Snakes are vertebrates, possessing a backbone or spinal column. This internal skeletal structure is composed of many small, interconnected bones called vertebrae. The backbone provides the necessary support and flexibility for the snake’s elongated body, enabling its unique movements. All snakes belong to the class Reptilia, which falls under the subphylum Vertebrata. The spinal column is the foundational element that allows a snake to function without limbs.
Confirmation and Scale of the Snake Spine
The most striking feature of the snake spine is its immense length and the sheer number of segments it contains. While humans typically have 33 vertebrae, snakes generally possess between 200 and 400 vertebrae, with some species reaching up to 600. This extraordinary count gives the snake the distinction of having more vertebrae than any other living animal.
The vertebral column is broadly divided into two main regions: the pre-caudal (body) vertebrae and the caudal (tail) vertebrae. The pre-caudal section is the longest, containing 100 to 450 segments, each paired with a rib. The caudal section, which starts after the cloaca, is shorter and typically contains 10 to 205 vertebrae that do not have ribs attached. This massive number of segments, each contributing a small degree of motion, allows the snake’s body to bend and coil with freedom.
Specialized Vertebral Structure for Flexibility
The remarkable flexibility of the snake spine is due to the specialized way the bones connect, not just the sheer number of segments. Each vertebra articulates with its neighbor at five distinct points, forming a complex and stable joint. The main connection is a ball-and-socket joint, where a convex projection from one vertebra fits into a concave depression on the next.
Beyond the central ball-and-socket, four additional connections stabilize the joint. These include the prezygapophyses and postzygapophyses, which are upper and lower articulating surfaces. A pair of structures called the zygosphene and zygantrum, almost exclusively found in snakes, adds even more security. The zygosphene fits snugly into the pocket-like zygantrum of the adjacent vertebra, allowing for lateral and vertical bending while limiting twisting or rotation.
Nearly every pre-caudal vertebra is attached to a pair of free-floating ribs, which are not joined to a breastbone as in humans. These ribs offer a protective “cage” for internal organs and support locomotion. The combination of interlocking joints and attached ribs creates a highly controlled skeletal system that withstands extreme contortion without dislocation.
How the Vertebral Column Enables Locomotion
The hyper-flexible vertebral column is the mechanical engine for all snake movement, enabling several distinct forms of locomotion. The most recognized method is lateral undulation, or serpentine movement, where the body forms a series of S-shaped curves. In this motion, the snake pushes against fixed points in the environment, such as rocks or grass clumps, using the bends for propulsion.
Another technique is concertina movement, often used in narrow spaces or when climbing. The snake anchors the back half of its body by bunching the vertebrae into tight coils. It then extends the front half forward, anchors the head, and pulls the rear part of the body to catch up, resembling an accordion.
Larger, heavier snakes often utilize rectilinear movement, or “caterpillar” crawling, to move in a straight line. This involves using the broad belly scales as anchor points against the ground. Muscles beneath the scales contract in waves, pulling sections of the body forward over the ribs and vertebral column. Sidewinding is a specialized motion used primarily on slick or loose surfaces like sand. Only two or three sections of the body contact the ground at any time, allowing the snake to move at an angle to its direction of travel.