Unit 7 · Life: Cycles, Traits and Survival

A puppy is born. It grows into a dog. That dog may have puppies of its own. Someday the dog dies. Its puppies grow up and have puppies. This pattern is called a life cycle. Every kind of living thing has one. A sunflower has one. A frog has one. So do you.

All life cycles share the same big stages. First comes birth, which for many living things means hatching or sprouting. Then comes growth. Next, an adult may reproduce, which means making new living things like itself. Last comes death. The cycle does not end there, because the young ones carry it on.

The stages are the same, but the details are wildly different. A mayfly may live as an adult for only a day. A bur oak can stand for 300 years. Some animals are born looking like tiny adults. Others, like frogs, change shape completely. Scientists compare life cycles to find what all living things have in common.

In April, an Illinois pond is full of frog eggs. Each egg is a black dot inside a ball of clear jelly. The eggs float in clumps near the shore. After one to three weeks, tiny tadpoles wiggle out. A tadpole does not look like a frog at all. It has a round body, a long tail and no legs. It breathes water through gills, like a fish.

The tadpole eats and grows. Then the changes begin. Two back legs sprout. Then two front legs appear. Lungs grow inside its body. The tail gets shorter and shorter. It is not cut off. The body slowly takes the tail back in and uses it as food. Finally, a small frog with no tail hops onto the shore and breathes air.

This big change in body shape is called metamorphosis. The word means change of form. How long it takes depends on the kind of frog. A spring peeper can finish in a couple of months. A bullfrog tadpole may stay in the water through a whole winter. The adult frog lays eggs in the pond, and the cycle starts again.

A monarch has four stages. It starts as an egg the size of a pinhead. A female lays each egg on a milkweed leaf. Only milkweed will do, because monarch caterpillars eat nothing else. In about four days, a caterpillar chews its way out. The caterpillar’s job is simple: eat. In about two weeks it grows roughly 2,000 times heavier than when it hatched.

Next the caterpillar hangs upside down and sheds its skin one last time. Underneath is a smooth green case called a chrysalis. Inside, the insect’s body is being rebuilt. Wings, long legs and a coiled drinking tube take shape. Nothing seems to happen on the outside. Inside, almost everything is changing.

After about ten days, the case splits and an adult butterfly climbs out. The adult’s job is different from the caterpillar’s job. It drinks nectar from flowers, finds a mate and lays eggs. It cannot chew leaves at all. Egg, caterpillar, chrysalis, adult: this is one full turn of the monarch’s life cycle.

Put a bean seed in a wet paper towel. In two or three days the seed swells and splits. A white root pokes out and heads down. A pale shoot heads up. This is germination, the sprouting of a seed. The seed’s food is stored inside it, so the plant can grow before it has leaves.

A bean plant grows fast. In a few weeks it has leaves and flowers. Each flower turns into a pod. Inside the pod are new bean seeds. By fall, the bean plant dies. The whole cycle takes one summer. But its seeds can wait in the soil and sprout next spring.

An oak starts the same way, from a seed called an acorn. But an oak does not hurry. It may grow 20 years or more before it makes its first acorns. A white oak, the state tree of Illinois, can live for several hundred years. Same stages: seed, seedling, adult, seeds. Very different clock.

Look at your hand. Your fingers, your skin color, the shape of your thumbnail: each is a trait. A trait is a feature a living thing has. Eye color is a trait. A cat’s stripes are a trait. A pea plant’s flower color is a trait. Traits can be seen, or measured, or both.

Many traits are inherited. That means they are passed from parents to offspring. A calico cat’s kittens are often calico. Two tall parents usually have tall children. Monarchs are orange and black because their parents were. The instructions for these traits are inside every living thing, and a copy goes to each of the young.

Farmers have known about inherited traits for thousands of years. They saved seeds from the sweetest corn. They bred the cows that gave the most milk. In the 1860s, a monk named Gregor Mendel counted thousands of pea plants. He showed that traits pass from parents to offspring in patterns you can predict.

A litter of puppies is born. Same mother, same father, same day. Yet one puppy is brown, one is black, and one has a white paw. One grows bigger than the rest. This is variation. It means members of a family, or of a kind, are different from one another.

Variation happens because each young one gets traits from both parents. The mix is a little different every time. One puppy may get its mother’s ears and its father’s color. Another may get the reverse. Only identical twins start with the same instructions. Even they can end up a bit different.

Variation is easy to see if you look closely. Pick up ten acorns from one white oak. Measure them. Some are long, some are round, some have thick caps. Look at monarchs from the same egg batch. Some have wider black lines. No two members of a kind are ever exactly alike. Those small differences are called variation.

Grow two bean plants from seeds off the same pod. Put one in a sunny window. Put the other in a dark closet. In a week, the sunny plant is short, green and sturdy. The closet plant is tall, thin and yellow-white. Same parents, same instructions, very different plants. The difference came from the environment, everything around the plant.

Animals are shaped by their environment too. A well-fed dog grows bigger than its hungry brother. A flamingo is pink only if it eats pink shrimp and algae. Without them, its feathers turn pale. Your skin gets darker in summer sun. Muscles grow when you use them. A scar comes from a cut. None of these were passed on by parents. They are acquired traits, picked up during life.

Some traits come from both. Height is inherited, but a child with poor food may not reach it. The language you speak is not inherited at all. You learned it from the people around you. Scientists ask a careful question about every trait: how much comes from the parents, and how much from the world?

Two great blue herons stand in a Chicago lagoon. One has slightly longer legs. It can wade into deeper water where more fish hide. It eats more. It stays stronger through the winter. A trait that helps a living thing survive is called an advantage. Long legs are an advantage for a heron. They would be no help to a mole.

Look at a bird’s beak and you can often guess its food. A cardinal’s thick beak cracks seeds. A woodpecker’s chisel beak digs insects out of bark. A hummingbird’s needle beak reaches nectar deep in flowers. Among woodpeckers, the ones with slightly stronger beaks may find more food in a hard winter.

Color can be an advantage too. A brown moth resting on brown bark is hard for a bird to see. This is camouflage, blending in. A white moth on the same bark stands out. The brown moth is more likely to live and have young. Its young are more likely to be brown. Over many generations, advantages like this add up.

A male cardinal in February is the reddest thing in a gray Illinois yard. That color costs him. Hawks can see him from far away. So why is he red? Female cardinals tend to choose brighter males to mate with. A bright male has more young. His sons tend to be bright too. The trait keeps going.

Sound works the same way. On a spring night, male frogs call from every pond. Each kind has its own call. Females move toward the loudest, clearest callers. On a summer night, male fireflies flash a code. A female that likes the pattern flashes back. Every song and flash is a way of saying, choose me.

The peacock’s tail is the famous example. It is heavy and hard to hide. But peahens choose the males with the biggest, brightest tails. A trait can make life harder and still help an animal win a mate. Scientists call this sexual selection.

How long is a life? For a monarch that hatches in June, about a month as a caterpillar and chrysalis, then a few weeks as an adult. For a monarch that hatches in late summer, much longer. That one flies to Mexico, rests all winter, and lives about eight months. Same kind of butterfly, very different life span.

Scientists measure life spans by tagging and counting. They put a tiny sticker, a tag, on a monarch’s wing. They band birds’ legs. They count tree rings in a stump; each ring is one year. A house mouse usually lives about two years. A dog, about twelve. A horse, close to thirty. A bur oak in an Illinois prairie can pass 300 years.

A long life is not always an advantage. A mouse lives a short life but can have several litters a year. An oak lives long but may not make acorns for twenty years. Each way of living has worked for a very long time. Comparing life cycles shows there is no single best way to be alive.

Pull a dandelion out of a sidewalk crack. Look at what comes up. A long, tough root went deep for water. A hollow stem held the leaves up toward light. Flat leaves caught sunlight to make food. A yellow flower made seeds. Each part is a structure, a part with a shape and a job.

Plants have parts for defense too. A rose has thorns, so deer think twice before biting. A cactus has spines and a thick waxy skin to keep water in. Some leaves are hairy or bitter. Milkweed leaks a sticky white sap that gums up the mouths of most insects. Monarch caterpillars are among the few that can handle it.

Structures help plants reproduce, too. Bright petals and sweet nectar bring bees. Bees carry pollen from flower to flower. A maple seed has a wing so it spins away from the parent tree. A burr has hooks that grab a passing dog’s fur. Each structure solves a problem the plant cannot walk away from.

A painted turtle basks on a log in a Chicago lagoon. When a shadow passes, it slides into the water. Its shell is a bone box that a raccoon cannot bite through. Its webbed feet push it fast through water. Its eyes sit high on its head so it can see while nearly hidden. Every outside part has a job.

Animals that live through Illinois winters have parts for the cold. A white-tailed deer grows a thick, hollow-haired coat in fall. A chickadee fluffs its feathers to trap warm air. A frog cannot do either. It rests at the bottom of a pond and waits. Its skin is thin so it can take in oxygen from the water.

Parts for eating are easy to spot. A hawk has hooked talons to grab and a hooked beak to tear. A beaver has orange front teeth that never stop growing, so it can chew wood all its life. A mosquito has a needle mouth. Look at an animal’s outside parts and you can often guess how it lives.

Press two fingers to the side of your neck. Feel the beat? That is your heart, a fist-sized pump. It pushes blood through tubes to every part of you. Blood carries oxygen and food to your body and carries waste away. A fish has a heart. So does a frog, a hawk and a mouse.

Take a deep breath. Your lungs filled with air. Oxygen from the air passes into your blood. A fish has no lungs. It pulls oxygen from water with its gills. Insects have no lungs either. They breathe through tiny holes along their sides. Different structures, same job: get oxygen in.

Your skeleton holds you up and guards what is soft. Ribs cage the heart and lungs. The skull is a helmet for the brain. A grasshopper wears its skeleton on the outside, like armor, and must shed it to grow. Your stomach breaks food into pieces small enough to use. Every inside part has a job, and the parts depend on each other.

Turn off every light in a closet. You see nothing. Not because your eyes are broken, but because there is no light. Seeing needs light. Light from the sun or a lamp bounces, or reflects, off things. Some of that bounced light enters your eye. A red apple reflects red light to your eye. That is why it looks red.

Light enters through the pupil, the black hole in the middle of the eye. In dim light the pupil opens wide to let more light in. In bright sun it shrinks. Behind it sits a clear lens. The lens bends the light into a sharp, upside-down picture on the retina at the back of the eye. The retina is a screen packed with cells that react to light.

The retina changes light into signals. A thick nerve carries the signals to the brain. The brain turns the signals into what you see, and flips the picture right side up. Animals’ eyes fit their lives. An owl’s huge eyes gather dim light. A rabbit’s eyes sit on the sides of its head, so it can watch for a fox almost all the way around.

A ball flies toward your face. Your hands come up before you have time to think. How? Your eyes sent a message to your brain along nerves, thin threads that carry signals. Your brain sent a message back down other nerves to your arm muscles. The whole trip took a fraction of a second.

Every sense works this way. Ears catch sound and turn it into signals. Skin has tiny sensors for touch, heat, cold and pain. The nose and tongue sense chemicals in air and food. All of the signals travel to the brain. The brain sorts them and decides. Loud bang? Jump. Smell of smoke? Look for fire. Hot pan? Let go.

Other animals lean on different senses. A dog’s nose has far more smell sensors than yours. A bat sends out high squeaks and listens for the echoes to find moths in the dark. A catfish tastes with its whole skin. Each animal’s senses fit how it gets food and stays safe. But a sense is only useful if its message reaches the brain.

A friend holds a ruler so it hangs just above your open fingers. Without warning, she drops it. You pinch. The ruler slides 15 centimeters before you catch it. That means it took about 0.17 seconds for your eyes to see it move, your brain to decide, and your fingers to close. That is your reaction time.

Try it ten times. Most people get a little faster. That is not because their nerves changed. It is because the brain learned. It stored a memory of the drop and got ready. Memory turns one experience into a better response next time. A squirrel that was chased by a hawk near the oak stays away from the oak.

The brain also decides what to ignore. Right now your skin feels your socks. Your brain is not bothering you about it. But if a spider crawled across your ankle, you would notice instantly. Sorting important signals from unimportant ones is one of the brain’s biggest jobs. Animals that sort well tend to survive.

In October, Canada geese fly over Chicago in a V. The goose in front breaks the air. The ones behind ride the smoother air and save energy. When the leader tires, another takes its place. Geese honk to keep in touch. A single goose would tire faster and have no one to watch for danger. The flock is a survival tool.

Wolves hunt in a pack. One wolf cannot bring down an elk. Six can. Prairie dogs live in towns with lookouts. When one spots a hawk, it barks, and everyone dives underground. Honeybees split the work. Some guard, some gather, some feed the young. Prairie dog towns and bee hives are kinds of colonies. A hive of thousands can do what no single bee could.

Groups have costs too. Food must be shared. Sickness spreads faster. So not every animal lives in a group. A bobcat hunts alone. Still, for many animals, the group means more eyes, more warmth and more help. Bison on an Illinois prairie stand shoulder to shoulder in a blizzard, sharing heat.

A summer with almost no rain. By August, a pond near Springfield is a cracked mud flat. What happens to the living things? The answer is not the same for all of them. Some survive right there. Some move. Some die. Scientists watch which is which, and why.

Frogs with strong legs hop to a creek half a mile away. They move. Turtles dig into the damp mud below the surface and wait. They survive in place. Tadpoles cannot leave and cannot wait. They die. Cattail seeds sit in the dry mud, alive but not growing, ready for rain. The trait that matters is whichever one fits the new conditions.

Big changes have happened in Illinois before. Two hundred years ago, tall grass prairie covered most of the state. Then farms replaced almost all of it. Prairie chickens, which need wide grassland, nearly vanished. White-tailed deer, which like the edges of fields, are now everywhere. A change that hurts one kind can help another.

Most dead things vanish. A fish dies in a river and other animals eat it. Bacteria finish the job. Even the bones crumble. But once in a while, a dead thing is buried quickly, by a flood of mud or a slide of sand. Under the mud, there is little oxygen. Rot slows to a crawl. This is how a fossil begins.

Over thousands of years, more layers pile on top. The mud is squeezed into rock. Water seeping through carries dissolved minerals. The minerals slowly replace the bone or shell, or fill the hollow it left behind. What remains is a copy in stone. Fossils can also be traces: a footprint, a burrow, a leaf print. These are trace fossils.

In 1669, a Danish doctor named Nicolas Steno made a simple claim. Rock layers form one on top of another, so the bottom layer is oldest. And the shark-tooth-shaped stones in the rocks were exactly that: sharks’ teeth, turned to stone. Fossils were not strange rocks. They were once alive. Every fossil is a life that the earth kept a record of.

Most fossils are hard parts: bones, shells, teeth. Soft parts rot before they can be buried. That makes Mazon Creek special. Its rocks hold jellyfish, worms, shrimp, insects, and the Tully monster, soft bodies and all. Collectors find these soft-bodied animals inside rounded rust-colored lumps called concretions. Split one and it may open like a book.

Why here? About 300 million years ago, rivers poured mud into a shallow sea. Storms and floods buried animals and plants in minutes. Iron in the water hardened the mud around each body into a shell of stone before it could rot. The rest of the mud stayed soft and later washed away or became coal.

Coal mining brought the concretions to the surface. For years, collectors walked the spoil piles after rain, when wet rocks showed their cracks. Some soak the lumps in water and freeze them, over and over, until they split. Many of the best finds went to the Field Museum in Chicago. You can see a Tully monster there today.

Illinois has no ocean. Yet limestone all over the state is packed with fossil sea creatures: corals, sea lilies, clam-like shells. Pick up a chunk of gravel and you may hold one. The only explanation is that a shallow sea once covered Illinois. The rocks are the evidence. Scientists infer the past from clues, the way a detective does.

Fossils also show what a place was like. Fern fossils from Mazon Creek match ferns that grow today only in hot, wet places. So Illinois was once hot and wet. Mammoth bones found in Illinois gravel pits show an ice-age world of cold grassland. Same ground, three different worlds: sea, swamp, ice.

Fossils show living things that are gone, too. No Tully monster swims anywhere today. No mammoth walks. Scientists say these kinds are extinct. Comparing an extinct animal’s structures with living animals gives clues. A sharp, claw-like mouth means a hunter. Big eyes mean it found food by sight. Fossils let us ask questions about a world we can never visit.