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You ever think about how there are little fish, and big fish, and big marine mammals, but no little marine mammals? Turns out there's a reason for that, and it's all about metabolism.

Hosted by: Hank Green (he/him)

Jesslyn Shields : Writer
Angela Reed: Fact Checker
Amy Peterson: Script Editor
Faith Evelyn Schmidt: Videographer
Savannah Geary: Script Supervisor, Editor, Associate Producer
Daniel Comiskey: Editorial Director
Sarah Suta: Producer
Nicole Sweeney: Executive Producer
Hank Green: Executive Producer

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Hank: We *can* have tiny fish, but we cannot have tiny dolphins, and it turns out, there's a pretty fundamental reason why.

Recent analysis shows that mammals balloon in size as they transition to a completely marine lifestyle. There's a small range of perfect body sizes for living in the ocean when you are a mammal, and it seems to be right around 500 kg or 1,100 lbs. And this magic number is all about heat and metabolism.

The ocean is cold, and mammals have to retain heat to survive. How much heat you can make is determined partly by how many cells you have, so a tiny dolphin could never eat enough to generate sufficient heat to replace what it loses to the water. On the other hand, if they get too much bigger than 500 kg, it's difficult to find enough calories to maintain their size without making some major feeding adaptations, which is how whales manage to tip those scales.

So that's why evolution says -