If a vertebrate animal has broken bones or lost the use of a leg, its chances of surviving for very long are usually small, and we seldom note their passage. Most die of starvation or predation. Sometimes a hardy or lucky individual manages to go on making a living for a considerable time, and there are a few scattered reports –a rabbit with a healed hind leg, or a shrew with a separated fracture in one leg, and an elk with a healing leg fracture. A couple of long-legged wading birds with misaligned leg fractures managed to survive. A few non-migratory curlews on a tropical island survived several years with broken wings and even a mallard (victim of hunting) is recorded with healed breaks in a wing. A rough-legged hawk was able to snag prey such as a rabbit even though it had only one functional leg; that leg later healed.
Much of the evidence for such exceptional individuals comes from museum specimens that became specimens long after their wounds had healed. A nice example is a lynx in Spain: an old female had lost one foot, yet she had recently produced a litter of kits. Gray squirrels in Georgia showed healed fractures in thirty-seven specimens of a sample of over ninety specimens, including seventeen with healed long-bone fractures. A study of several species of small mammals in northeastern U.S. found that thirteen to twenty-five percent of adults had healed broken bones.
These critters had survived despite serious impairments. They lived by the motto of a centenarian who (as various body systems faltered, one by one, and began to fail) said: “you just have to get on with it!” A coyote in Gustavus has been doing just that: this one has only three functional legs; one hind leg dangles uselessly. But for over a year, it had been hunting for itself, even making those wonderful leaps that pounce down on some rodent in the grass.
Many animals are able to self-amputate a leg or a tail—the list includes some spiders, crabs, centipedes, true bugs, salamanders, and lizards (notably NOT birds or mammals). This capacity is well-documented as a means of distracting or escaping predators or, in the case of arthropods, to free a molting individual from being stuck in its old exoskeleton. A self-amputated leg is even used by certain male invertebrates as copulatory plug after he’s mated with a female, to prevent other males from inseminating her.
There is, however, another possible function for self-amputation: reducing the damages associated with a serious injury. All the animals listed above are known to break off an injured appendage upon occasion, but the adaptive value of doing this has been little studied. A study of the leaf-footed cactus bug showed that there is, in that species, an advantage to cutting off a seriously injured leg: individuals that self-amputated survived better than those that did not. However, it was not clear exactly by what means survival was improved.
Self-amputation has costs, of course: the animal has to function without that limb at least until it regenerates or do without it. There may be some loss of body fluid and a risk of infections, although self-amputation takes place at particular places where healing may occur quickly. And there is the additional metabolic cost of regenerating the lost limb, if that is possible. To be adaptive, the benefits have to outweigh such costs.