animals

Can you identify the animals in the pictures?

Animals are living organisms that have a body made of many cells and can move, reproduce, grow, eat and breathe.

Let's discuss animals and their characteristics in more detail.

Characteristics of animals

Animals are made up of cells

Organisms are made of cells. Cells are the basic building blocks of all living organisms. Some organisms are made up of one cell. They're called unicellular organisms. Other organisms are made up of more than one cell. They're called multicellular organisms. Animals are multicellular organisms. They are made up of millions of cells. An animal cell is a type of cell that is exclusively observed in animal tissues. Different types of cells are designed to perform specific functions. Skin cells, muscle cells, blood cells, nerve cells, and fat cells are types of animal cells.

Animals need food

All animals need food to survive.  

• Many animals mainly eat plants (or parts) such as fruits, leaves, seeds, and roots. These animals are called herbivores. Cows, goats, deer, elephants, and horses are examples of herbivores. 
• Some animals eat other animals or eat meat solely and are called carnivores. Lions, tigers, and polar bears are examples of carnivores.
• A few animals eat both plant and animal food. They are called omnivores. Birds, dogs, and foxes are examples of omnivores.
• And some animals eat dead or rotting food and waste. Those animals are called detritivores. Millipedes, slugs, and sea stars are examples of detritivores.

Here's a fun fact!

Did you know that giraffes spend most of their day eating? Giraffes can eat up to 75 pounds (34 kilograms) of food per day. 

Animals breathe (respire)

Breathing supplies organs and muscles with oxygen. Animals need oxygen to survive. They get their oxygen through the process of breathing. But not every animal breathes the same way.

• Some animals breathe with their lungs. Such animals are dogs, cats, pigs, horses, sheep, monkeys, and rats.
• Fish and crabs breathe with the help of gills.
• Spiders and insects breathe with the help of the trachea. 
• And some animals, like frogs, breathe with the help of the skin.

All breathing methods aim to transport oxygen into the cells and the remaining carbon dioxide out of the cells.

Here's a fun fact!

There are lots of air-breathing animals that are built for breath-holding. Turtles can spend all winter long at the bottom of a frozen lake in deep hibernation without using their lungs. 

Animals reproduce

Animals reproduce mainly in two ways.

• One way is when a baby animal develops inside a mother's body (pregnancy), and a live young is born. Animals that reproduce this way include cats, dogs, horses, cows, elephants, goats, pigs, lions, squirrels, etc. These animals are called viviparous. 
• The other way of reproduction is when animals lay eggs, and the baby animals hatch from the egg. Animals that reproduce this way are birds, fish, lizards, insects, etc. These animals are called oviparous.

Here's a fun fact!

The elephants' gestational period (the length of time that they're pregnant) is longer than that of any other animal. It lasts for more than a year and a half or 640 to 660 days.

Animals move

Animals move differently. Some are fast, and some are slow. 

Why do animals move? They move from one place to another to find food, to escape from other animals that are hunting them, or to find shelter. 

How do animals move? There are many ways animals move. They jump, run, slide, glide, fly, slither, swim, climb, hover, creep, crawl, wiggle, and so much more. To move, animals use different body parts. Some, like rabbits, use their legs. Some, like fish, use fins. And some, like birds, use wings. They can also use other organs, like opossums that use their tail to climb.  

Where do animals move? Animals move through four different types of environments:

• water (fish, sea turtles, etc.)
• ground or other surfaces (cow, cat, dog, monkey, etc.)
• underground (moles, worms, etc.)
• air (birds, butterflies, etc.)

Animals are capable of moving through more than one type of medium. 

Here's a fun fact!

The fastest land animal is the cheetah, which has a recorded speed between 109.4 km/h (68.0 mph) and 120.7 km/h (75.0 mph).

Animals respond to stimuli

Anything in the environment that causes a change is called a stimulus. Animals can sense and respond to stimuli around them. Stimuli can be external or internal. Animals react to many stimuli, including light, temperature, water, odor, pressure, etc. An example is when a dog salivates in response to the smell of its food.

Animals maintain a stable internal balance in their body

Homeostasis is the way animals maintain a stable internal balance in their body. Cells require appropriate conditions that are not constant (temperature, ph, etc.) to function properly. But, despite the changes in the environment, animals can maintain internal conditions within a narrow range.

Now that we learned about animals and their characteristics, let's discuss how we can group them.

Types of animals

Animals are organisms, further broken into sub-groups. Animals are broken down into:

• vertebrates
• invertebrates

Vertebrates

Vertebrates are animals with a backbone. They are further broken down into five classes: amphibians, birds, fish, mammals, and reptiles.

 

1. Frog skeleton anatomy, where the presence of the backbone can be easily detected.

• Amphibians are small vertebrates that need water, or a moist environment, to survive. The species in this group include frogs, toads, and salamanders. All can breathe and absorb water through their skin.
• Birds are vertebrate animals that have feathers and wings and lay eggs. Most birds can fly. Eagles, pelicans, condors, parrots, and penguins are birds. 
• Fish are an aquatic group of vertebrates that live in water and respire with gills. Sharks, clownfish, eel, and carp are fish.
• Mammals are a group of vertebrate animals. They are characterized by having fur or hair and precise temperature regulation. They are the only animals that produce milk for their young through their mammary glands. Humans belong to this group. Other mammals are lions, monkeys, foxes, pigs, and mice.
• Reptiles are vertebrate animals that breathe air and usually have their skin covered with scales or bony plates. Snakes, lizards, and turtles are reptiles. 

Invertebrates

Invertebrates are animals that neither possess nor develop a backbone. Familiar examples of invertebrates include arthropods, mollusks, annelids, and cnidarians.

2. Jellyfish - an example of an invertebrate. This animal doesn't possess a backbone.

• Arthropods are invertebrates with jointed limbs, a segmented body, and an exoskeleton made of chitin. These include ants, spiders, bees, crabs, lobsters, scorpions, and centipedes.
• Mollusks are invertebrate animals with soft, unsegmented bodies and often a shell secreted by a fold of skin. These include snails, squids, and octopuses.
• Annelids are characterized by the possession of a body cavity and movable bristles. Their body is divided into segments by transverse rings or annulations. These include earthworms and leeches.
• Cnidarians are a group of vertebrate animals containing over 9,000 species found only in aquatic and marine environments. Cnidarians include hydras, jellyfish, sea anemones, and corals.