information encoding and transfer seem inescapable.

Jon Richfield

Somerset West, South Africa

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 Balanced lifestyle

Why do some birds stand on one leg?

Alexander Middleton

Moorooka, Queensland, Australia

Thanks to all those who offered the answer: ‘If they picked up the other leg they’d fall over.’ The old jokes are still the best – Ed.

It has been proposed that the reason that flamingos stand on one leg is so ducks don’t swim into them as often! The most likely answer, though, has to do with energy conservation. In cold weather, birds can lose a lot of heat through their legs because the blood vessels there are close to the surface. To reduce this, many species have a counter-current system of intertwined blood vessels so that blood from the body warms the cooler blood returning from the feet. Keeping one leg tucked inside their feathers and close to the warm body is another strategy to reduce heat loss.

I imagine the converse is true in hot climates – blood in the legs will heat up quickly, so keeping one leg close to the body will reduce this effect and help the birds to maintain a stable body temperature.

Another factor in long-legged birds is that it may require significant work to pump blood back up the leg through narrow capillaries. Keeping the leg at a level closer to the heart may reduce this workload.

It is also worth remembering that birds’ legs are articulated differently to ours; what looks like the knee is in fact more like our ankle. Many birds have a mechanism to ‘lock’ the leg straight, so for them it is much easier to stand for hours on end on just one leg – on numerous occasions I have seen birds take off, and even land, on one leg.

Rob Robinson

Senior population biologist

British Trust for Ornithology

Thetford, Norfolk, UK

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 Do the twist

All the stems of the morning glory plants growing on my balcony coil in the same direction. When I moved some of the plants, I re-coiled them by hand onto the strings they creep around. Those that I had coiled in the ‘wrong’ direction started to coil in the ‘right’ direction as soon as they could. Why is this?

Judit Zádor

Budapest, Hungary

Some winding plants such as morning glory and wisteria wind counter-clockwise (CCW). Others, such as hops and honeysuckle, wind clockwise (CW). Supposedly, you should not force them to wind in the ‘wrong’ direction or they will wither.

Although it is said that hops wind CW to follow the sun, the actual direction of winding is determined by the plant’s genes and the pull of gravity.

A Japanese team at Kobe University led by T. Hashimoto chemically mutated straight-growing vines until some wound CCW, then looked at the molecular structure of the twisty bits. They found that a slight change in the structure of tubulin, a microtubule protein in cells, determined the winding direction (Nature, vol. 417, p. 193).

Another Japanese team, this time led by Daisuke Kitazawa at Tohoku University, found that gravity-sensing cells are crucial for shoot circumnutation – the bending and bowing of the tip – and the winding response (Proceedings of the National Academy of Sciences, vol. 102, p. 18742).

So a plant’s gravity sensors tell it which way is up, and its tubulin structure determines whether it winds CW or CCW in relation to the vertical.

Quinn Smithwick

Cambridge, Massachusetts, US

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 It’s a dog’s life

Why do dogs like jumping into cold ponds, while cats and humans generally do not?

James Scowen

London, UK

Your questioner appears to be confusing willingness with enjoyment. Most dogs are prepared to dive into cold water, but they may not like the experience. And in referring to cats, your questioner is almost certainly referring to the domesticated species, which is not necessarily representative of its genus.

Nonetheless, the canine tolerance for cold water, and feline intolerance, lie in their respective evolutionary histories. The dog (Canis lupus familiaris) originated in central Asia during the aftermath of the last ice age, at least 15,000 years ago. It is descended from the grey wolf (Canis lupus), with all the evolutionary baggage that implies. Ice-age wolves preyed on sub- Arctic herd animals such as elk, reindeer and caribou, which would have migrated in search of better grazing, crossing fast-flowing rivers swollen by meltwater when required.

Any animal – including the ice-age wolves – fording or swimming these rivers would have had to develop considerable physical and psychological resistance to low temperatures. Those that weren’t prepared to get their feet wet wouldn’t have lasted long enough to pass on their genes. Those that did bequeathed their doggy descendants a tolerance for cold water.

Some 5,000 years after the big bad wolf began the transition to being man’s best friend, a group of wild cats (Felis sylvestris) in what is now western Asia apparently attached themselves to the local human population in a semi-symbiotic relationship. Significantly, the closest living relative of the proto-kitties is believed to be the sand cat (Felis margarita), a denizen of regions of extreme heat and aridity, such as the Sahara.

This ancestry was never likely to cultivate a hereditary tolerance for getting wet, even if natural selection had not already instilled a wariness of bodies of water, whatever their temperature. Large mammals have no freshwater predators in the sub-Arctic, but animals originating in the tropics have good reasons for not going into the water, most of them possessing very powerful jaws. A prehistoric African water hole was a fast-food outlet for large predators, both in and around the water.

The behavioural heritage of these widely differing ancestries can be most clearly observed when our modern-day pets are drinking. A dog will generally lap up its water enthusiastically, albeit with the occasional sideways glance at any animal that could attack. A cat, on the other hand, displays far more caution, constantly looking around suspiciously and keeping its body as far back as possible from the liquid.

Hadrian Jeffs

Norwich, Norfolk, UK

Dogs, like humans and cats, exhibit a homeothermic mode of temperature regulation – their body temperature remains constant in spite of fluctuations in the temperature of their environment.

Dogs are covered with thick hair to conserve internal heat, and regulate their body temperature through panting, an extremely efficient method. On a hot day it is quite common to see a dog with its mouth wide open and tongue hanging out.

Recent research has also indicated the presence of a complex network of blood vessels in the basal part of a dog’s neck. This region functions as an efficient temperature regulator. In addition, dogs have relatively large spleens. When a dog is active or under stress, the spleen contracts and releases blood into the circulatory system, which provides yet another mechanism for carrying excess heat to the skin.

All this means that dogs are better adapted than humans or cats to withstand cold shocks or hypothermia.

Saikat Basu

Lethbridge, Canada

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 Flight of the butterfly

My 4-year-old daughter asked me how high butterflies fly. I was stumped. Can anyone tell us?

Jacque (and Tara) Lawlor

Chelmsford, Essex, UK

Unlike humans, butterflies are not disposed to seeking altitude records. Indeed, they will not fly higher than is strictly necessary in their everyday lives, whether looking for a mate, food or somewhere to lay eggs, avoiding predators or migrating.

Worldwide there are many thousands of species of butterfly, each adapted to its own particular habitat and needs. Some spend their whole lives on a patch of coastal grassland, the larvae feeding on low plants or living in ants’ nests, and the adults never flying more than a few feet above the ground. Others spend all their time in the tree canopy many metres above ground level.

Still others are only found on high mountains. So even though they don’t actually fly very high above the ground locally, butterflies that live on the mountains of Peru spend their whole lives at altitudes of around 6,000 metres.

Butterflies that migrate tend to fly the highest in general. The most famous migratory butterfly is probably the monarch, Danaus plexippus. These leave Mexico each year and fly north to Canada, albeit taking several generations to get there. Monarchs have been sighted by glider pilots flying as high as 1,200 metres. Interestingly, they seem to fly in the same way as a glider, using updrafts to gain sufficient altitude so that they can glide for quite a distance before needing to use energy to climb again.

Europe also has plenty of migratory species. The painted lady, Vanessa cardui, makes its way to southern France from north Africa. It has to leave Europe in winter as no development stage of this insect can survive a frost.

To get to France many will cross through the mountain passes of the Pyrenees, which in general lie at about 2,500 metres. During late summer and autumn one can observe butterflies drifting southwards. If they encounter a high building, they just fly straight upwards and over it. If they encounter a high mountain range, they will do the same. So you need only to stand for a while on any mountain pass during the migration period to see them coming over either singly or in swarms, flying close to the ground as they travel.

The mountain passes of the Caucasus are higher, while those of the Himalayas are higher still at 7,500 metres. I wouldn’t be surprised if migratory butterflies could fly straight over Everest if they encountered it in good weather.

However, insects of any kind cannot fly if they are too cold. Butterflies can keep warm to a certain extent by beating their wings, though if they fly too high in the wrong conditions, they may become too chilled to maintain a wingbeat.

On average, the air temperature reaches freezing at an altitude of just below 8,000 metres, suggesting that this would be their physical altitude limit. They might on occasion be carried higher on updrafts, but this surely doesn’t count as autonomous flight.

Terence Hollingworth

Blagnac, France

The greatest acknowledged height achieved by migrating butterflies is 5,791 metres, set by a flock of small tortoiseshells, Aglais urticae, crossing the Zemu glacier in the eastern Himalayan mountains.

Not only is this an altitude record for butterflies, it is also the highest that any insect has been observed in controlled flight, comfortably exceeding the more frequent altitudes of between 3,000 and 4,000 metres at which monarch butterflies have been sighted by commercial airline pilots.

Hadrian Jeffs

Norwich, Norfolk, UK

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 Brainy breeding

Dog breeding often gets a bad press, including the apparently unfounded assertion that breeding for looks has an adverse effect on intelligence in dogs. But has anyone ever bred dogs, or any other species, purely for intelligence? Just how intelligent could any species get through selective breeding? And how quickly?

John Schofield

London, UK

Intensive breeding for looks in any animal adversely affects intelligence and every other attribute – eventually including those very looks. This is intrinsic to selection, whether natural or artificial.

The effectiveness of selection depends on the range of relevant genes in the population: the larger the natural population, the greater the range of genes is likely to be. Selection for any desired attribute rapidly reduces that range: in a single generation, less than 1 per cent of a population might be selected, immediately reducing the range of ‘irrelevant’ genes, including genes for mental or physical health or functionality.

Dogs bred for show are commonly selected so obsessively that any harmful genes they carry become fixed in their populations. In competitive show breeding, selection is particularly stringent, with the result that gene pools shrink rapidly. Most mutations and recessive genes in small, closed populations are harmful, so progress is overwhelmingly negative.

The closest we come to breeding for intelligence and functionality in dogs is in certain working breeds. Breeding companion animals specifically for desirable behaviour, intelligence and health should be gratifying, but it is also challenging and commercially precarious. People who need companions prefer to buy mongrels.

Jon Richfield

Somerset West, South Africa

Asking if anyone has bred a variety of dog purely for intelligence begs the question of what is meant by intelligence. The psychologist Robert Sternberg has shown that what we think of as ‘intelligent’ depends on what we value – specifically what we think people should be good at. So what we consider to be a clever dog would be one that does a good job at what we want it to do: herd sheep well or guard the house effectively. We have no need for dogs that are adept at calculus or playing the futures market, so we have never tested our capacity to breed this into them.

Sternberg identified three signs of intelligence: the ability to adapt to environments, the ability to shape environments and the ability to understand that the environment is not optimal, thus facilitating a move to a more congenial niche. On this basis, you could make the argument that almost all species are intelligent, even bacteria, because at the very least they are adapted to their environments. In addition, many can up sticks when things are not so good and move elsewhere, and some can even shape their environment in some way congenial to them.

Maybe dogs deserve special mention because they have shaped their environment by making themselves useful and appealing to humans, in return for food and shelter.

Catherine Scott

Surrey Hills, Victoria, Australia

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 In the green

The benefits of camouflage would suggest that there should be green mammals. Are there any – and if not, why not?

A. C. Henderson

Braco, Tayside, UK

There is only one green mammal, the three-toed sloth. This is because a coat of algae covers the sloth’s fur. Because of the sloth’s tardiness and lack of personal hygiene, this is never cleaned off. No known mammal is capable of producing its own green epidermal pigment. The main reason for the absence of green mammals seems to be an ecological one. In general, mammals are simply too big to use a single colour for camouflage as there are no blocks of green large enough to conceal them. Most mammals have an environment that is made up of patches of light and dark and composed of many different colours. This means that those mammals which are camouflaged tend to be dappled or striped. Animals that do use green coloration for camouflage, such as frogs and lizards, are small enough to use solid blocks of green – leaves and foliage – for cover.

Paul Barrett

Department of Earth Sciences

University of Cambridge, UK

The main predators of most mammals are other mammals, especially the carnivores, such as the cat, dog and weasel families. Carnivores are all colour-blind or, at best, have very limited colour vision. Hence effective camouflage against them is not a matter of coloration but of a combination of factors such as brightness, texture, pattern and movement.

Graeme Ruxton

Scottish Agricultural Statistics Service

Edinburgh, UK

Your answers to this question only mention the tree sloth, which is not truly green, just covered with algae. There is actually a real green mammal – the green ringtail possum (Pseudocheirus archeri) – and what a lovely animal it is. The possum is a marsupial endemic to a small area in northeastern Australia. You can see a fine colour portrait of it in The Complete Book of Australian Mammals (Angus and Robertson, 1983). The article accompanying the portrait is by J. W. Winter, an expert on the possums of that part of Australia. He writes: ‘This remarkably beautiful ringtail is aptly named: a mixture of black, grey, yellow and white hairs confers a most unusual lime-green colour to its thick, soft fur.’

I would add that it is also the most docile wild animal I have ever encountered. A scientist studying possums during the 1960s who caught one and kept it for a day before returning it to the wild allowed me to photograph it. It made no attempt to struggle, scratch or bite when taken out of the cage, nor did it try to escape.

Winter makes no comment as to whether the green colour has any apparent advantages, but he does report that ‘its daytime roost, unlike that of other possums, is usually on an open branch. It sleeps upright, curled into a tight ball, gripping the branch with one or both hind feet and sitting on the base of its coiled tail, with the forefeet, face and tip of the tail tucked tightly into its belly.’

A motionless, amorphous green ball among the multitudinous shades of green in the rainforest would be far from obvious. The only predators Winter reports (apart from Aboriginal humans in the past) are nocturnal: the rufous owl (Ninox rufa) and the spotted-tailed quoll (Dasyurus maculatus). The latter is a marsupial carnivore, with a head and body length of about half a metre, found over much of eastern Australia including Tasmania.

H. S. Curtis

By email, no address supplied

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 Back on track

While working in the garden, I saw a beetle walk past, take a wrong step and land on its back. Without my intervention it would have stayed in this position and probably died. Why is it that millions of years of evolution have not eradicated this basic and potentially lethal design fault?

Greg Parker

Brockenhurst, Hampshire, UK

If your correspondent had left the beetle in place on its back it probably would not have remained as it was until death. Beetles and other insects have a variety of mechanisms which they can use for righting themselves in these circumstances which, as the writer presumes correctly, must arise often and hazardously.

The most famous mechanism is used by the click beetles (Elateridae), which are able to launch themselves into the air by the sudden release of a blunt spine which is kept under pressure in a specialised groove on the venter.

As many readers will have noticed, the click beetle often makes several attempts before it lands on its feet, but its success, given time, is assured.

Other less sophisticated beetle correcting mechanisms include spreading the wings, reaching out with the legs, and rocking the body in a forward-aft or side-to-side motion.

Christopher Starr

Department of Zoology

University of the West Indies

St Augustine, Trinidad and Tobago

Only a minority of beetles possess a body plan that poses such a problem. For example, I have worked with several species of ladybeetle (Coccinellidae) in the laboratory, and most are able to right themselves with relative ease.

The species that do find themselves stranded on their backs tend to be the larger varieties that possess strongly convex elytra (the first pair of hardened, protective wings).

Ladybeetles that do become stranded on a smooth surface will eventually unfold their membranous hind wings, which are normally hidden beneath the elytra, and then use these to right themselves. Part of the answer, then, is that very few species become stranded and those that do eventually flip themselves over by means of their hind wings.

Over the long course of evolution it was probably quite rare for beetles developing in temperate forests and grasslands to encounter totally smooth surfaces or bare soil that was devoid of plant litter. Under normal circumstances, grass blades, fallen leaves and plant stems would offer a convenient hold for beetles that happened to become overturned.

The reduced rate of predation and numerous other benefits that are conferred by a hard protective covering, which far outweighs the occasional stranding, has contributed to the enormous evolutionary success of beetles. In terms of both absolute numbers and numbers of species, beetles are the most successful group of animals on the planet.

Tom Lowery

Pest Management Research Centre

Ontario, Canada

I doubt whether the beetle was a healthy specimen that just happened to fall over and was unable to right itself. It is more likely that it was an old, sick or diseased specimen that was nearing the end of its life. When this happens in beetles, they lose a great deal of their mobility and coordination and they become very unstable when walking. They frequently fall over when placed on a hard flat surface and are unable to right themselves.

I have observed this countless times in a number of beetle groups. In fact, while growing up I lived near Milwaukee, Wisconsin, in the US. We had a fairly large population of Carabus nemoralis, which is a ground beetle that was introduced from Europe into the US. I would frequently find beetles on the sidewalks on their backs. No matter how many times they were righted, they would invariably end up on their backs again, soon to die. I also observed beetles stagger out of the vegetation bordering the sidewalk, only to fall onto their backs. If these beetles were placed on their feet, even in the vegetation, they would stagger about and would fall onto their backs again when they encountered the sidewalk.

So, I suspect that the poor design is really a combination of dying beetles coupled with a smooth, hard surface – one that is not normally found in nature. Considering that roughly one out of every five living creatures is a beetle, and that they occupy virtually every niche and habitat known, I would suggest that beetles are, in fact, very well designed animals.

Drew Hildebrandt

By email, no address supplied

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 Myopic mammals

I would estimate that about 40 per cent of people that I know need glasses or contact lenses for distance vision. Assuming that this sample is typical of the human race, I would like to know why it is that eye problems prevalent in humans such as myopia (shortsightedness) seem very rare in wild animals. As far as I know, myopia is a genetic condition and so is not usually acquired by habits such as reading small print (otherwise one would expect recovery after stopping the habit). Obviously, it is not easy to test the eyesight of an animal, but if the incidence of myopia is as high in wild animals as it is in humans then how can the animals survive?

Stephen White

Surbiton, Surrey, UK

For most nonhuman mammals the ocular refraction (the lens power required to form a clear retinal image of an object at infinity) for optimal distance vision tends strongly towards emmetropia, or perfect vision. Similarly, the variation of distance refraction and the presence of astigmatism is lower than for humans. It should be noted, however, that the sample tested is far smaller for mammals than for humans.

These trends have been noted by my co-workers Clive Phillips, Jacob Sivak, Robin Best and Bill Muntz, and myself, in a number of different studies using standard clinical optometric techniques (obviously avoiding those that require a verbal response).

The studies have covered such disparate species as domestic sheep, guanaco (a llama), polar bear, manatee and three-toed sloth. Such findings would support the suggestion that a myopic mammal would be at a natural disadvantage.

David Piggins

Bangor, Gwynned, UK

There probably is a genetic factor in short-sightedness, but that does not explain why it is so common in modern society. Those who regularly focus their eyes over longer distances, such as sailors and mountaineers, are apparently less likely to become myopic. It seems likely that the muscles on either side of the eye can be trained to contract the eye, thus overcoming short-sightedness. Once a person starts wearing glasses, the need for such adjustment disappears.

Brynjolfur Thorvardarson

Southampton, Hampshire, UK

The fact that about 40 per cent of people you know wear glasses or contact lenses does not really indicate a malfunction in the entire human race. If you travel among primitive peoples of the world, you will find numerous examples of keen sight that seem almost super-human to the Western mind.

Kevin Wooding

Oxford, UK

Myopia often has a genetic component, but this isn’t the whole story. People who do close work are often myopic (tailors are the classic example) but in the past it was usually assumed that it was their myopia that attracted them to such jobs: the idea that myopia could be acquired seemed too far-fetched. However, several decades ago it was observed that university students with better than average grades (who presumably read more) tended to be myopic, as were laboratory animals that were raised in a confined environment.

C. R. Cavonius

University of Dortmund, Germany

Primates brought up in captivity do tend to become myopic. Myopia is caused by the axial length of the eye, but changes in corneal power also affect sight. Most change is likely to occur in the growth phase during the few years after birth, but may continue to a lesser extent for the next three decades in humans.

Better evidence comes from chicks, where 10 dioptres of myopia or hypermetropia can be induced by contact lenses, and reversed, over a few weeks. There is a dramatic change in the posterior segment of the eye, which is accounted for by alterations in the rate of growth of the eye as the chick ages. Just like humans who squint to improve their vision, chicks have the capacity for corneal and lenticular accommodation, but this appears to exert little influence upon growth.

Whether human myopia can be arrested or reversed is the subject of some debate in ophthalmology. There does seem to be something approaching an epidemic of myopia, especially in the Far East, which cannot be explained purely by genetic or occupation selection.

Matt Cooper

Brighton, Sussex, UK

Among older people, acquired defects of vision are more common than inherited myopia. At ages beyond those that would have been achieved in the wild, human eye tissues distort and lose flexibility, causing astigmatism and presbyopia. The lens may go opaque from cataracts, or it may darken so that it needs more light.

Domestic animals may show similar defects when they get beyond the life expectancy in the wild, and old dogs or cats often go blind from cataracts. Replacing a pet dog’s lens is practically a routine operation nowadays. One need not give the dog glasses afterwards – just as long as it can see clearly enough to get at its food dish. It doesn’t have to read the brand name on the tin.

Jon Richfield

Somerset West, South Africa

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 Milking the issue

Cows eat lush grass in summer but prepared dried feed in winter. Does the milk that I pour on my breakfast cereal differ in any way throughout the year?

Graeme Mawson

Newcastle upon Tyne, UK

More than fifty years ago, I spent some time living on my aunt’s croft in Sutherland.

In winter, her cow Bella was fed largely on turnips, and during those months Bella’s milk also tasted of turnip. But this taste disappeared in summer when she was fed on grass.

Nowadays, dairy cows are fed on a fermented stored grass called silage during the winter and this is supplemented with processed feed containing animal protein.

The winter milk no longer smells or tastes of turnip, but I assume that if turnip used to contribute to the composition of milk then the modern winter feed probably does too. In retrospect, I think I would prefer the turnip.

Ian Sutherland

Birmingham, UK

Your correspondent probably assumes that the milk he pours onto his breakfast cereal every morning is much the same as the milk that comes out of a cow. In fact, most drinking milk is pasteurised and standardised for fat, so this quality parameter is invariable throughout the whole year. Milk may in future also be standardised for protein – in effect removing high-value solids from the milk for further processing.

As a dairy farmer drinking raw milk, both I and my family might detect slightly higher levels of fat and lactose at certain times of the year but, unless the cows graze on a patch of wild garlic, the taste of the milk remains the same.

I believe that in parts of France, certain cheeses are only made from milk produced in a particular season and from cows that graze on pastures with specific herbs.

Pasteurising milk probably removes taste by damaging natural enzymes. If anyone could achieve the antibacterial effects of pasteurisation without also damaging the taste, the process would be invaluable to the dairy industry and would also enable your correspondent to experience the taste of milk straight from the cow.

Mark Pearse

South Molton, North Devon, UK

The effect of different feeds on the content of milk is actually only slight. The nutrients affected are the fat-soluble vitamins A and E, folate and iodine. There is no significant difference in any of the macronutrients such as protein, carbohydrate and fat.

Vitamin A in whole milk varies from 69 micrograms per 100 millilitres in the summer to 44 micrograms in the winter. Whole milk is a useful source of vitamin A, particularly for young children, and this is one of the main reasons why milk is recommended for children under two years.

The variations in vitamin E and folate are both small: vitamin E averages 0.1 milligrams per 100 millilitres in summer and 0.07 milligrams in winter, while folate rises from 4 micrograms per 100 millilitres in summer to 7 micrograms in winter.

Iodine content averages 7 micrograms per 100 millilitres in summer and 38 micrograms in winter because cattle consume greater amounts of iodine-supplemented manufactured feed in the winter months.

The widespread addition of iodine to animal feed has meant that milk and dairy products are now major sources of iodine in the British diet – a factor which has helped significantly to eliminate goitre.

Sarah Marshall

National Dairy Council, London, UK

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 In-flight meal

During migration the ruby-throated hummingbird (Archilochus colbris) tanks up with a few drops of nectar for the last time on the northern shores of the Gulf of Mexico. It then flies non-stop for at least 800 kilometres to reach the shores of the southern Gulf. Can anyone calculate the metabolic fuel efficiency of these birds that fly so far on so little, and how does this compare to a human?

Martin Bradfield

Lohhof, Germany

This question may involve some erroneous assumptions. Before departing for the Yucatán peninsula, a hummingbird spends weeks gorging on arthropods and does not merely consume ‘a few drops of nectar’. It puts on enough fat to nearly double its weight: a female can grow from 3.2 grams to around 6 grams, and can barely get airborne. When, after anything up to 22 hours, it reaches its destination, it will weigh around 2.7 grams, having consumed the fat and often some muscle tissue. Many do not complete the trip.

The average metabolic rate for the black-chinned hummingbird is 29.1 ± 6.3 kilojoules per day. A man, metabolising energy at the same rate, would have to consume twice his weight in meat a day, or 45 kilograms of glucose, and his body temperature would rise to over 400 °C.

Lanny Chambers

St Louis, Missouri, US

The basal metabolic rate is the measurement of how much oxygen an organism uses when at rest. Just the fact that the hummingbird has a very high ratio of body mass to surface area gives it a basal metabolic rate that is 12 times as high as a pigeon’s and 100 times that of an elephant.

The metabolism of the ruby-throated hummingbird is much lower when in torpor than in flight. To go from torpor to an active state takes it about an hour. The heart rate rises from 50 beats per minute to 500, and its temperature from 10 °C to 40 °C. When in full flight its heart pumps at 1,260 beats per minute and its wings beat 50 to 200 times per second. And this is still more energy-efficient than a human taking a walk.

Ruby-throated hummingbirds belong to a group of birds known as passerines, which have three toes facing forwards and one toe back. Passerines tend to have a metabolic rate that is as much as 70 per cent higher than either non-passerine birds or mammals. Their muscles are made up of about 35 per cent mitochondria with densely packed cristae – infoldings of their inner membrane – which makes them at least twice as efficient as human mitochondria.

To achieve the same efficiency, humans would have to have muscles composed of 70 per cent mitochondria. And even then the muscles could not work because there would be too few myofibrils in them.

While wintering in Mexico, the bird doubles its weight by building up considerable fat reserves to use on its long journey north. In fact, most ruby-throated hummingbirds travel along the edge of the Gulf of Mexico, eating a little along the way, but some do take the short cut from Florida to Yucatán using their fat reserves and catching gnats along the way.

A number of key factors make the ruby-throated hummingbird so efficient. Its pectoral muscles are red meat, which is rich in the oxygen-carrying protein myoglobin; the muscle has a high capillary-to-fibre ratio, giving it a good blood supply; it has an energy-rich diet of nectar that is stored as fat; it can eat nectar from any species of flower so it does not waste time looking around for a particular source, and will also eat any insects; its tongue is fringed so that the nectar is effortlessly drawn up by capillary action; and it operates at high temperatures to make its metabolic reactions more efficient.

It has been calculated that this species eats as much as three times its own weight in a day, during which it is awake for 16 hours on average. This is the equivalent of an 83-kilogram human eating 125 kilograms of hamburgers every day, or 1,335,000 kilojoules.

Mike Ball

Gorinchem, Netherlands

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 Burnt offerings

Almost all dog food contains ash as an ingredient. Sometimes it makes up as much as 14 per cent by weight. I have always thought of ash as being toxic waste, containing all sorts of noxious elements, so why is it added to dog food and what type of ash is it?

William Davidson

Strathaven, Lanarkshire, UK

You will be relieved to hear that ash is not added to pet foods. It is a way of describing the mineral content of pet food. The ash you see listed is part of the guaranteed nutrient analysis: legally the pack must state how much of the food is protein, fat, fibre, water and ash.

Ash is measured by heating the pet food to temperatures of around 550 °C, and burning off all the organic components to leave just the inorganic or mineral residue. If the mineral content of pet food sounds high, it is important to remember that our domestic carnivores were designed to eat carcasses that are full of bones containing minerals, and a well-designed pet food will reflect this in its composition.

Kim Russell

Registered pet nutritionist

North Molton, Devon, UK

This is a misreading of the label on the product. Ash is usually given under ‘typical analysis’ or a similar heading, not under the ingredients list.

Foods are often described in terms of their nutritional content by carrying out a proximate analysis. This is done because it is much quicker and cheaper than carrying out a detailed analysis of the nutrients.

The protein content is determined by analysing the nitrogen content, using a technique called the Kjeldahl method, and multiplying by a conversion factor to obtain a crude protein figure.

The fat content is derived by gravimetric extraction using a suitable non-polar solvent (usually petroleum ether), while the water content is obtained by drying, and the mineral content is found by burning off all the organic material in a muffle furnace to obtain the ash. The carbohydrate content is often estimated by subtracting the former components from the total weight.

So ash is not added as an ingredient but is instead an indicator of mineral content. These minerals will be chiefly potassium and phosphorus with smaller amounts of calcium, iron, magnesium, sodium and zinc, and trace amounts of many others. Historically, manufacturers often boosted the mineral content of dog food with bone meal to raise calcium levels but, because of concerns about BSE, they now tend to use fish meal instead.

You might see ash levels of 14 per cent in a dry meal for dogs, but tinned products often have around half this level. The composition of the food will affect the ash content, but the elements are likely to be beneficial or neutral to the dog’s health and not noxious or toxic at the concentrations in the product. It is worth pointing out that dogs should always have access to fresh water to ensure they can urinate away any excess potassium or sodium.

Brian Ratcliffe

Professor of Human Nutrition

The Robert Gordon University

Aberdeen, UK

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 Floundering about

How do certain animals, such as the flounder fish, change their colour to match their background? More specifically, if you made a tiny blindfold for the flounder, would it still be able to match its surroundings?

Nick Axworthy

By email, no address supplied

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’s 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. 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 even produce a chequerboard appearance if the fish is lying over a chequered surface.

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 ‘black’ plaice sometimes sold in the UK, which have come from the sea around the dark volcanic seabed off Iceland.

Cliff Collis

London, UK

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

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 Biosphere

Hypothetically (because otherwise my mum would get mad), if I were to put my brother in a perfectly sealed room, how much plant life would I need in that room in order to maintain a balance of oxygen and carbon dioxide such that both my brother and my beloved plants may continue to live?

Gene Han

Iowa, US

To simplify matters, you could supply his meal through an airtight hatch. The plants would then only need to provide his oxygen. If he spent all his time eating and dozing, he would need about 350 litres of oxygen per day (the amount of oxygen in 1.7 cubic metres of air). This much oxygen is produced in full sunlight by typical vegetation covering a floor area of between 5 and 20 square metres. Using the most productive ‘C4 plants’ such as sugar cane, you could reduce the area needed to 2.5 square metres. Your brother would exhale 350 litres of carbon dioxide per day, which would enable the plants to grow with an increase of dry weight of 430 grams per day.

Now let’s muddy the waters. If his windows plus artificial lights supply 10 per cent of full sunlight, multiply the required area of greenery by a factor of 10. If the lights go out at night, double the area – more in winter. Plants photosynthesise during the day more rapidly than they respire at night. Therefore, as a reasonable approximation, you can neglect the extra oxygen that plants consume at night.

If you don’t intend to feed your brother, but hope he will survive by eating the plants, remember that most material a plant synthesises is indigestible, so double the area again. The inedible parts of the plants plus your brother’s faeces would need to be decomposed or burnt to carbon dioxide to recycle the carbon they contain. So, if your brother is a well-trained plant physiologist, this ambitious biosphere might need to be a plant-filled room about 20 metres square.

Here is the basis of my calculations:

The daily energy requirement of an adult dozing is 1,750 kilocalories per day. The energy content of 100 grams of sucrose is 400 kilocalories. Therefore an adult needs 1,750/4 = 438 grams of sucrose per day = 1.28 moles sucrose per day.

Respiration of this requires 1.28 × 12 = 15.36 moles of oxygen per day. One mole occupies 22.4 litres, so this corresponds to 15.36 × 22.4 = 344 litres of pure oxygen per day.

Photosynthesis rates of plants in the field under optimal lighting are between 10 and 30 micromoles (up to 70 in C4 plants) of carbon dioxide fixed per square metre per second (0.86 to 6.05 moles per square metre per day). For each mole of carbon dioxide the plants fix, they liberate a mole of oxygen.

Therefore the area required is somewhere between 18 square metres for the less productive plants down to 2.5 square metres for C4 plants.

Stephen Fry

Institute of Cell and Molecular Biology

Edinburgh University, UK

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 Blood brothers

At the risk of flogging a dead, er, penguin. Why don’t polar bears’ feet freeze?

Paul Newcombe

Zurich, Switzerland

Unlike the penguin with its fancy internal plumbing, the reason that polar bears’ feet do not freeze is good insulation, pure and simple.

Polar bears (Ursus maritimus) are just about the best-insulated animals on the planet, certainly among those species of mammal that do not live primarily immersed in water. An adult bear has 10 centimetres of blubber beneath its skin, which in turn is covered by a thick coat of fur. This fur relies not only on its density, but also on its unique structure: each hair is a hollow tube, so that air is trapped inside the hairs as well as between them. Even without covering its nose with its paws (as it is reputed to do, although the evidence is very limited) a polar bear is almost invisible to heat-sensitive infrared photography or the latest military image-intensification technology.

The polar bear also has very hairy pads on its feet, and the tough skin is extremely callused on the underside of the paws, so there is a sturdy layer of dead tissue between the ice and any blood vessels.

There may also be another factor at work. The underside of a polar bear’s paw is dotted with dozens of papillae – small nipple-shaped extrusions of even more callused skin – which provide extra grip in the same way as the studs on a footballer’s boot. It is these papillae that enable a polar bear to accelerate to a very respectable pace on the ice and overcome its awesome inertia. They also prevent it skating out of control, past a potential meal.

On really compacted ice, the bears tend to lift part of the underside of the paw clear of the surface. The papillae enable an additional cushion of insulating air to be trapped between most of the pad and the ice.

Such highly developed thermal adaptations can, however, be a double-edged sword. A bear attempting a brisk trot in ambient temperatures of 10 °C or greater would succumb, almost immediately, to a fatal attack of heat stroke. During the Arctic summer it can often be far hotter than that, limiting the polar bear’s ability to function as a hunter.

This potential cramping of the polar bear’s style may prove as fatal to the species’ chances of survival as the actual destruction of its territory. If global warming causes the polar bear to die out, it would surely be the most terrible irony that this was because it had mastered the art of conserving the very energy that a profligate humanity has squandered so obscenely.

Hadrian Jeffs

Norwich, Norfolk, UK

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 Carriers of death

Do mosquitoes get malaria? Do rats catch bubonic plague? If not, why not?

Year 5, Christopher Hatton School

London, UK

Congratulations to the children for asking such a penetrating question.

Rats can get quite sick from plague fleas and some will die, but usually not too quickly. Plague-carrying rats are at their most dangerous when they are about to die, because their fleas leave them as soon as they are dead to find new hosts.

The malaria parasite Plasmodium does not usually kill its host mosquitoes, though it may take a high enough toll that it is better for the mosquitoes not to get infected.

If we could breed mosquitoes that were resistant to the parasite we might find that they outcompete ordinary mosquitoes, and this might ultimately help get rid of malaria. This kind of strategy would not work with yellow fever, as the mosquitoes that carry the virus responsible for the illness in humans hardly seem to be affected.

Jon Richfield

Somerset West, South Africa

Plasmodium, the parasite that causes malaria in humans, infects mosquitoes. The mosquitoes then transmit it to people when feeding on their blood. As for the plague microbe, Yersinia, it blocks the gut of the flea that transmits it. As a result, when an infected flea feeds on the blood of a human or rat, it will regurgitate some blood containing the microbe and so spread the germ to a new host.

To address the question directly, the important thing to note is that being infected with a microbe or other parasite does not necessarily cause disease, because it is often in the interest of the microbe to cause no harm to its hosts.

However, the mosquitoes that transmit Plasmodium are affected by it, as the parasite grows in their salivary glands. Such infection can reduce the ability of the salivary glands to function and thus the viability of the mosquitoes.

A related parasite called Theileria, transmitted between cattle by ticks, can damage the gut and salivary glands of the ticks, and can even kill them in the laboratory. Epidemiologists make a point of studying the extent of such effects under natural conditions.

Alan R. Walker

Edinburgh, UK

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 Topsy-turvy world

Why don’t bats get dizzy when they hang upside down? Or do they?

Year 5, Christopher Hatton School

London, UK

Dizziness is a sensation humans describe when they feel a sense of motion, even when not moving. It can be associated with queasiness or nausea, and sometimes vomiting. Other types of dizziness include motion sickness and vertigo, which often manifests itself as a spinning feeling, or other sensations such as light-headedness or heavy-headedness.

It is impossible to know for sure whether or not an animal is dizzy, because it cannot communicate such feelings. However, it is possible to infer an animal is dizzy from how it behaves. For example, if an animal is aimlessly walking in circles, it is probably dizzy.

Motion sickness occurs when there is excessive stimulation of the inner ear or from a conflict between sensory information from different sources, such as from the inner ear and the eyes. The balance mechanism of the inner ear is complicated, and includes sensors that detect both movement and orientation with respect to gravity, even when an individual is not moving. Bats have such a balance mechanism, and in addition use echolocation.

The parts of the inner ear that are important for orientation with respect to gravity are called the otolith organs: the utricle and the saccule. It is these parts of the inner ear that would be activated while the bat was hanging upside down. Stimulating these parts of the inner ear, however, would not necessarily lead to dizziness, especially in a dark cave where there is no conflict between information from the inner-ear balance mechanism and vision.

The bottom line is that bats are used to hanging upside down without showing any behavioural changes that would suggest dizziness or motion sickness. But because we cannot ask a bat directly whether or not it is dizzy, we cannot be certain about the effects of hanging upside down.

Joe Furman

Editor of the Journal of Vestibular Research

University of Pittsburgh

Pennsylvania, US

When you think of bats, you usually think of them in one of two conditions: hanging upside down resting, or flitting about pulling high-g turns in the dark. So why don’t they get dizzy?

Bats have evolved a number of adaptations to allow them to hunt and hang without the problems that humans would face. First, some bats have specialisations in the vestibular portion of their inner ears – the portion that generates sensory signals for controlling balance. Their sacculus, which in humans acts as a gravity sensor to help us stand upright, is slightly rotated forwards. This enables it to act more as a pitch detector, which is more useful in flight. Second, their semicircular canals, which sense rotation of the head, have an internal structure more like a bird’s than a human’s. This probably allows them to make high-speed turns without the fluid in the canals sloshing back and forth too much. Lastly, if you photograph bats in flight with a high-speed camera, you notice that they keep their heads very stable except in the most violent turns.

But it is the way in which bats sense the world that probably gives them immunity to dizziness. All the vestibular system does is tell you about changes in acceleration of your head. It requires other senses to pin down your position and motion in the outside world. We primarily use vision to do this, but vision is very slow. Anything you look at that takes a second or less to cross 30 degrees of your vision appears smeared. Echolocating bats, while not blind, rely more on biosonar, an especially precise form of hearing that lets them build up 3D images from echoes.

Echolocating bats emit brief sonar chirps from 30 to more than 150 times per second, and respond to changes in echoes of less than a microsecond. These bats integrate echolocation with their vestibular system, so they are working with a faster, more precise positioning system than humans do with vision. Because dizziness and motion sickness usually arise when signals from the vestibular system conflict with those from other sensing systems, bats are less likely to show motion sickness than other mammals.

Seth Horowitz

Assistant professor of neuroscience

Brown University, Rhode Island, US