Animal Life Cycles
There are a big number of different types of animals on the planet Earth. There are real big ones like blue whales, elephants or polar bears. There are also so small species that the human eye cannot see them. Their life cycles vary. All living organisms are divided into groups.
Animal Life Cycles by Rachel Bladon
CHAPTER 1: Life Cycles
There are millions of different species of animal in the world. Some animals, like the blue whale, are very big. Others are so small that we almost can’t see them. These animals all have very different life cycles.
Scientists put animals into groups. The animals in each group are the same in many ways, and they often have very similar life cycles. The two biggest animal groups are invertebrates and vertebrates.
Invertebrates are animals that have no backbone. More than 95% of the animal species in the world are invertebrates, and most of them are very small. There are many different groups of invertebrates. The biggest group is called arthropods. Insects, spiders, and animals like crabs that have a hard shell, are all arthropods.
Animals that have a backbone are called vertebrates. Only a few percent of the animals in the world are vertebrates, and most of them are much bigger than invertebrates. Because of this, we see vertebrates more often than invertebrates. Scientists put vertebrates into five big groups – they are called fish, amphibians, reptiles, birds, and mammals.
All animals need to find food to stay alive. Some animals are carnivores. This means that they eat other animals. Herbivores eat plants, and omnivores eat plants and animals.
There are many dangers for animals. They have to keep safe from predators – animals that hunt and eat other animals. People hunt birds and other animals. People also cut down trees and build houses and roads on land. When they do this, animals lose their homes and their food. Many animals also become sick or die because of pollution of the land, oceans, rivers, and air. Life is very dangerous for animals. Many animals die before they are old enough to produce young.
Different animals live for different amounts of time. A Galapagos tortoise can live for 150 years, but an adult mayfly usually lives for just a few hours.
Most species of animal have males and females. Males produce sperm, and females produce eggs. Baby animals are made when sperm from the male joins together with an egg from the female. This is called fertilization.
Some animals give birth to baby animals. Other animals, like reptiles and birds, lay eggs. Their babies hatch – they break the egg open and come out.
Most reptiles and birds care for their babies. Most insects and fish do not care for their eggs or their babies. All mammals care for their babies and feed them milk. Some big mammals stay with their babies for many years.
CHAPTER 2: Insects
Insects are small animals like butterflies, beetles, flies, bees, and ants. There are more than a million species of insect in the world. Many insects change in amazing ways during their life.
Larvae and Nymphs
Almost all insects start their life in an egg. The young insect grows inside the egg and then it hatches. Some baby insects, like bees, look very different from their parents. These baby insects are called larvae. Other baby insects, like locusts, look more like their parents. These baby insects are called nymphs. Larvae and nymphs are very small when they hatch.
A young insect eats a lot of food and grows very quickly. It has a hard cover, called an exoskeleton. When the exoskeleton is too small for the young insect, it breaks open and comes off. A new exoskeleton grows under the old one. This is called molting.
Some insects grow wings that get bigger every time they molt. After the insect molts for the last time, it becomes an adult that can fly and produce young. This is called incomplete metamorphosis.
Most insects change by complete metamorphosis. When the larva is ready to change into an adult, its exoskeleton comes off. Under the exoskeleton there’s a pupa – it’s like a closed case. Inside the pupa, an amazing change happens. The larva changes into an adult that looks completely different!
Finding a Mate
When insects become adults, they are ready to breed and produce young. First they need to find a mate. Male crickets, cicadas, and grasshoppers sing to find females that they can mate with.
Female butterflies and moths produce special chemicals called pheromones that have a strong smell. Male butterflies and moths can smell the pheromones from up to 2 kilometers away. They then fly toward the smell.
Male fireflies make special lights in their body. They turn these lights on and off so that female fireflies know that they are looking for a mate.
Some insects only lay a few eggs during their life, but others lay thousands. Many insects are careful to keep their eggs safe. The cardinal beetle makes a hole in dead wood and lays her eggs inside. When the eggs hatch, you can’t see the larvae very well. Cockroaches lay their eggs in a special case, and beetle eggs are usually yellow, green, or black so that they are camouflaged. When these insect eggs hatch, the life cycle starts again!
Termite queens lay up to 30,000 eggs every day!
CHAPTER 3:Other Invertebrates
There are many other groups of invertebrate, and they all have very different life cycles. Did you know that earthworms, octopuses, spiders, and lobsters are all invertebrates?
All earthworms lay eggs, because they are hermaphrodites. This means that they all have male parts that produce sperm and female parts that produce eggs, but they cannot fertilize their own eggs. After earthworms mate, their bodies make special rings with eggs inside. The ring moves forward over the worm’s body and then it comes off. Then the ends of the ring join together. The eggs hatch after about three months, and young earthworms can mate when they are 12 months old.
Baby octopuses live in the open ocean at first, but when they grow, they move down to the bottom of the ocean. Here they live on their own in dens.
Octopuses are ready to mate when they are one or two years old. Some female octopuses lay about 200,000 eggs, and many put them in strings from the top of their dens. The females keep the eggs safe from predators and they carefully push water over them so that they get enough oxygen. The octopuses do not eat for up to ten months when they are caring for their eggs. After the eggs hatch, the female octopuses die.
Spiders produce silk, and they use it to make webs, where they catch flies and other insects. Most species of spider also put lots of silk around their eggs to make a special cover, called a cocoon.
Most spider eggs hatch after a few days or weeks. The baby spiders, or spider lings, grow fast and they molt a few times. They can soon make their own webs and catch their own food.
Many spiders die after they lay their eggs. Some spider lings eat their mother’s body after she dies!
A female lobster can lay up to 10,000 eggs, but only about ten of them will live. The female lobster carries her eggs hidden under her tail for almost a year. When they are ready to hatch, she lifts her tail and they float away.
When the eggs hatch, the larvae move to the top of the ocean. Life here is very dangerous because of predators and difficult currents. Lobster larvae look very different from adult lobsters. When they are about one month old, the larvae move to the bottom of the ocean. They molt about 25 times before they become adults.
Lobsters often eat their old shell after molting. This helps their new shell to become hard.
CHAPTER 4: Fish
Fish are vertebrates that are cold-blooded. This means that their body is as hot or cold as the water around them. All fish live and breathe in water, and most baby fish hatch from eggs.
Female fish can lay hundreds of eggs. Inside every egg there’s a bag of food called the yolk. The unborn baby fish eats the yolk, so that it can grow bigger and stronger.
Fish often try to hide their eggs because many other animals like to eat fish eggs. Female seahorses lay their eggs into a special pouch on the male’s front, where they stay safe until they are ready to hatch.
Some species of shark lay their eggs in a hard egg case to keep the eggs safe until they hatch.
When young fish hatch, the yolk bag is still stuck to their body. The young fish use this as food at first.
Life is very dangerous for young fish. Most of them don’t have fins, so they can’t swim well. They often float into colder water or places where there’s no food. Also, many predators, like seals, birds, frogs, and other fish, hunt them. This is why fish lay so many eggs. A large female tuna fish can lay up to six million eggs at one time, but only about two of these will grow into adult tuna!
Most fish never see their parents because they swim away after they have laid their eggs. The young fish have to find food for themselves so that they can grow bigger. They eat small plants and insect eggs or larvae.
Most small fish grow into adults just a few months after hatching, but whale sharks do not grow into adults until they are 25 years old!
The whale shark is the biggest fish in the world. An adult whale shark can be about 12 meters long and it can weigh more than 21 metric tons.
Fish need to find mates so that they can breed. When the male three-spined stickleback is ready to breed, he builds a nest.
Part of his body changes to a red color. Females see his bright colors and they lay their eggs in his nest.
Many fish go back to the same place to breed every year. Salmon live in the ocean, but to breed, they swim back to the rivers where they hatched. Some salmon travel hundreds of kilometers up rivers, and sometimes they have to jump up waterfalls. They change color from silver to red when they are swimming back to their breeding places.
CHAPTER 5: Amphibians
Like fish, amphibians are cold-blooded vertebrates. Most amphibians live for some of their life in water and for some of their life on land. Salamanders, toads, and frogs are all amphibians.
All amphibians have two different parts of life. At first they are larvae that breathe in water through special gills. Then they change into adults with lungs. This change is called metamorphosis. Usually, amphibians live in water for the first part of their life and on land for the second part. Some salamanders live in water all the time.
Eggs and Larvae
Amphibians often lay thousands of eggs. The eggs have a sticky cover, and they float in water. Most amphibians do not care for their eggs, but the male midwife toad carries them on his back until they are ready to hatch.
The young that hatch from eggs are called larvae, but young frogs and toads are also called tadpoles. At first, tadpoles don’t have legs or feet and they breathe through gills. It’s easy for predators to catch and eat eggs and tadpoles. Frogs can lay about 2,000 eggs, but probably only 5% live to become adults.
The male Darwin’s frog keeps his tadpoles safe and wet in his throat, until they are big enough to swim away.
When amphibians begin to change into adults, they grow lungs inside their body and their gills close. At the same time, most amphibians grow legs and feet.
An adult frog looks completely different from a tadpole!
Most adult amphibians can breathe through their skin and their lungs! They can only breathe through their skin if it’s wet, so most amphibians live near water.
Amphibians are cold-blooded, so when it gets very cold they have no energy. Some amphibians in colder countries hibernate for the winter. They hide in a safe, wet place and go into a special, long sleep.
Most amphibians go to wet places like ponds, lakes, or rivers to breed, because their eggs and larvae live in water. Many go back to the same place every year, and some go back to the pond or stream where they were born. Male frogs croak and shout to tell the females that they are ready to breed. They push their throats out so that their calls are louder. When the female has chosen a mate, she lays her eggs in the water, and the male covers them with sperm.
CHAPTER 6: Reptiles
Reptiles are cold-blooded, and their skin is dry and covered with special scales. Lizards, snakes, crocodiles, alligators, and turtles are all reptiles. Some live on land and some live in water.
Most reptiles hatch from eggs, but a few are born as living animals. Females lay their eggs in a warm, wet place. They usually lay a lot of eggs, but many eggs are eaten by predators. Most reptiles lay their eggs and then they leave them, but pythons put their body around their eggs to keep them warm. Alligators make a big nest from mud and leaves. They lay their eggs and stay near the nest. They don’t eat for weeks when they are keeping their eggs safe.
Crocodiles and alligators are very good parents after their eggs hatch. The female Nile crocodile is very careful – she puts her babies inside her mouth to carry them to water. She stays near them to keep them safe for up to two months.
When snakes and lizards grow, they get too big for their skin. The old skin comes off and there’s a new one underneath. This is called sloughing. Most snakes lose their old skin three or four times every year.
When reptiles are ready to breed, they have to find a mate. Male crocodiles lift their head up and make a loud sound. They also blow bubbles in the water so that females will see and hear them. Snakes find their mate by smell. The females leave a special smell that the male follows. Other male reptiles, like monitor lizards, fight each other for a mate. The females choose the winner of the fight because he’s the strongest.
Some whiptail lizards can produce young without mating! The eggs are not fertilized, but they still hatch.
Reptiles are in danger from many predators, and they have lots of ways of keeping safe. If a predator catches a lizard by its tail, the lizard can break its tail off and escape. It then grows a new tail. The stinkpot turtle makes a horrible smell from its legs to make predators go away. Many reptiles also use camouflage to keep safe. Some chameleons can change color so that predators do not see them, and the leaf-tailed gecko can hide easily because its body looks like a leaf.
CHAPTER 7: Birds
There are about 9,000 different species of bird, and they all lay eggs. Baby birds called chicks come from eggs. Birds are the only animals that have feathers. All birds have wings and most birds can fly.
Courtship is when male birds try to find female birds that they can mate with. Male birds do many different things to find a mate. Some males show females their colors. The male frigate bird pushes his red throat out like a balloon. The blue-footed booby shows his big blue feet.
Songbirds sing for their females, and the bower bird builds a special place with twigs and puts bright colored things in it so that it looks pretty.
Birds work very hard to build nests – special homes where they can keep their eggs and chicks safe and warm. They find grass, mud, and twigs, and they push them together. Many birds build nests in high places like trees and cliffs. Others build them on the ground. These birds are usually brown and speckled, so they are well camouflaged.
The female bird lays her eggs in the nest. Then the female or the male sits on the eggs to keep them warm. When the chick is ready to hatch, it breaks the shell with a special ‘tooth’ on its beak.
Cuckoos lay their eggs in other birds’ nests. When the baby cuckoo hatches, it pushes the other eggs or chicks out of the nest so that it can have more food. It often grows much, much bigger than the adult bird that is feeding it!
Ostrich eggs are the biggest bird eggs, and they can weight about 2 kilograms. If you stand on one, it will not break!
Many chicks have no feathers, and they can’t see or move when they hatch, but some can run, swim, and find food. Birds feed their chicks and keep them safe until they can care for themselves. Birds often have to fly to and from the nest hundreds of times a day to bring enough food for their chicks.
Some small birds can fly just two weeks after they hatch, but bigger birds take longer. Chicks watch their parents and learn to fly, feed, sing, and keep safe. Some baby birds do not become adults for months or even years.
Many birds that migrate every year have to make long, dangerous journeys when they are still very young. These birds fly to warmer places for winter and come back to their breeding places for summer.
CHAPTER 8: Mammals
All mammals have fur or hair and they produce milk to feed their babies. Giraffes, bears, kangaroos, elephants, and people are all mammals.
It’s usually the female mammals who choose a mate, so the males want the females to look at them. Leopards make marks on trees with their claws so that the females will know they are there. Other animals make special smells.
Males often fight with each other to win a mate. Male giraffes fight with their head and neck to show which animal is the strongest.
Most mammals do not lay eggs. The babies grow inside the mother’s body, and they get food and oxygen from a special part of the body called the placenta. This is called gestation.
The gestation time for shrews is only two weeks, but for African elephants it’s 22 months!
When mammals are ready to have their babies, many look for safe, quiet places. Polar bears dig special dens in the snow so that their babies will be warm and safe.
Some baby mammals grow a lot inside their mother’s body, and they can stand up as soon as they are born. Other baby mammals can’t see or move when they are born. Mammals produce milk for their babies, and this is the only food or drink that they need at first.
Marsupials are animals like kangaroos and koalas. Baby marsupials only grow for a short time inside their mother’s body. After they are born, they continue to grow on the outside of their mother’s body, usually in a pouch. Baby kangaroos are only 5 centimeters long when they are born. They climb into their mother’s pouch, where they stay for up to a year.
Mammals are different from many other types of animal because they feed and care for their babies. Anteaters carry their babies on their back for the first year, and elephants stay near their mother until they are ten years old. Baby mammals learn from their parents, and they learn by playing, too.
Life is very dangerous for a lot of animals. Many are killed by predators. Also, people hunt them and damage their habitats – the places where they live.
Many animals die before they become adults, but if they grow up, then they can produce young. This is all part of their life cycle. Because of this, there will always be millions of species of animal on Earth, if we care for them and their habitats.