The 8 Main Animal Characteristics

What, exactly, is an animal? The question seems simple enough, but the answer requires understanding some of the more obscure characteristics of organisms, such as multicellularity, heterotrophy, motility, and other hard-to-pronounce words biologists use. In the following slides, we'll explore the basic characteristics all (or at least most) animals share—from snails and zebras to mongooses and sea anemones: multicellularity, eukaryotic cell structure, specialized tissues, sexual reproduction, a blastula stage of development, motility, heterotrophy, and possession of an advanced nervous system.

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Multicellularity

Multicellular organism, conceptual image — Science Photo Library - ANDRZEJ WOJCICKI / Getty Images

If you're trying to distinguish an animal from—say—a paramecium or an amoeba, it's not hard: animals, by definition, are multicellular creatures, though their number of cells varies greatly across species (for example, the roundworm C. elegans, widely used in biology experiments, consists of exactly 1,031 cells—no more and no less— while a human being is composed of literally trillions of cells). However, remember that animals aren't the only multicellular organisms; plants, fungi, and even some species of algae also share that honor.

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Eukaryotic Cell Structure

The split between prokaryotic and eukaryotic cells is possibly the most important split in the history of life. Prokaryotic organisms lack membrane-bounded nuclei and other organelles and are exclusively single-celled; for example, all bacteria are prokaryotes. Eukaryotic cells, by contrast, have well-defined nuclei and internal organelles (such as mitochondria) and can group to form multicellular organisms. While all animals are eukaryotes, not all eukaryotes are animals: this hugely diverse family also includes plants, fungi, and the tiny marine proto-animals known as protists.

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Specialized Tissues

internal organs illustration — SCIEPRO / Getty Images

One of the most remarkable things about animals is how specialized their cells are. As these organisms develop, what seems to be plain-vanilla "stem cells" diversify into four broad biological categories: nervous tissues, connective tissues, muscle tissues, and epithelial tissues (which line the organs and blood vessels). More advanced organisms display even more specific levels of differentiation; the various organs of your body, for example, are made up of liver cells, pancreatic cells, and dozens of other varieties (sponges, which are technically animals but have virtually no differentiated cells, are the exceptions that prove the rule here).

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Sexual Reproduction

Insemination, 3D Rendering — Westend61 / Getty Images

Most animals engage in sexual reproduction: two individuals have some form of sex, combine their genetic information, and produce offspring bearing the DNA of both parents (exception alert: some animals, including certain species of sharks, are capable of reproducing asexually). The advantages of sexual reproduction are huge, from an evolutionary perspective: the ability to test out various genome combinations allows animals to adapt quickly to new ecosystems, and thus out-compete asexual organisms. Once again, sexual reproduction isn't restricted to animals: various plants, fungi, and even some very forward-looking bacteria also employ this system!

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A Blastula Stage of Development

This one is a bit complicated, so pay attention. When a male's sperm encounters a female's egg, the result is a single cell called a zygote; after the zygote undergoes a few rounds of division, it's called a morula. Only true animals experience the next stage: the formation of a blastula, a hollow sphere of multiple cells surrounding an inner fluid cavity. Only when cells are enclosed in a blastula can they start differentiating into different tissue types, as described in slide #4 (if you're interested in further study or just a glutton for punishment, you can also explore the blastomere, blastocyst, embryoblast, and trophoblast stages of embryonic development).

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Motility (The Ability to Move)

A lion running — bucky_za / Getty Images

Fish swim, birds fly, wolves run, snails slide, and snakes slither—all animals can move at some stage in their life cycles, an evolutionary innovation that allows these organisms to more easily conquer new ecological niches, pursue prey, and evade predators (yes, some animals like sponges and corals are virtually immobile once they're fully grown, but their larvae can move before they become rooted to the sea floor). Motility (the ability to move) is one of the key traits distinguishing animals from plants and fungi (if you ignore relatively rare outliers like venus flytraps and fast-growing bamboo trees).

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Heterotrophy (The Ability to Ingest Food)

chipmunk eating corn — Juan De Dios Sanchez / EyeEm

All living things need organic carbon to support basic life processes, including growth, development, and reproduction. There are two ways to obtain carbon: from the environment (in the form of carbon dioxide, a freely available gas in the atmosphere) or by feeding on other carbon-rich organisms. Living organisms that obtain carbon from the environment, such as plants, are called autotrophs, and living organisms that obtain carbon by ingesting other living organisms, such as animals, are called heterotrophs. However, animals aren't the world's only heterotrophs; all fungi, many bacteria, and even some plants are at least partially heterotrophic.

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Advanced Nervous Systems

Human brain, illustration — SEBASTIAN KAULITZKI / Getty Images

Have you ever seen a magnolia bush with eyes or a talking toadstool mushroom? Of all the organisms on earth, only mammals are sufficiently advanced to possess more-or-less acute senses of sight, sound, hearing, taste, and touch (not to mention the echolocation of dolphins and bats, or the ability of some fish and sharks to sense magnetic disturbances in the water using their "lateral lines"). These senses entail at least a rudimentary nervous system (as in insects and starfish) and, in the most advanced animals, fully developed brains—perhaps the one key feature distinguishing animals from the rest of nature.

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Strauss, Bob. "The 8 Main Animal Characteristics." ThoughtCo, Sep. 27, 2024, thoughtco.com/the-main-animal-characteristics-4086505. Strauss, Bob. (2024, September 27). The 8 Main Animal Characteristics. Retrieved from https://www.thoughtco.com/the-main-animal-characteristics-4086505 Strauss, Bob. "The 8 Main Animal Characteristics." ThoughtCo. https://www.thoughtco.com/the-main-animal-characteristics-4086505 (accessed November 11, 2024).

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