Eva Crosson of Balclutha asks :-

How and why do lizards drop their tail when under attack?

Alison Cree, a zoologist at Otago University, responded.

Lizards have a fascinating ability to drop and regrow their tails When a lizard drops its tail, it does so at a pre-determined plane of weakness. Many of the tail vertebrae (the bones that make up the spine) have fracture planes within them. Muscles arranged in blocks alongside a vertibra contract away from each other, the vertibra splits at the fracture plane, the spinal cord breaks and blood flow stops. Meanwhile the dropped piece of tail squirms on the ground, leaving a distracting morsel for the (hopefully) confused predator.

So, a lizard's tail comes pre-prepared for the possibility that it might have to be dropped. Some species have many fracture planes along the tail, so these lizards can decide how much of the tail to drop. This seems to be the case in the lizards you are probably familiar with in Otago. You may even be able to see the potential sites of fracture as small indentations on the surface of a brown gecko's tail. Some species have only a few sites at which the tail can break, or they may lose their tails very infrequently. For example, New Zealand's green geckos are unlikely to have broken tails. These are climbing species that use their tail as a "fifth limb".

The tail stores fats and other nutrients, so its loss has other costs. Lizards that have lost their tails often grow more slowly. Males that lose their tails sometimes lose their ability to hold a territory, and females of egg-laying species may fail to produce eggs that year.

These "costs" may be temporary, because the tail can usually regrow. But the regenerated portion is never identical with the original. The supporting rod is made of cartilage, not bone, and because it has no fracture planes it cannot drop segments in the same way as the original. Often the regenerated skin differs in scale pattern and colouring. If the original tail breaks incompletely, a new piece may grow alongside the original, producing a forked tail.

Tuatara can also drop and regrow their tails. This is one of the features that convince scientists that tuatara and lizards evolved from a common ancestor about 230 million years ago. Fossil relatives of tuatara also sometimes show evidence of fracture planes in their tail vertebrae, so it is a feature of great antiquity.