Xochimilco is an extensive semi-rural demarcation in the south of Mexico City with a vast network of canals surrounding agricultural plots called chinampas. Since the year 900 AD, this labyrinth of land and water has produced food for the xochimilcas, a Nahuatl-speaking people who were one of the first to populate the region and design their wetlands.
Currently, early in the mornings, you can see farmers – many of them descendants of the original inhabitants of Xochimilco – loading canoes with lettuce and flowers grown in the rich sediments dredged from the canals. On weekends, hundreds of brightly colored trajineras, as the festive boats of the area are known, crowd the waters, filled with city dwellers seeking an escape.
The Mexican axolotl – a dark-colored amphibian with the extraordinary habit of neoteny, or retaining a juvenile body throughout its life – once thrived in these canals. Although axolotls have been widely bred as laboratory animals and in the aquarium trade, where they are often pink or yellow due to genetic mutations, it is now questionable whether a significant wild population remains. In the last count, a decade ago, there were 35 axolotls per square kilometer in the wetlands of Xochimilco, compared to the thousands that were present in the 1990s. Pollution, urbanization, and introduced fish species had made life almost impossible for them.
In the early 2000s, Luis Zambrano, an ecologist at the National Autonomous University of Mexico (UNAM), was studying the effects of invasive carp when the government commissioned him to conduct a census of axolotls. After decades of constant environmental degradation in Xochimilco, Mexico wanted to know how many axolotls were left in the last stronghold of the species. Axolotls have been of great cultural importance, as they were part of the traditional diet and cosmology of the region. And laboratory biologists around the world, who had used axolotls for over a century to study tissue regeneration, were concerned that their animals were becoming inbred, without a wild population from which to extract new lineages.
As an ecologist, Zambrano never considered any strategy to save the axolotl that did not first involve restoring its habitat. But “we are not in the middle of Borneo or the vast Serengeti,” he said. The habitat was Mexico City, with 22 million inhabitants and growing. The number of factors against them was overwhelming.
The springs that historically fed the wetlands of Xochimilco were long ago diverted for urban use, replaced by treated wastewater. The introduced carp and tilapia ate the axolotl eggs. New roads pushed urbanization further south, threatening the last remnants of the unique pre-Columbian agricultural culture whose canals had housed the axolotls for over a millennium. The trajineras not only caused noise and more pollution, but also tempted farmers to turn their chinampas into restaurants, bars, and soccer fields and allowed the small canals to dry up.
In Mexico City, there are representations of axolotls everywhere – their enigmatic faces adorn street murals, crafts, and even a recently issued 50-peso bill – but the natural history of the animal is unknown. Almost everything that has been learned about axolotls comes from specimens in tanks.
The only way to save and study the wild axolotl, as determined by Zambrano and his colleagues, was to promote a revival of ancestral agricultural practices and then convert segments of the farmers’ canals into sanctuaries for axolotls, with the hope that they could eventually interbreed. For over a decade, Zambrano and his colleagues have extensively published on the philosophy and logistics of this approach. A major conservation group now supports their efforts, while some of their axolotl research colleagues consider them almost quixotic.
Now, Zambrano and his team have put their ideas to the test by releasing a small number of animals. Twelve, to be exact.
“How is the oxygen?”
Axolotls must be kept cool, and Zambrano’s laboratory at UNAM, home to a breeding colony of about 150 animals of wild lineages, is kept at 18 degrees Celsius. One mid-October morning, with his colleague Carlos Sumano at the helm, Zambrano and a group of students boarded a boat with six laboratory-bred animals in coolers. They were all lively hatchlings; under proper conditions, axolotls can live up to 20 years.
In 2017, Zambrano’s team placed radio frequency tags on 10 axolotls and released them in an artificial lake on the UNAM campus. They noticed that the amphibians, who were not thought to be very social, often gathered in the evenings for about an hour and then dispersed. They observed a male and female who never strayed more than a few meters apart. They also saw one end up in the stomach of a water snake. But the animals grew, had no trouble finding food.
The axolots to be released that day in October were placed in submerged cages made of bamboo and shrimp mesh, allowing them to move and hunt without becoming prey. The cages would go to canals equipped with biofilters made of volcanic rock and native plants to prevent contaminants and invasive fish from entering. Each canal had to be cool and oxygenated and contain abundant tiny crustaceans for the axolotls to feed on. Only six animals would be released in two canals. In a week, the group would release another six. Even thinking about their reproduction was too much for now: the animals were separated by sex. It was enough for them to survive.
Maria Huitzil, a doctoral student at the Metropolitan Autonomous University in Xochimilco, was working on a study that contributed to Zambrano and his colleagues’ conservation effort. She planned to retrieve the axolots monthly and take samples of their skin to look for “bacteria, fungi, viruses, protozoa, all the eukaryotes and prokaryotes that perform important functions of nutrition, osmoregulation, nutrient acquisition, and defense against pathogenic organisms,” she said. Since most axolotls in the world are bred in fish tanks, no one really knew what their natural microbiota consisted of. However, they seem to resist the infectious chytrid fungus, which has wreaked havoc on amphibian populations worldwide. What other secrets would the skin samples reveal?
With the boat moored, the students disembarked on their first chinampa, a farm filled with vegetables with rows of sunflowers, corn, leafy greens, and tomatoes. Javier del Valle, a fourth-generation co-owner and chinampero, watched as Zambrano and Sumano dug a protrusion in the black soil of the edges of his canal and began to sink one of their nearly two-meter-tall bamboo cages, which were difficult to handle. The students submerged their instruments to measure dissolved oxygen, turbidity, and conductivity. “How is the oxygen?” Zambrano wanted to know.
A native of Xochimilco, Del Valle grew up eating axolotls, especially in the form of tlapiques, tamales that combine fish, amphibians, and vegetables from the chinampas. Unlike many of his neighbors, who have dedicated their plots to other uses, he believes in the virtues of traditional chinampa farming, which does not use pesticides or chemical fertilizers. Alongside his family, they cultivate 80 varieties of flowers and vegetables in their chinampa, including an uncommon red spinach that he plucked for the team to try.
His axolotl refuge, at first glance little more than a trench, took five years to build. UNAM researchers provided interested farmers with a 70-page manual describing how the canals should be constructed.