The ability of many lizard species to shed and regrow their tails is a remarkable survival mechanism known as caudal autotomy. When threatened by a predator, a lizard can instantly detach part of its tail. The detached tail continues to writhe and twitch, creating a distraction that allows the lizard to escape. For most species, the tail does grow back, though the replacement is never an exact replica of the original.
Why Lizards Drop Their Tails
Tail autotomy functions as immediate self-defense against a predator attack. When the tail is grabbed, a deliberate reflex action causes it to break off at a specific, pre-designed point. This requires the lizard to contract muscles surrounding specialized weak points in the tail structure.
These weak points are called fracture planes and are built directly into the vertebrae. In most lizards, these are intravertebral fracture planes, meaning the break occurs across the middle of a single vertebra. The muscle, nerves, and blood vessels are designed to separate cleanly along these planes, minimizing blood loss. The detached segment can thrash for up to 30 minutes, successfully diverting the predator’s attention and maximizing the lizard’s chance of survival.
The Biological Process of Tail Regrowth
Initial Healing and Blastema Formation
After the tail is dropped, the exposed wound is quickly sealed by a clot, and a specialized wound epidermis migrates over the stump within days. This rapid closure prevents infection and prepares the site for cellular reconstruction. Beneath this protective cap, a mass of undifferentiated cells accumulates, forming a structure called the blastema. The blastema is a collection of progenitor cells that multiply rapidly and transform into the various cell types needed for the replacement tail.
Cartilage and Tissue Construction
A key structure guiding this regrowth is the ependymal tube, an outgrowth of the original spinal cord stump extending into the blastema. The regenerating spinal cord releases molecular signals, which activate the blastema cells to form new tissues. Instead of regenerating complex segmented bony vertebrae, the blastema forms a single, flexible, unsegmented tube made of cartilage. This cartilaginous rod provides the central support for the new tail, while new muscle, skin, and nervous tissue are constructed around it. The entire process takes several weeks to a few months, depending on the species and environmental conditions.
Comparing the Original and Regrown Tails
Skeletal and Muscular Differences
The regenerated tail is a functional replacement, but it is not a perfect copy of the original appendage. The primary difference lies in the skeletal structure: the original tail is supported by interlocking, bony vertebrae, while the new tail contains only a single, continuous rod of cartilage surrounding the spinal cord. This cartilaginous rod makes the replacement tail less flexible, as it lacks the numerous joints provided by individual vertebrae. Furthermore, the muscle arrangement is simpler in the regrown tail, consisting of elongated fibers spanning the length of the structure.
Appearance and Energy Storage
Externally, the regenerated tail may also differ in appearance, often exhibiting differences in scale pattern, color, or texture. The original tail also frequently serves as a storage organ for fat, which is sacrificed upon autotomy. The regrown tail typically has a reduced capacity for fat storage, meaning the lizard loses a valuable energy reserve when it sheds its tail.
Species Variation and Limitations of Regeneration
Species Variation
While the ability to regrow a tail is common among lizards, it is not universal across all species. Larger lizards, such as monitor lizards and chameleons, generally lack the biological mechanisms for regeneration. Even within groups that can regenerate, certain species, like the crested gecko, have lost the ability and heal with a smooth stump.
Costs of Regeneration
Tail regeneration comes with significant costs and limitations. Growing a new tail requires a substantial investment of energy, diverting resources away from functions like growth or reproduction. The regenerated tail is often shorter than the original, and its quality may diminish with repeated autotomy events. Losing the tail also temporarily impairs the lizard’s balance and locomotion, making it more vulnerable until the replacement is fully grown.