Functional cervicothoracic boundary modified by anatomical shifts in the neck of giraffes

Giraffes gained extra neck movement without adding another neck bone. Their familiar seven neck vertebrae are not the whole moving structure: the first bone of the chest can contribute too. That matters when a long neck must carry a head between high branches and water at ground level.

Megu Gunji and Hideki Endo compared giraffes with their shorter-necked relative, the okapi. They moved the necks of two newborn giraffe carcasses and one newborn okapi through their bending range, recording the joints with photographs and scans. They also examined adult skeletons and dissected muscles to identify what might permit movement where ribs usually stiffen the spine.

The giraffes' first chest vertebra moved more than the chest bones behind it. Modified attachments of the first two ribs left neighbouring joints freer, while a neck-bending muscle extended farther back than in the okapi. The movement measurements came from newborn carcasses, not living adults, but the adult anatomy supported the same functional interpretation. This does not establish an eighth anatomical neck vertebra. It shows how small changes to ribs, joints and muscles can make a chest bone participate in moving a giraffe's neck.