The Mystery of the Mexico City Tiny Terror Frog of Xochimilco

On a July night in 2023, in the Xilotepec cemetery in Xochimilco, four UNAM biologists were seen using their cell phone lights to search through the undergrowth and graves for the source of a powerful and clear whistle that seemed to come from a human being. The task took them a couple of hours, but in the end they found the animal: a tiny green frog that, at two centimeters long, would easily fit on a thumbnail.

Although common in states like Morelos or Guerrero, these whistles had not been heard in Mexico City, or at least there was no record of them until a year ago, when in the middle of the rainy season, Diego Iván Sánchez Aguilar — a graduate of the Biology degree from the Faculty of Higher Studies (FES) Zaragoza — heard them behind his house and immediately recorded them with his cell phone.

Diego was almost certain that it was a frog of the Eleutherodactylus genus; he had worked with them before and knew that they were creatures from warm areas, so he was surprised to hear them south of the capital, and also in such a densely urbanized area. The next day, he played the recordings to his professor, Uri García Vázquez, who suspected the same as the young man.

To find out more, they invited the students André Montero Vachier and Víctor Castillo Sánchez to join the search party. They equipped themselves with lamps, recorders, and cold-weather clothing, and went to the Xochimilco cemetery to confirm or rule out whether it was the animal they suspected.

“In the cemetery, every time we heard a song we approached slowly. It was not easy to detect where the whistle came from, due to the acoustics of the place it was easy to get disoriented. It took them a couple of days to collect a specimen. When they caught it, they transported it to their laboratory to proceed with the relevant studies,” García Vázquez recalled.

Physically, the snooping frog has an almost triangular head, its body has small tubercles, its limbs are thin and long, with long fingers but lacks an interdigital membrane. Only the males have a vocal sac.

Once in the Molecular Systematics Laboratory of the FES Zaragoza, the specialists performed a DNA analysis and a morphological review of the small amphibian, and any doubt was cleared: it was an Eleutherodactylus nitidus, also known as the snooping frog.

“It is an endemic species of Mexico distributed mainly in the states of Oaxaca, Guerrero, Veracruz, Puebla, Tlaxcala, State of Mexico, and Morelos, especially in tropical places. The most characteristic feature of this frog is how the males attract the females, as they use a very clear whistle that people sometimes confuse with insects. It is able to camouflage itself with the environment and is usually found among vegetation or on rocky places,” said García Vázquez.

Another characteristic of the snooping frog –he indicated– is that it reproduces directly, that is, it does not have a tadpole phase and, therefore, it does not need places with permanent water. From its eggs comes a well-formed frog, and for a much wider distribution.

The whistle –said Sánchez Aguilar– is an invitation from the males to the females so that they deposit their eggs in humid areas so that, later, they can come to fertilize them. This is called external fertilization and usually occurs during the rainy season, when their song is heard more. When the rains end, these animals hide, hibernate, and are not seen again until the next rainy season.

“We know that in Mexico City there is a very similar species of frog, also from the Eleutherodactylus genus, which whistles and has the same type of reproduction, but this one is endemic to the San Ángel Pedregal Ecological Reserve and, in addition, is protected,” said Uri García.

This frog is known as the greater snouted frog; it is smaller than a Mexican peso coin and enjoys low temperatures, starting to sing when the environment is cold. It generally hides among stones, cracks, and holes, making it almost impossible to find.

“The snouted frog in particular caught our attention because there were no reports of it in the capital; until now its presence was associated with warmer climates,” said the specialist.

One hypothesis for why it is seen (and heard) in Mexico City is that perhaps the snouted frog was already present, but biologists had no knowledge of it. Another hypothesis is that it arrived in pots to Xochimilco (a major producer of ornamental plants) and began to thrive. Both possibilities are being evaluated by experts, as it is necessary to know if its arrival affects fauna such as the Pedregal frog, or if it only established itself in the city and should be classified as an “introduced species.”

In this regard, Sánchez Aguilar said that a review of the bibliography in a public database found no record of the snouted frog, so now it is time to investigate how and why it arrived in such a highly urbanized region as the country's capital. To learn more about this animal, it is necessary to monitor what happens in the following years.

“These frogs have already been registered in various places in the capital, such as the Xochimilco cemetery, the Madreselva plant market, the Nativitas forest or near the Military College. We face complications with permits and security concerns, as collecting specimens requires searching for them at night, and there are people who do not understand our work. “We must always be careful,” said Diego Sánchez.

The results of this research were published on June 11, 2024 in the European journal CheckList.