Which animal can regrow a lost limb?

An axolotl can regrow a leg, a tail, parts of its jaw, sections of its heart, and portions of its brain and spinal cord, repeatedly, without scarring. It does it by forming a blastema - a mound of cells at the wound that revert to an unspecialised state, work out from their position what is missing, and rebuild it to the correct size and orientation.

It is a salamander that never grows up. Most amphibians metamorphose and leave the water; the axolotl keeps its feathery external gills and its larval body for life and breeds in that form, a condition called neoteny. Give it the right hormone and it will reluctantly turn into a land salamander it would never have become on its own.

There is a bleak irony in its status. It is one of the most-studied animals in laboratories on Earth and is critically endangered in the wild, surviving only in the remnants of the lake system beneath Mexico City, where surveys have struggled to find any at all. Its name comes from the Aztec god Xolotl, who was said to have escaped sacrifice by turning into one.