What mammal has the most cervical vertebrae (nine)?

The three-toed sloth is the mammal with the most cervical vertebrae, with as many as nine. Cervical vertebrae are the small bones forming the neck, and most mammals—including humans—have seven of them.

This unusual skeleton helps three-toed sloths rotate and angle their heads while hanging beneath branches. Their flexible necks are especially useful when the animals search for leaves or watch for danger without moving their bodies. The extra vertebrae are linked to developmental changes involving Hox genes, which help organize the body during growth.

The answer is easy to confuse with the two-toed sloth. Despite their similar names and slow, tree-dwelling lifestyles, two-toed sloths generally have six cervical vertebrae. Manatees also have six, while giraffes have the standard seven; their necks look extraordinarily long because each vertebra is greatly elongated, not because they have many more neck bones.

Three-toed sloths belong to the genus Bradypus and are native to Central and South America. Their slow metabolism, algae-tinted fur, and powerful curved claws are other adaptations to life in the forest canopy.