How many times can a lizard lose its tail and grow it back?

Most people know that lizards can drop their tails. Fewer people know what actually happens after that or how far the biology goes. The tail keeps twitching on the ground, distracting the predator. The lizard escapes. And then, quietly, over the following weeks, something remarkable begins: the tail grows back. What sounds like a neat party fact turns out to be one of the more fascinating puzzles in regenerative biology, with implications that reach from the study of spinal cord injuries all the way to how artificial intelligence processes language. But first, the basic question. How many times can a lizard actually do this?The honest answer is: there is no hard limit. But the full answer is far more interesting than that.The process has a name, caudal autotomy, and it is not accidental. Specifically in lizards, caudal autotomy refers to the self-amputation of the tail. Not all lizards have this adaptation, but species that do include the Green Anole lizard, most salamander species, geckos, and iguanas. The degree of stress or predation required to drop a tail varies depending on the species of lizard. The mechanics are precise. The series of bones that make up a lizard's spine extends along the length of its tail. When the tail is shed, it breaks along a fracture plane within one of the bones. These fracture planes are essentially pre-built weak points, the body's own emergency exit. Once the tail detaches, it continues to twitch for several minutes, giving the predator something to chase while the lizard runs. Evolution designed the distraction. Then comes the regrowth. Once the tail is detached, the lizard's body initiates rapid wound healing to prevent infection and blood loss. Stem cells, the body's master cells, migrate to the site of the injury. Unlike the original tail, which contains vertebrae made of bone, the regrown tail is typically supported by a cartilage rod. Scientists have identified at least 326 genes in specific spots in the tail that are activated during regeneration, indicating that lizard DNA has a genetic recipe that it uses to regenerate its tails.The regrown tail works, but it is not the original. In place of the original tail that includes a spinal column and nerves, the replacement structure is an imperfect cartilage tube. The bones are gone permanently. When a new tail grows, the bones do not regenerate, but are replaced with cartilage. There are consequences to this. The quality of regeneration often decreases with each subsequent regrowth. The first regrowth is usually the most complete and closely resembles the original tail. Subsequent regrowths may be shorter, have a different colour or texture, and lack the bony internal structure of the original.This degradation is not just cosmetic. The tail in many lizard species is a fat storage organ, essentially the body's pantry. When a lizard autotomizes its tail, all resources, that is, fat reserves stored in the tail, are also lost. The long-term metabolic cost of regenerating the tail imparts an additional energetic burden upon the animal. Research on female geckos found that caudal energy reserves represented 60% of the total reserves of tailed females and were one-third greater than the total energy reserves of tailless females. Losing the tail, in other words, is not just losing a tail. It is losing a significant portion of the body's total stored energy in one moment. Technically, there is no limit to how many times a lizard can regrow its tail. However, according to Dr. Damian Lettoof, a research fellow at the School of Molecular and Life Sciences at Curtin University, once the original bony tail is lost, it will never be the same again. The removal of the replacement cartilage rod is less likely to trigger regeneration, but observations show that lizards can regenerate their tails multiple times if they have enough nutrients. Working from lifespan and regrowth rates, if we assume that a fully grown lizard lives around four years on average in the wild, and it takes around four months for a tail to regrow, the maximum number of times the lizard can regrow its tail will be 12 times. While a lizard could theoretically regrow its tail 12 times, it really depends on factors such as the lizard species, age, its environment, and its health. And because lizards are disadvantaged without a tail, lizards usually do not survive to regrow their tails multiple times. There is an even stranger wrinkle. Some lizards do not just regrow one tail; they regrow several simultaneously. In some cases, lizards may even regrow more than one tail, a phenomenon known as abnormal regeneration. Researchers have compiled reports of lizards growing back two, three, or even more tails, with multiple small tail branches emerging from the original site of tail loss. A scientist even discovered a case where a lizard had regrown as many as nine tails. This branching, called bifurcation, occurs when the original tail remains partially attached, and the body begins regenerating alongside it, giving the lizard the appearance of something out of mythology. The implications of this biology extend well beyond the lizard. Scientists are using this to study how humans might regrow body parts in the future. Researchers hope to apply insights from lizard tail regeneration toward regenerative medicine. Understanding how stem cells or satellite cells are mobilised could help treat spinal cord injuries or aid limb repair. In 2021, a USC-led study published in Nature Communications went further. Lizards can regrow severed tails, making them the closest relative to humans that can regenerate a lost appendage. Now, for the first time, a USC-led study describes how stem cells can help lizards regenerate better tails, ones with actual dorsoventral patterning, something no lizard had managed in 250 million years of evolution, until researchers intervened. The lizard dropping its tail to survive a hawk is doing something that the most sophisticated medical science on Earth is still trying to replicate. Every regrowth, imperfect, costly, and remarkable, is a small window into what biology is capable of, and what it still has not figured out how to do perfectly.