An interactive biophysics game where designing a creature means making real biological trade-offs.
Built by Dan Aridor with Claude Opus 4.7 for Adam Aridor (age 9) — who already knows biology is the best subject and that the math of life is even cooler than the animals themselves.
🎮 Play it: https://daridor9.github.io/critter-forge/
Critter Forge is a creature-design game where every choice — body size, warm or cold blooded, how long the legs are, whether to give it wings or armor or sonar — has a real biological consequence. Then you take your creature into one of six survival arenas and see whether it lives.
The numbers and rules aren't made up. They come from real biology — the same equations that govern why cheetahs are 50 kg and not 5000 kg, why hummingbirds eat their body weight in nectar every day, and why a T-rex built like a chihuahua would collapse under its own weight.
The game is inspired by Geoffrey West's book Scale (2017), which shows that biology is built on a small set of beautiful mathematical patterns. Most of those patterns are wired into Critter Forge.
Daily energy use ≈ 70 × (body mass)^¾
A 1-gram shrew burns much more food per gram than a 100-tonne whale. Bigger bodies are more efficient per kilogram. Whales eat a lot in total, but mice eat much more for their size.
In game: drag the size slider and watch the Food / day stat grow less than proportionally with mass — that's the ¾ exponent at work.
Body weight scales with length³. Bone strength scales with length². Double a creature in every direction → 8× heavier but only 4× stronger. The bones snap.
Real example: the largest land animal alive today is the African elephant (~6 t). The largest dinosaurs were ~70 t — and they had specially-evolved hollow bones to survive. Anything much bigger physically can't exist on land.
In game: drag the size slider up and watch the Bone-break risk meter go red.
Surface area grows as length²; volume grows as length³. So small bodies have lots of surface for their volume → heat leaks out fast. Big bodies have little surface relative to their bulk → heat stays in.
Real example: musk-oxen thrive in the Arctic; tiny shrews can't (they'd burn through fuel faster than they could eat). Elephants would overheat in deep snow — but die in the desert for the opposite reason: they can't shed heat fast enough.
In game: this controls Cold tolerance. Tiny creatures freeze in The Climb arena.
Almost every mammal — mouse to whale — gets about 1.5 billion heartbeats over its lifetime.
- Small creatures have fast hearts and short lives.
- Big creatures have slow hearts and long lives.
Real example: mouse heart rate ~600 bpm, lifespan ~2 years. Blue whale heart rate ~6 bpm, lifespan ~100 years. About the same total beats.
In game: see Heart rate and Lifespan — they always tell the same story.
Top sprint speed on land peaks around 50 kg of body mass. Both smaller (cat, mouse) and larger (elephant, rhino) animals are slower.
Real examples: cheetah (50 kg, 110 km/h), pronghorn (50 kg, 88 km/h), ostrich (90 kg, 70 km/h). The fastest land animals all live in a narrow size band.
In game: drag the size slider — Top speed peaks in the middle and drops off both ways.
Warm-blooded animals (mammals, birds) maintain constant body temperature. It costs about 10× more food per kg than being cold-blooded (reptiles, fish, insects).
Real example: a crocodile can fast for a year. A wolf the same size needs several kg of meat every day.
In game: toggle Blood = cold and watch food/day drop dramatically. Cold-blooded creatures dominate The Drought.
A human brain is ~2% of body mass but burns ~20% of total energy. The same is true of dolphins, ravens, octopi — every brainy animal pays a huge metabolic tax for being smart.
Real example: big-brained animals always evolve in rich-food environments. You can't have a big brain on a starvation diet.
In game: the Brain tier adds +10–25% to food per day. Pays off in The Maze.
Wing loading (mass per unit wing area) caps powered flight at small body sizes. Above about 12 kg, sustained flapping flight is essentially impossible.
Real example: swans (~12 kg) are at the upper limit for living fliers. Pterosaurs got bigger only because they had hollow bones AND glided most of the time — no flapping-elephant in any era.
In game: the Wings hybrid trait is locked out above 2 kg. Try to wing an elephant — denied.
| Arena | Wins when you have… | Real animals that thrive |
|---|---|---|
| 🦌 Chase | speed + stamina balance | wolves, pronghorns, cheetahs |
| 🌳 Hunt | stealth, armor, or escape speed | rabbits (hide), turtles (tank), pronghorns (flee) |
| 🏔 Climb | cold tolerance + manageable body mass | snow leopards, musk-oxen, yaks |
| ☀️ Drought | low metabolism + fat reserves | camels, crocodiles, tortoises |
| 🌊 Deep | gills, or huge lungs + pressure tolerance | fish, sperm whales (with blubber) |
| 🧩 Maze | brain power + sharp senses | apes, ravens, octopi, bats |
Every body-plan choice and every hybrid pays off somewhere — and is a liability somewhere else. That's the whole point of biology.
Each "hybrid" power is something a real animal evolved to solve a problem:
- 🦇 Echolocation (bats, dolphins) — pings the world with sound; works in darkness and underwater. Bonus in The Maze.
- 🦅 Wings (bats, birds) — flight. Hard 2 kg mass cap because of wing-loading limits.
- 🐍 Venom (snakes, octopi, platypus) — lets a small predator drop huge prey. A 50 g snake can kill a 100 kg deer.
- ⚡ Electric organs (electric eels) — 600 V shocks. Producing them costs ~⅓ of the eel's daily energy. Fish-only.
- 🦎 Camouflage (octopi, chameleons) — change skin colour in ~0.3 seconds using millions of pigment cells. Huge bonus in The Hunt.
- 🧊 Antifreeze blood (arctic fish) — sugar proteins that block ice-crystal formation. Survival at -2°C salt water. Cold-blooded only.
- 🦣 Thick fur (musk-oxen, mammoths) — ~8× warmer per gram than sheep wool. Heavy, drag-prone. Warm-blooded only.
- 🐟 Gills (fish) — pull oxygen from water. Lets a land creature handle The Deep — but water carries ~30× less O₂ than air, so gill flow must be very fast.
- Build a creature in the left panel — size, body plan, blood, legs, brain, defense, sensors, hybrids.
- Watch the live stats update as you change things. Hover the ℹ icons to see exactly which formula produced each number.
- Pick an arena in the right panel and hit ⛶ for fullscreen play.
- Win? An "🥚 Next generation" button spawns three mutant offspring — pick one to continue the lineage.
- Save your favorites to the Family album at the bottom of the page — they persist between visits in your browser.
Use the 🎲 Random button up top for surprise starters, ✏️ Name to auto-name a creature based on its traits, ↺ Reset to start over, 🔊 to mute sound.
Built in May 2026.
- 🧠 Game design & direction: Dan Aridor
- 🤖 Pair-programmer: Claude Opus 4.7 (1M context) by Anthropic
- 🎯 Dedicated to: Adam Aridor (age 9), who loves animals and the math of life
- 📚 Inspired by: Geoffrey West, Scale (2017) — the book showing that biology runs on a small set of beautiful equations
"Bigger isn't better. Faster isn't better. Smarter isn't better. Everything has a cost. The art of biology is choosing which costs to pay."
git clone https://github.com/daridor9/critter-forge
cd critter-forge
npm install
npm run devOpen http://localhost:5173/critter-forge/ in a browser.
To deploy a new version to the public site:
npm run build && npx gh-pages -d distBuilt with Vite, React + TypeScript, and plain SVG for the creature and arena art. All physics formulas live in src/physics.ts — change one line and every arena recomputes.