IELTS Academic Reading Practice Test 15: Passage 1

Few animals combine fame and rarity as strikingly as the axolotl. Its wide, smiling mouth and feathery gills have made it a familiar sight in home aquariums and even in video games, and many thousands are kept in laboratories around the world. In the wild, however, it survives almost entirely in one place, a network of canals and small lakes on the southern edge of Mexico City, and there it is on the verge of disappearing.

The axolotl is a kind of salamander, an amphibian related to frogs and newts. Most amphibians undergo a dramatic transformation known as metamorphosis: a frog, for example, begins life as a tadpole that breathes through gills, and later develops lungs and legs and leaves the water. The axolotl usually skips this stage. Even when fully grown, at a length of around 23 centimetres, it keeps the features of a juvenile, including a fin that runs along its back and three feathery external gills on each side of its head. It is able to breed in this state, and it normally spends its whole life under water. This retention of youthful features into adulthood is known as neoteny.

This does not mean that the axolotl has lost the ability to change. Metamorphosis in amphibians is triggered by hormones produced by the thyroid gland, and when axolotls are given these hormones, or iodine, which the body needs in order to make them, they lose their gills, develop eyelids and become land-dwelling salamanders. This was discovered almost by accident. In 1864 a group of axolotls was shipped from Mexico to Paris, where the animals bred successfully. In 1865 the zoologist Auguste Duméril was astonished to find that a few of the offspring had turned into creatures that looked like an entirely different species. Axolotls that are forced to transform in this way, however, tend to live shorter lives than those that do not, and in the wild metamorphosis appears to be extremely rare.

The axolotl's name comes from Nahuatl, the language of the Aztecs, and is usually linked with Xolotl, a god associated with fire and lightning. According to one legend, Xolotl turned himself into an axolotl in an attempt to escape being sacrificed. The animals were also an everyday food. The Aztecs, whose capital, Tenochtitlan, stood on an island in a great lake, caught them in large numbers, and axolotls continued to be sold in the markets of Mexico City for centuries afterwards.

What has made the axolotl so valuable to science, however, is its extraordinary power of regeneration. If an axolotl loses a leg, it can grow a complete new one, with bones, muscles and nerves in the right places, and it can do the same with its tail, parts of its spinal cord and heart, and even parts of its brain. Shortly after an injury, cells gather at the wound to form a structure called a blastema, from which the missing tissue develops. Remarkably, the process leaves no scar. Researchers hope that by understanding how the axolotl achieves this, they may one day find ways of improving the healing of wounds in humans.

Studying the animal at the genetic level, however, has not been easy. When its genome was finally sequenced in 2018, it turned out to be around ten times the size of the human genome. Much of this extra length consists of sequences that are repeated many times over, and some of its genes contain unusually long stretches of DNA, which made the task of assembling the genome considerably more difficult.

While laboratory colonies flourish, the wild population is in serious trouble. Mexico City has spread across the lakes of the Valley of Mexico, most of which were drained long ago, and Xochimilco, the axolotl's last stronghold, receives wastewater from the city and runoff from farms. Fish such as carp and tilapia, introduced in the late twentieth century to provide food for local people, compete with axolotls and eat their eggs and young. Surveys suggest that the density of axolotls in Xochimilco fell from around 6,000 per square kilometre in 1998 to about 35 in 2014, and the species is now classed as critically endangered.

Conservationists are now working with the farmers of Xochimilco, some of whom still cultivate chinampas, narrow artificial islands built up from lake mud using a technique that dates back to before the Spanish conquest. In some places, the canals beside the fields have been fitted with simple filters that keep out the introduced fish and clean the water, creating refuges where axolotls can breed. Farmers who take part agree to reduce their use of chemicals and are helped to sell their produce at a better price. The axolotl's popularity may help too: in 2021 it appeared on a new Mexican banknote, a sign of how closely the country has come to identify with this unusual animal.