role in courtship displays. The extra-stiff tail feathers of other species, including woodpeckers and woodcreepers, allow them to brace themselves firmly against tree trunks. The tails of grazing animals, such as horses, are used both to sweep away insects and positioned or moved in ways that indicate the animal's physical or emotional state. Human tails In humans, tail bud refers to the part of the embryo which develops into the end of the spine. However, this is not a tail. Infrequently, a child is born with a "soft tail", which contains no vertebrae, but only blood vessels, muscles, and nerves, but this is regarded as an abnormality rather than a vestigial true tail, even when such an appendage is located where the tail would be expected. Fewer than 40 cases have been reported of infants with "true tails" containing the caudal vertebrae, a result of atavism. Humans have a "tail bone" (the coccyx) attached to the pelvis; it comprises fused vertebrae, usually four, at the bottom of the vertebral column. It does not normally protrude externally - humans are an acaudal (or acaudate) species (i.e., tailless). Gallery See also Empennage, the tail of an aircraft Rump (animal) References External links Animal anatomy