Animals & nature, by the numbers
The animal kingdom is full of numbers that sound invented — and tidy "facts" that turn out to be wrong. A centipede can never have exactly 100 legs. A shark isn't born with its 3,000 teeth. And of an estimated 8.7 million species on Earth, we've only managed to name about one in seven. This hub rounds up the counts worth knowing, with the real figures behind them.
The one-line version
~8.7 million species estimated, ~1.2 million named. Spider: 8 legs. Bee: 5 eyes. Dog: 42 teeth. Shark: ~3,000 teeth. Centipede: never 100 legs.
Guess the number
tap to revealSix animal numbers that sound made up. Guess first, then tap.
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Bodies built different
Legs, eyes and teeth — the counts that separate one branch of the tree of life from another.
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How long, how much
Lifespans and outputs — the numbers that change the most depending on how the animal lives.
The big counts
Numbers so large that nobody has actually finished counting them.
Letters, names & games
Two interactive toys: every animal sorted by its first letter, and a rare-animal guessing game where a letter and a type are your only clue.
The animal "facts" that are actually wrong
- Centipedes have 100 legs. They can't. Centipedes always have an odd number of leg pairs, so the total is always 4n+2 — values like 30, 38, 42, or 354, but mathematically never 100.
- A goldfish has a 3-second memory. Goldfish remember things for weeks to months; they can be trained to respond to cues and even push levers for food.
- Bats are blind. All bat species can see; many see well. Echolocation supplements vision, it doesn't replace it.
- Cats have nine lives. Folklore, not biology — though an indoor cat's 13–17 years can feel like several.
Why the big numbers keep moving
The most-cited estimate of life on Earth — 8.7 million eukaryotic species (± 1.3 million) — comes from Camilo Mora and colleagues in PLOS Biology, 2011. About 1.2 million had been catalogued at the time; the rest remain undescribed, most of them insects and other invertebrates in tropical forests and the deep sea. Even Jupiter's moon count behaves the same way: better telescopes keep finding more, so any "final" number has a short shelf life. When an animal number sounds suspiciously round and final, it's usually neither.
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Did you know?
Sources
- Mora C, Tittensor DP, Adl S, Simpson AGB, Worm B (2011) — How Many Species Are There on Earth and in the Ocean?, PLOS Biology 9(8): 8.7M ± 1.3M.
- Stork NE (2018) — How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth?, Annual Review of Entomology.
- Arthur W & Chapman AD — centipede segmentation; leg pairs are always odd (so never 100 legs).
- Australian Museum / Smithsonian Ocean — shark dentition (multiple rows, lifelong replacement).
- AAHA & RSPCA — cat and dog lifespan and sleep figures.
- NASA / IAU Minor Planet Center — Jupiter confirmed-moon count.
Keep exploring
When an animal number sounds round, tidy and final, that's your cue to check it.