The vertebrae that lie between the lumbar and the caudal vertebrae in the vertebral column. The function of the sacral vertebrae is to articulate securely with the pelvic girdle, and they are usually fused to form a single bone (the sacrum) to provide a firm support. The number of sacral vertebrae varies from animal to animal. Amphibians have a single sacral vertebra, reptiles have two, and mammals have three or more.