Animals

The oldest argument in the field

Why Do Zebras Have Stripes?

People argued about zebra stripes for a hundred and fifty years. The best answer involves biting flies, not lions — plus blurring the herd.

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Turning round in mid-air

Why Does a Cat Always Land on Its Feet?

A falling cat has nothing to push against, and turns over anyway: it tucks one end in and leaves the other out, then swaps, twisting each half in turn.

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Rebuilt from the inside

How Does a Caterpillar Become a Butterfly?

Inside the chrysalis the caterpillar dissolves into a soup of cells, and clusters that have waited since it hatched build a butterfly out of the wreckage.

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And it stops one of them to swim

An Octopus Has Three Hearts and Blue Blood

An octopus runs on copper-based haemocyanin rather than iron-based haemoglobin: blue, good in cold water, and inefficient enough to need three hearts.

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Four layers and a good idea

How Do Penguins Stay Warm in the Snow?

An emperor penguin stands through months of the coldest dark on Earth without eating, on trapped air, fat, cold feet and a rotating huddle.

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By about sixty million years

Sharks Are Older Than Trees

Sharks appear in the fossil record 65 million years before the first true trees — and they are older than Saturn's rings and older than the North Star.

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A filter, not a plan

Why Do Animals Fit Where They Live?

Individuals vary, their surroundings quietly filter which survive, and the traits that won get passed on. Repeat that and the fit becomes very close.

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Never far from water

What Is an Amphibian?

Frogs, toads, newts and salamanders share bare damp skin they breathe through and jelly eggs with no shell, which keeps them all near water.

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Smelling with your forehead

What Are an Insect's Antennae For?

An insect's antennae are not feelers but mostly its nose: pitted surfaces catching single scent particles, two of them giving direction.

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The fastest-growing bone there is

Why Do Deer Grow New Antlers Every Year?

Antlers are bone grown and discarded every year, which lets a deer show its condition honestly and cheaply repair a broken one.

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Chemical trails

How Do Ants Find Food?

Ants lay scent trails that evaporate steadily. A short route gets walked more often and so stays strongest, and the colony finds it with nobody in charge.

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Ready to run, or helpless and blind

Why Are Baby Animals Born So Different?

Some babies are born nearly finished and some barely started, and which one depends on whether the mother has a safe place to leave them.

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Shouting at the world and listening back

How Does a Bat See in the Dark?

A bat crosses a pitch-black cave by shouting too high for you to hear and timing the echoes — and its wings are hands with enormous fingers.

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A tool for one job

Why Are Bird Beaks All Different Shapes?

A beak is a bird's only hand, mouth and tool, so its shape is a direct record of one food — pliers for seeds, a straw for nectar.

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Making the pond it needs

Why Does a Beaver Build a Dam?

A beaver cannot defend itself on land, so it floods the land instead. The dam makes a pond deep enough to put its front door safely under the surface.

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A deal, not a favour

Why Do Bees Visit Flowers?

A flower makes nectar purely to give it away, and the bee carries pollen entirely by accident, on hairs that are branched and slightly charged.

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One animal in every four

Why Are There So Many Kinds of Beetle?

Of every four kinds of animal that have ever been named, roughly one is a beetle — largely thanks to a pair of armoured wing cases.

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Everything about it is built for lightness

How Does a Bird Stay Up?

A bird bends passing air downwards off its curved wings, and almost every part of it — hollow bones, enormous chest muscles — pays for that.

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Instinct plus practice

How Do Birds Build Nests?

Nest building is instinctive, but birds genuinely improve with practice — and where the nest goes matters as much as how carefully it is woven.

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The steadiest place around

Why Do Animals Dig Burrows?

A metre of soil holds one temperature through a heatwave and a frost alike, and the door is narrow — both worth an enormous amount of digging.

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Breaking up the outline

How Does Camouflage Work?

Camouflage is less about matching a colour than about destroying a recognisable outline and cancelling the animal's shadow. It fails the moment it moves.

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Teeth in the wrong place

Why Do Chickens Swallow Stones?

A chicken has no teeth at all. It swallows small stones on purpose and grinds its food against them in a muscular gizzard further down.

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Both, in a way

Is Coral a Rock or an Animal?

A reef is not rock. It is millions of tiny animals living in stone cups they built themselves, fed by algae farmed inside their own tissue.

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A stomach with four rooms

Why Does a Cow Chew All Day?

Grass is nearly indigestible, so a cow ferments it with microbes in a huge first stomach, brings it back up, and chews the same mouthful twice.

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Legs that bend the wrong way

Why Does a Crab Walk Sideways?

A crab is not being awkward. Its leg joints only hinge side to side, so sideways is the fast way to travel rather than the strange way.

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A comb and a scraper

How Does a Cricket Chirp?

A cricket does not chirp with its mouth. It scrapes a hard wing edge over a comb of ridges, and listens with eardrums in its front legs.

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An hour underwater, without trying

How Does a Crocodile Hold Its Breath So Long?

A crocodile lies on a riverbed for an hour by slowing its heart to a crawl and sending blood only where it matters — brain and heart.

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Reading bones

How Do We Know About Dinosaurs?

Everything we know about dinosaurs comes from bones, teeth and footprints, read against animals alive today — birds and crocodiles above all.

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A nose that catches smells

Why Is a Dog's Nose Wet?

The bits of stuff that carry a smell stick to damp skin, so a wet nose holds on to what a dry one would let blow straight past.

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A picture built from echoes

How Do Dolphins See With Sound?

A hollow ball and a solid one ring differently, so a dolphin hears what a thing is made of and not merely where it is — hundreds of times a second.

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Four wings, four motors

How Does a Dragonfly Fly Backwards?

Each of a dragonfly's four wings has its own muscles, so it can aim the push anywhere — which is how it hovers, reverses and turns on the spot.

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A bird that sits on water all day

Why Does a Duck Stay Dry?

A duck stays dry with zipped, oiled feathers over trapped air, and keeps its feet deliberately cold so that it loses no heat through them.

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Shout, listen, measure

How Do You See With Sound?

Echolocation is measuring distance by timing an echo — and bats, dolphins, ships and hospital scanners all do the same arithmetic.

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A pond in a box

Why Do Animals Lay Eggs?

An egg carries water, food and air for a growing animal inside a shell strong enough to sit on and thin enough to breathe through.

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A nose with forty thousand muscles

What Can an Elephant Do With Its Trunk?

An elephant's trunk is a boneless nose and upper lip with tens of thousands of muscles, used to breathe, drink, grab, smell and talk.

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Counting down

What Does Endangered Mean?

Endangered is about the trend and the risks, not just the number: a shrinking, scattered population is in danger long before it is small.

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One structure, many jobs

Why Do Birds Have Feathers?

A feather is a branching shaft whose barbs hook into one another to make a sheet — light and stiff for flight, tangled and loose for warmth.

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Taking oxygen out of water

How Do Fish Breathe Underwater?

Fish do not breathe water. They pull the oxygen dissolved in it through hugely folded gills, with blood running the opposite way to catch more of it.

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You are what you eat

Why Are Flamingos Pink?

The pink is not theirs. Pigment from the algae and shrimp they eat is carried into each feather as it grows, so the colour is eaten first.

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Catching what you cannot see

How Does a Fox Hunt Under Snow?

A fox catches a mouse it never sees by comparing what each ear hears, tilting until it has the depth, then diving head-first at the sound.

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Two lives, one animal

Why Do Frogs Start as Tadpoles?

A tadpole is built for water and a frog for land, so the animal rebuilds itself halfway through — and the young never compete with the adults for food.

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Holding onto still air

Why Do Animals Have Fur?

Fur works by trapping a layer of still air against the skin, and animals fluff it, shed it and grow it back as the seasons change.

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Everyone gets a tow except one

Why Do Geese Fly in a V?

Each bird rides the swirl of rising air spilling off the wingtip ahead, which is real work saved. They take turns at the front, where there is none.

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Seven bones, same as yours

Why Is a Giraffe's Neck So Long?

A giraffe has the same seven neck bones you do, each one enormous, plus a very strong heart and valves that let it survive bending down.

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Standing on almost nothing

How Do Goats Climb Cliffs?

A goat stands on ledges narrower than its own hooves. Each hoof splits into two toes with a hard gripping rim around a soft rubbery pad.

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An argument without a fight

Why Does a Gorilla Beat Its Chest?

Chest-beating is an honest advertisement of size, carried far enough that a rival can decide to back down long before anybody has to get hurt.

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Fitting one place very well

Why Does Every Animal Live Somewhere Different?

A polar bear would die in a desert and a camel on the ice: being superb in one place is exactly what makes an animal hopeless anywhere else.

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Two ways to rent a stomach

Why Do Grass-Eaters Have Such Different Insides?

No animal can digest grass, so every grass-eater rents microbes — and whether it keeps them before the stomach or after changes everything else about it.

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Turning the engine down

Why Do Animals Hibernate?

Hibernation is not deep sleep but a controlled shutdown — body temperature and metabolism dropped far enough that stored fat can outlast a foodless winter.

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Sunscreen made at home

Why Do Hippos Sweat Red?

A hippo has no fur, so it oozes a red secretion that is sun cream and antiseptic at once, and stays submerged through the heat of the day.

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From somewhere it has never been

How Does a Pigeon Find Its Way Home?

A pigeon works out where it is from magnetism and smell, then takes a bearing from the sun — so a shifted body clock sends it confidently wrong.

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Everything else was left behind

Why Do Horses Run on One Toe?

A hoof is a single huge toenail, and losing the other toes made the leg light at the bottom, which is what makes a horse fast.

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A leg that locks

Why Do Horses Sleep Standing Up?

A hook of bone clicks a horse's knee into a catch, so standing costs no muscle and it can doze upright, lying down only for its deepest sleep.

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Sticky feet and a very fast eye

How Does a Fly Walk on the Ceiling?

A fly lands upside down on clawed, glue-footed pads, sees your hand coming far sooner than you expect, and tastes whatever it stands on.

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Running the engine hot

Why Does a Hummingbird Eat All Day?

Wings beating fifty times a second and a heart running past a thousand a minute add up to nearly its own weight in nectar taken in before dark.

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Bread would be fine

What If Every Bee Vanished?

The crops that feed the most people are pollinated by wind, not bees — so losing bees costs us the colour of the plate rather than the calories.

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No brain, no heart, no bones

How Does a Jellyfish Live With No Brain?

A jellyfish has drifted the seas since before there were trees with no brain, heart, bones or blood — just a net of nerves and a bag of water.

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Getting your energy back

Why Does a Kangaroo Hop?

A kangaroo's tendons store and return energy like springs, so hopping fast costs it almost nothing more than hopping slowly does.

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A meal nothing else wants

How Does a Koala Eat Poisonous Leaves?

Eucalyptus is toxic to almost everything, which is exactly the point: a koala took the job nobody was competing for, and its liver pays the bill.

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A warning, not a decoration

Why Are Ladybirds Red?

That red is a warning, backed by a foul fluid the beetle leaks from its knees. It works because every ladybird has agreed to look the same.

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The only big cat that shares

Why Do Lions Live in Groups?

Lions are the one big cat that lives in a group, because open grassland makes sharing the hunt worth more than keeping the food.

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Borrowing warmth instead of making it

Why Do Lizards Sit in the Sun?

A lizard eats a small fraction of what a mammal its size needs, because it buys its warmth from the sun instead of paying for it with food.

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Milk, hair and a warm inside

What Makes an Animal a Mammal?

A whale, a bat and you are all in one group. Milk is the test that never fails, with hair and home-made body heat as the other two marks.

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Two lives, one animal

Why Do Some Animals Grow a Second Body?

Some animals grow a whole second body because the job of eating and the job of travelling and breeding need completely different machines.

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If the head fits, the mouse fits

How Does a Mouse Fit Through a Gap?

A mouse pushes through a hole the width of a pencil because nothing behind its skull is locked at a fixed width, not even the shoulders.

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Following the food

Why Do Birds Migrate?

Birds fly thousands of miles because the food supply moves with the seasons, and they find their way using several overlapping systems at once.

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A fifth hand

Why Can a Monkey Hang by Its Tail?

African and Asian monkeys never evolved one in thirty million years of similar forest — a reminder that evolution does not find every good answer.

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It is not the bite, it is you

Why Does a Mosquito Bite Itch?

A mosquito bite does not hurt going in. The insect spits to stop your blood clotting, and the itch is your own reaction to that spit.

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A very old rule, badly broken

Why Do Moths Fly Into Lights?

A moth is not drawn to your porch light. It is holding a steady angle to a distant one, and a close lamp turns that rule into a spiral.

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Growing out of your own container

Why Do Animals Shed Their Skin?

A snake crawls out of its skin in one piece because it cannot grow inside it — and the hours before the new one hardens are its most dangerous.

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A whole second shift

Why Do Some Animals Only Come Out at Night?

Night is a shift, not an empty time: cooler air, fewer eyes on you, and food nobody else is taking, for animals built to sense the dark.

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Skin that works like a screen

How Does an Octopus Change Colour?

An octopus matches a rock in under a second by squeezing millions of tiny colour bags open and shut — and it is colour-blind while doing it.

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Hearing in three dimensions

How Do Owls Hunt in the Dark?

An owl's ears sit at deliberately different heights, so a sound reaches one a fraction before the other. That gap is enough to locate a mouse in the dark.

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A bear with a meat-eater's stomach

Why Do Pandas Only Eat Bamboo?

A panda has a meat-eater's short gut and eats only a tough grass, so it copes by chewing for half the day and spending almost no energy.

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No lips, no teeth, no vocal cords

How Does a Parrot Talk?

A bird's voice box sits at the fork of its windpipe, so each branch works separately and some birds hold two notes at once — a duet with themselves.

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A net, not a bag

What Is a Pelican's Pouch For?

Eleven litres is about three times what a pelican's stomach could hold, and the same thin skin flutters in the heat to cool the whole bird down.

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Choosing the tame ones

How Did Wild Animals Become Pets?

Nobody set out to invent the dog. People kept the calmest wild animals to breed, over and over, until calm was ordinary and the looks changed too.

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Cooling down without sweat

Why Do Pigs Roll in Mud?

A pig can barely sweat, so it wears mud instead. Mud dries slowly enough to carry heat away for hours, which plain water could never manage.

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Eyes in front, and one good chance

What Makes a Good Hunter?

You can tell a hunter from its skull: eyes at the front, pointing the same way. It buys depth judgement at the price of ever seeing behind itself.

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Radiators that also listen

Why Do Rabbits Have Such Big Ears?

Related animals in hot places grow bigger ears, legs and noses the world over — a desert jackrabbit's run to about a quarter of its own body length.

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Seeing by touch

Why Do Raccoons Feel Everything With Their Hands?

A raccoon's front paws sense more detail than its eyes do, so it explores by touch — especially at night, and especially with wet hands.

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The group that left the water

What Makes an Animal a Reptile?

Scaly skin and an egg with a shell are what made reptiles the first backboned animals able to live a whole life away from water.

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Two animals, one shape

Is a Salamander a Lizard?

A salamander looks like a lizard and is not remotely related to one. The wet skin gives it away: it is an amphibian, and it breathes through that skin.

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The shift nobody watches

Who Clears Away Dead Animals?

A dead animal vanishes from a forest in days: birds, then insects, then microbes take it apart in a reliable order and hand it to the soil.

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Senses you do not have

How Does a Shark Find Its Food?

A shark swings its head to compare nostrils, and its teeth sit in gum rather than bone, so a fresh row rolls forward through thousands in a lifetime.

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Wool that never stops

Why Do Sheep Need a Haircut?

A wild sheep sheds its coat every year. Thousands of years of breeding took that shedding out of farm sheep, so somebody now has to do it for them.

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One foot, one shell, no hurry

How Does a Snail Carry Its House?

Snail slime grips when it is pressed slowly and turns slippery when pushed hard, which is how a snail climbs glass without ever sliding back down.

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A whole body doing the work of feet

How Does a Snake Move With No Legs?

A forked tongue samples two places at once, so a snake smells in stereo, and its jaw comes in two halves that walk over a meal wider than its head.

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Drawing a line that nature blurs

What Makes Two Animals the Same Kind?

A great dane and a chihuahua are one species; two identical birds can be two. The test is whether the young can have young of their own.

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Only some threads are glued

Why Don't Spiders Stick to Their Own Webs?

Only the spiral threads of a web are glued. The spider walks the dry spokes instead, on oily feet that touch as little of the silk as possible.

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Thousands of hiding places

How Does a Squirrel Find Its Buried Nuts?

A squirrel remembers roughly where it buried each nut, then finds the exact spot by smell — and the ones it forgets become trees.

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An animal built the wrong way round

How Does a Starfish Grow a New Arm?

A starfish cannot swallow a mussel, so it prises the shell open a crack, pushes its own stomach out through its mouth, and digests the meal in place.

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Nobody is in charge

How Does a Flock Turn All at Once?

A thousand birds turn together in a fraction of a second with no leader at all, because each one follows a few rules about its neighbours.

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One part, a dozen jobs

What Is a Tail For?

A tail can be a rudder, a balance pole, a fifth limb, a fly swat, a blanket or a shout across a field — whatever that animal needs most.

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Half a millimetre long

The Animal That Was Left Outside a Spacecraft and Lived

A tardigrade can dry itself into a barrel called a tun, stop being metabolically alive, and survive vacuum and radiation — then rehydrate and carry on.

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Orange that disappears

Why Are Tigers Striped?

A tiger is bright orange and nearly invisible, because stripes break up its outline and the deer it hunts cannot tell orange from green.

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A shell that is part of the animal

Can a Tortoise Come Out of Its Shell?

Because the ribs grew outwards, a tortoise carries its shoulder blades inside its own ribcage — true of no other backboned animal on the planet.

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It depends on who does the delivering

Is Venom the Same as Poison?

Venom is injected by the animal, poison is only dangerous if you eat or touch the animal — and both are made of the same kinds of chemicals.

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Eating what would kill you

Why Don't Vultures Get Ill?

A vulture eats meat that has been dead for days and walks away well, because its stomach acid destroys the bacteria and their poisons.

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The biggest animal there has ever been

Why Is a Whale Not a Fish?

A blue whale is longer than three buses and would drown if it could not surface: a warm, milk-feeding mammal that went back to the sea.

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Measuring the dark

What Are Whiskers For?

A cat's whiskers are not for stroking. They are levers in nerve-packed follicles, measuring gaps and air movement in total darkness.

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Hitting a tree, all day

Why Doesn't a Woodpecker Get a Headache?

It hammers wood twenty times a second and walks away fine. Being small is most of the reason: a tiny brain packed tight has no room to slosh.

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Free soil engineering

Why Are Earthworms Useful?

Worms eat dead leaves and pass them out enriched, drill the soil full of air and drainage, and quietly mix its layers year after year without stopping.

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