January 11, 2018

The axolotl, an unusual Mexican salamander, is one of the most studied animals in the world. It is also one of the most endangered animal species, and the axolotl may soon be extinct in the wild. Axolotls are popular as pets kept in home aquariums, and they are used in research laboratories worldwide. But in the wild, the amphibian’s numbers are depressingly small and getting smaller.

The axolotl is an unusual salamander with gills on the outside and a large tail fin. Unlike most salamanders, axolotls remain in the water throughout their lives. Credit: © Arco Images/Alamy Images

Axolotls are large compared with most salamanders. Axolotls can reach 12 inches (30 centimeters) long. The adult axolotl has a large tail fin and feathery external gills that extend out from behind the head. These characteristics are common to salamander larvae (young), which live in water. Most adult salamanders lose these features when they move to live on land. Adult axolotls, however, retain the gills and continue to live in the water. The axolotl is an example of pedomorphism <<PEE doh MAWR fihz uhm>>, the retention of juvenile characteristics by an adult. It’s as if they never grow up!

Scientists study the axolotl for its remarkable ability to regenerate body parts. The animals can grow back missing limbs, tails, organs, parts of the eye, and even portions of the brain. This ability makes the animal an important lab model in the study of tissue repair and development, as well as in the search for a cure for cancer. Axolotls are simple to study: they have very large cells and thrive in captivity. In the early 1900’s, axolotls were essential to understanding how organs develop and function in vertebrates (animals with backbones). Axolotls helped scientists unravel the causes of spina bifida in humans. In spina bifida, the spinal cord does not form properly and the vertebrae and skin cannot form around it.

In the wild, however, axolotls are having a much tougher time of it. The axolotl once thrived in lakes Chalco and Xochimilco in the area of Tenochtitlan, the ancient capital of the Aztec Empire and current site of Mexico City. The lakes do not exist anymore, and the few remaining wild axolotls live in the canals and other wetlands of the sprawling Mexican capital. These waters are badly polluted, however, by human waste, ammonia, heavy metals, and other toxic substances. Axolotls are defenseless against these lethal forms of pollution.

Invasive fish species such as carp, perch, and tilapia are also reducing wild axolotl numbers, competing with the unique amphibians for food and living space. To help preserve and expand the axolotl’s shrinking habitat, scientists are trying to revitalize traditional farming techniques that create floating islands in the canals and wetlands. These organic farming islands provide a healthy habitat for the axolotls, and they also help filter the polluted water. Whether or not this will be enough to save the axolotl from extinction in the wild remains to be seen.

April 12, 2012

Each year, people in the Australian state of Queensland take up the fight against a massive army of dangerous, alien invaders: cane toads. The event is called Toad Day Out, and in most years it results in tens of thousands of toads being caught and, later, humanely killed. Prizes are awarded for the heaviest toad caught, as well as for the heaviest total weight of toads caught by an individual. This year, volunteers are facing a toad population estimated at 200 million, and one which may in fact be spreading out of Queensland into New South Wales and the Northern Territory.

The cane toad is one of the largest toads. An adult can grow as long as 9 inches (23 centimeters). (World Book illustration by Oxford Illustrators Limited)

The cane toad, whose range originally extended from the southern United States to tropical South America, is one of the largest toads. It can reach 9 inches (23 centimeters) in length. It is also an impressive breeder. Females are capable of producing up to 20,000 eggs at a time, sometimes several times a year.

Cane toads were originally brought to Australia from Hawaii in 1935 to eat cane beetles, whose larvae were destroying sugar cane crops. The toads multiplied rapidly and soon became a threat to native Australian species. They eat some native animals and insects and compete with others for food. They also secrete toxins through their skin from large glands at the back of the head, making them poisonous to predators, pets, and human beings. Some animals, including quolls (a small marsupial), snakes, and crocodiles, have died after eating cane toads, causing populations of native species to decline. Human beings have experienced intense pain, temporary blindness, and inflammation after handling cane toads. Worst of all, it quickly became clear that cane toads do not eat adult sugar cane beetles, and the larvae live underground, where the toads cannot reach them. The experiment was a failure.

Cane beetle larvae eat the roots of sugar cane, stunting or killing the plants. (World Book illustration by James Teason)

Scientists have been trying for years to find a biological control, such as a genetically engineered virus, to eradicate the cane toad in Australia. However, so far, no solution is in sight. For the time being, the government is focusing on limiting the spread of the toad, and Toad Day Out appears to be the most effective method of dealing with this robust amphibian pest.

Additional World Book article:

• Biology 2009 (Back in Time article)