A dog is an animal! Animals are heterotrophs (they must eat other organisms to get energy). They are also motile (can move) at some stage of their life. Animals range in size from microscopic (zooplankton) to enormous (blue whale). The animal kingdom contains over 1.5 million described species, but scientists estimate there may be 8-10 million total. Animals are divided into vertebrates (about 65,000 species) and invertebrates (over 1.4 million species). Invertebrates include insects, spiders, worms, snails, jellyfish, and crustaceans. Vertebrates include mammals, birds, reptiles, amphibians, and fish. All animals are eukaryotes (cells with a nucleus). The study of animals is called zoology. Animals live in nearly every habitat on Earth, from deep oceans to high mountains to deserts. Animals have evolved a vast range of adaptations for survival. Humans are animals too!

A frog is a vertebrate! Vertebrates have an internal skeleton (endoskeleton) made of bone or cartilage. Fish are the oldest vertebrates (evolved over 500 million years ago). Amphibians (frogs, salamanders) can live on land and in water. Reptiles (snakes, lizards, turtles) have scaly skin and lay eggs on land. Birds have feathers, beaks, and lay eggs. Mammals have fur/hair, produce milk for their young, and are warm-blooded (maintain constant body temperature). Vertebrates have complex organ systems: circulatory (heart and blood vessels), respiratory (lungs or gills), digestive, nervous (brain and spinal cord), and sensory organs (eyes, ears). The human backbone has 33 vertebrae. Vertebrates can be found in all habitats. Some vertebrates (like fish) live only in water; others (like birds) can fly. Vertebrates are more complex than most invertebrates, but invertebrates are far more numerous and diverse.

A whale is a mammal! Whales and dolphins (cetaceans) evolved from land mammals that returned to the ocean about 50 million years ago. Bats are the only mammals that truly fly (others glide). Mammals are endothermic (warm-blooded), meaning they can maintain a constant body temperature regardless of the environment. This allows them to live in cold climates (polar bears, arctic foxes). Mammals have a four-chambered heart and a highly developed brain. The largest mammal is the blue whale (up to 100 feet, 200 tons). The smallest mammal is the bumblebee bat (about 1 gram). Humans are mammals. Mammals first appeared about 200 million years ago (during the age of dinosaurs). After the dinosaurs went extinct (66 million years ago), mammals diversified into many forms. Mammals have specialized teeth (incisors, canines, premolars, molars) adapted to their diet (herbivores, carnivores, omnivores). Monotremes (platypus, echidna) are egg-laying mammals. Marsupials (kangaroos, koalas) give birth to tiny young that develop in a pouch. Placental mammals (including humans) have a placenta that nourishes the developing young inside the mother.

A penguin is a bird! All birds have feathers – no other animal has feathers. Feathers provide insulation, waterproofing, and enable flight. Birds have a beak (bill) instead of teeth, which reduces weight. Birds have a four-chambered heart and are endothermic (warm-blooded). The ostrich is the largest bird (up to 9 feet tall, 350 pounds). The bee hummingbird is the smallest bird (about 2 inches, 1.6 grams). Birds are descended from theropod dinosaurs (the same group that includes T. rex). The closest living relatives of birds are crocodilians. Birds have excellent vision (eagles can spot prey from a mile away). Many birds migrate long distances (Arctic terns fly from the Arctic to Antarctica and back each year). Birds build nests to protect their eggs and young. Bird songs are used to attract mates and defend territory. Some birds (crows, parrots) are highly intelligent and can use tools. Birdwatching (ornithology) is a popular hobby. Many bird species are threatened by habitat loss and climate change. The domestic chicken is the most common bird species (over 23 billion chickens at any time).

A turtle is a reptile! Reptiles first appeared about 310 million years ago. They were the dominant land animals during the Mesozoic Era (age of reptiles). Dinosaurs were reptiles. Modern reptiles include crocodilians (alligators, crocodiles), lizards, snakes, turtles, and tuatara (a lizard-like reptile from New Zealand). Reptiles have scales made of keratin (same protein as human fingernails). Reptiles breathe air with lungs (even sea turtles come to the surface). Most reptiles lay eggs (oviparous); some snakes and lizards give live birth (ovoviviparous). Reptiles are cold-blooded (ectothermic) – they cannot generate their own body heat, so they must bask in the sun or seek shade. This is why you see lizards on sunny rocks. The largest reptile is the saltwater crocodile (up to 23 feet, 2,200 pounds). The smallest reptile is the dwarf gecko (about 1 cm). Reptiles are found on every continent except Antarctica. Some reptiles can go a long time without eating (snakes can go weeks or months). Alligators and crocodiles have been around for over 80 million years (they survived the asteroid that killed the dinosaurs).

A frog is an amphibian! Amphibians evolved about 370 million years ago and were the first vertebrates to live on land. They have permeable skin that must stay moist; they can absorb water and even breathe through their skin. This is why they live near water. Amphibians are ectothermic (cold-blooded). They lay jelly-like eggs in water (no shell). The eggs hatch into larvae (tadpoles) that have gills and tails. During metamorphosis, the tadpole grows legs, loses its tail, develops lungs, and becomes an adult. Some salamanders retain gills and stay aquatic (axolotl). Amphibians are indicators of environmental health because their permeable skin makes them sensitive to pollution. Many amphibian populations are declining worldwide due to habitat loss, pollution, climate change, and a fungal disease (chytridiomycosis). The largest amphibian is the Chinese giant salamander (up to 6 feet long). The smallest is the Paedophryne amauensis frog (about 7.7 mm). Amphibians eat insects (helping control pests). There are about 7,000 species of amphibians. The study of amphibians is called herpetology (also includes reptiles).

A salmon is a fish! Fish are the oldest vertebrates (evolved over 500 million years ago). Fish have gills that extract oxygen from water. Most fish have scales (bony plates that protect the skin). Fish are ectothermic (cold-blooded), so their body temperature matches the water temperature. They have a two-chambered heart. Fish swim by moving their tail (caudal fin) side to side. They have a swim bladder to control buoyancy (except sharks, which have oily livers). The largest fish is the whale shark (up to 60 feet, 40 tons) – but it is a fish, not a whale. The smallest fish is the Paedocypris (about 7.9 mm). Fish reproduce by laying eggs (spawning). Some fish give birth to live young (guppies, sharks). Fish are an important food source for humans and other animals. Overfishing threatens many fish populations. Some fish can breathe air (lungfish, walking catfish). There are jawless fish (lampreys, hagfish), cartilaginous fish (sharks, rays, chimaeras), and bony fish (most fish). Bony fish have a skeleton made of bone; cartilaginous fish have a skeleton made of cartilage (like your nose). The coelacanth is a "living fossil" – thought to be extinct until discovered in 1938.

A spider is an invertebrate! Invertebrates have no backbone. Instead, many have an exoskeleton (hard outer shell) for support (insects, arachnids, crustaceans). Mollusks have a muscular foot and often a shell. Jellyfish have a soft, gelatinous body. Invertebrates are found in nearly every habitat. Insects are the most diverse group of animals (over 1 million described species, possibly 5-10 million total). Some invertebrates are microscopic (rotifers, water bears). The largest invertebrate is the colossal squid (up to 45 feet long, 1,100 pounds). Invertebrates are essential for ecosystems: insects pollinate flowers, earthworms aerate soil, bees produce honey, and crustaceans recycle nutrients. Many people keep invertebrates as pets (tarantulas, hermit crabs) or raise them for food (shrimp, crabs, lobsters). Invertebrates have a wide range of body plans. Some (like octopuses) are highly intelligent. Spiders spin silk webs to catch prey. Many insects undergo metamorphosis (egg → larva → pupa → adult). Invertebrates are an important food source for vertebrates (birds eat insects, fish eat crustaceans). The study of insects is entomology; the study of spiders is arachnology; the study of mollusks is malacology. Invertebrates have existed for over 600 million years (fossils of trilobites, ancient marine arthropods).

Insects have six legs! The three body parts are: head (eyes, antennae, mouthparts), thorax (legs and wings attached), and abdomen (digestion, reproduction, stinger in some). Insects have an exoskeleton made of chitin (a tough, flexible material). They molt (shed their exoskeleton) to grow. Insects undergo metamorphosis: complete (egg → larva → pupa → adult) or incomplete (egg → nymph → adult). Bees, ants, and termites are social insects (live in colonies with division of labor). Some insects are beneficial (pollinators, predators of pests, producers of honey and silk). Others are pests (crop damage, disease vectors). Mosquitoes are the deadliest animals to humans (transmit malaria, dengue, Zika). The largest insect is the giant weta (up to 2.5 ounces) and the atlas moth (wing span up to 12 inches). The smallest insect is the fairyfly (about 0.14 mm). Insects are found on every continent, even Antarctica (there are insects that live in Antarctica, like the Antarctic midge). Insects have been around for over 400 million years. They are a crucial part of food webs. Many insects are in decline due to habitat loss, pesticides, and climate change. The study of insects is entomology. Insect collecting is a popular hobby.

Mammals are the correct answer! Mammals also have a four-chambered heart and a highly developed brain. They give birth to live young (except monotremes). There are about 5,500 species of mammals. Birds have feathers, lay eggs, and are also warm-blooded. Reptiles have scaly skin and are cold-blooded. Amphibians have smooth, moist skin and undergo metamorphosis. Fish live in water, breathe with gills, and are cold-blooded. Invertebrates include insects, arachnids, crustaceans, mollusks, worms, jellyfish, and sponges. You have learned about the major groups of animals, their characteristics, and examples. The animal kingdom is incredibly diverse. Scientists estimate there may be 8-10 million animal species on Earth, but only about 1.5 million have been described. Many species are unknown to science. Animals are amazing – from the smallest insects to the largest whales. Keep exploring and learning about the incredible diversity of life on Earth!