What Are Animals?
Taught in US schools
Key Takeaways
- Animals are classified into two broad groups: vertebrates (with a backbone) and invertebrates (without a backbone).
- The five groups of vertebrates are fish, amphibians, reptiles, birds, and mammals - each with distinct characteristics.
- Warm-blooded animals (mammals and birds) regulate their own body temperature; cold-blooded animals (fish, amphibians, reptiles) rely on the environment.
What Are Animals?
Animals are living organisms that share several key characteristics:
- They are made of many cells
- They cannot make their own food - they are consumers that eat plants or other animals
- Most can move on their own (at least at some point in their lives)
- They reproduce to create offspring
Scientists have identified over 8 million species of animals on Earth, from microscopic sea creatures to blue whales.
Vertebrates vs. Invertebrates
The biggest division in the animal kingdom is between vertebrates and invertebrates.
Vertebrates: Have a backbone (spine) - ~3% of all species
Invertebrates: No backbone - ~97% of all species
Invertebrates
Invertebrates include insects, spiders, worms, snails, crabs, jellyfish, and starfish. Many have an exoskeleton (hard outer shell) instead of an internal skeleton.
The Five Vertebrate Groups
1. Fish
- Breathe through gills (extract oxygen from water)
- Have scales and fins
- Almost all are cold-blooded
- Most lay eggs in water
Examples: salmon, goldfish, shark, clownfish
2. Amphibians
- Live in water and on land (amphi = both, bios = life)
- Have moist, smooth skin
- Begin life in water (larvae breathe with gills), then develop lungs for land
- Cold-blooded
Examples: frogs, toads, salamanders, newts
3. Reptiles
- Have dry, scaly skin
- Lay leathery eggs on land
- Cold-blooded; need sun to warm up
- Breathe with lungs
Examples: snakes, lizards, turtles, crocodiles
4. Birds
- Have feathers and wings (not all can fly)
- Lay hard-shelled eggs
- Warm-blooded
- Have beaks (no teeth); breathe with lungs
Examples: eagles, penguins, owls, hummingbirds
5. Mammals
- Have fur or hair
- Warm-blooded - regulate own body temperature
- Nurse young with milk (mammary glands)
- Almost all give live birth (exceptions: platypus, echidna)
Examples: dogs, whales, bats, elephants, humans
Warm-Blooded vs. Cold-Blooded
Groups: Mammals, Birds - Fish, Amphibians, Reptiles
Temperature control: Internal (stays constant) - External (matches environment)
Needs to bask in sun?: No - Yes
Active in winter?: Usually yes - Often hibernate or become dormant
Animals in the Food Chain
Animals are consumers. They eat producers (plants) or other consumers. Animals are classified as:
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Herbivores - eat only plants (deer, rabbits)
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Carnivores - eat only animals (lions, sharks)
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Omnivores - eat both plants and animals (bears, humans, crows)
Practice Activities
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Vertebrate sorting: Give students animal picture cards and have them sort into the five vertebrate groups.
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Vertebrate vs. invertebrate: Students sort 20 animals into the two categories using a T-chart.
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Animal classification book: Each student creates a mini-book with one page per vertebrate group, with drawings and facts.
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Warm vs. cold-blooded discussion: Show pictures of animals in cold environments; predict which ones can stay active.
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Animal adaptations match: Match each animal to one structural adaptation that helps it survive.
Frequently Asked Questions
What is the difference between a vertebrate and an invertebrate?
Vertebrates have an internal backbone (spine) and an internal skeleton. They include fish, amphibians, reptiles, birds, and mammals. Invertebrates have no backbone and make up about 97% of all animal species - including insects, spiders, worms, jellyfish, and crabs.
What are the five groups of vertebrates?
The five vertebrate groups are fish (breathe with gills, live in water), amphibians (live in water and on land, moist skin), reptiles (dry scaly skin, lay leathery eggs), birds (feathers, wings, lay hard eggs), and mammals (warm-blooded, fur or hair, nurse young with milk).
What does warm-blooded mean?
Warm-blooded animals (mammals and birds) can regulate their own body temperature internally, staying warm even in cold environments. Cold-blooded animals (fish, amphibians, reptiles) cannot regulate their temperature and depend on their surroundings - they need sun to warm up.
Free Animals Worksheets
Curriculum-aligned printable worksheets for Kindergarten – 3rd Grade. Download free.