|Juvenile Five-lined Skink|
There must be biological advantage to having a tail to justify the energy expended in regeneration. It turns out that individuals with tails can run faster than those without tails. The lizards tail also stores fat at the base, a backup supply of energy. Some lizards fight for dominance of their territory and may lose stature in their territory if they lose their tail.
The loss of the tail has no immediate impact on our common lizard's survival. According to Dr. Craig Guyer at Auburn.edu, losing their tail is a defensive adaptation.
"There are two ways that lizards lose their tails. In all lizards around Auburn, the tail bones have central regions that break easily when the tail is pulled. The muscles of the tail pull apart easily and the blood vessels constrict to stop the wounded tail from bleeding. So, if a predator attacks a local lizard, the tail is designed to separate from the body, allowing the lizard to escape while the predator eats the tail.
In species like Broad-headed Skinks, the tail is brightly colored (blue in this case) and is frequently twitched by the lizard so that predators see and attack the tail but not the body. Lizards that lose their tails in this way can grow them back but the replacement tail is never as long as nor as colorful as the original one. Replacement tails grow back in as little as three months or as long as two years."Large lizards lack this defensive adaptation and do not regenerate their tails if they are bitten off. To date, no one know how the regeneration occurs. Imagine the ramifications to treating human injuries if we understood this. One more reason why all "all God's creatures got a place in the choir."
An extended discussion of autotomy (self-amputation) is at http://en.wikipedia.org/wiki/Autotomy