What is the tallest animal in the world?

A giraffe reaches about five and a half metres, and does it with seven neck vertebrae - the same number as a human, a mouse and almost every other mammal. Each one is simply enormously elongated. The exceptions to the rule of seven are so few they are worth naming: sloths and manatees.

Holding blood pressure across that height requires serious engineering. A giraffe's heart generates roughly twice the pressure of ours to push blood up to the brain, and the walls of the left ventricle are correspondingly thick. Lowering the head to drink ought to cause a catastrophic surge, and does not, because valves in the neck veins and a spongy network of vessels at the base of the skull absorb it. The skin of the legs is tight enough to act as a compression stocking, which stops fluid pooling in the feet - an idea that has been examined for spacesuit design.

The neck's main use may not be feeding. The rival explanation is sexual selection: males fight by swinging their heads like clubs, in a contest called necking, and the biggest neck wins. Males have thicker skulls and heavier necks than females, which is the kind of difference that usually points to combat rather than diet. Also, in 2016 genetic work concluded that giraffes are not one species but probably four, which had gone unnoticed in one of the most recognisable animals on Earth.