In Sierra Morelos State Park, north of Toluca , a group of researchers has been investigating a phenomenon for months that has set off alarm bells: the recurring appearance of frogs with obvious malformations. What began as an isolated observation during field sampling ended up revealing a situation that has gradually been confirmed as a problem affecting several amphibian species within this protected natural area.
The affected frogs exhibit missing or deformed legs, fused toes, and striking changes in pigmentation . Given these findings, the scientific team has moved their work to the laboratory to determine whether, in addition to the visible anomalies, there is also DNA damage in the animals and whether environmental contamination, radiation, or a mutation process spreading through local populations could be behind it all.
An entire population of frogs with physical abnormalities
The research focuses primarily on the species Dryophytes eximius, known as the mountain tree frog , which inhabits forests and wetlands in the park. During field surveys conducted between 2024 and 2026, researchers began to detect an increasing number of individuals with deformities, eventually confirming that it was not an isolated case, but rather a recurring issue within a population where specimens with anomalies appeared repeatedly.
According to data collected during various field trips, between 12 and 15 tree frogs with some type of malformation have been identified —a modest number in absolute terms, but unusual for an apparently healthy population in a protected area. Experts emphasize that, while the frequency is not yet considered catastrophic, it is anomalous enough to warrant thorough monitoring and detailed genetic analysis.
The observed physical abnormalities include partial or total absence of limbs, stiff legs, shortened or missing toes, and varying degrees of abnormal pigmentation. Some frogs exhibit mobility problems, especially when jumping, which could reduce their ability to feed, escape predators, or reproduce normally, ultimately affecting population stability.
The first specimen with deformities was recorded in 2024 by Liliana Gutiérrez, an Environmental Biology student at the Autonomous Metropolitan University, Lerma Campus. Initially, it was thought to be an isolated incident, but subsequent sampling revealed new cases, eventually confirming that the phenomenon was recurring in the area.
Researchers have detected these cases in both juvenile and adult specimens , suggesting that the causes may be acting continuously on the population and not just in a specific generation. This pattern reinforces the idea that there could be an environmental or genetic factor that persists over time and warrants further investigation.
Two affected species within the same ecosystem
Although fieldwork and laboratory work focuses primarily on Dryophytes eximius , the anomalies are not limited to this species. During surveys in the Sierra Morelos, several specimens of Lithobates montezumae , popularly known as Montezuma's frog or Montezuma's leopard frog, have also been found, exhibiting similar deformities.
In this second species, researchers have identified at least three individuals with anomalies within the park. So far, these cases have been documented primarily through visual observation, but their mere presence in another distinct species suggests that the problem may not be exclusive to a single population or a single genetic lineage.
The fact that the alterations are being recorded in more than one species that shares habitat and resources increases the concern of specialists, who hypothesize that something may be happening in the environment —whether in the water, soil or air— that is affecting the amphibian fauna of the area more generally.
Amphibians, scientists remind us, are considered first-rate environmental bioindicators . Their delicate, permeable skin, their life cycle linked to both aquatic and terrestrial environments, and their sensitivity to changes in water and soil quality make them a very fine barometer of ecosystem health. When malformations begin to appear repeatedly, it is usually a sign that something in the environment is not quite right.
In this context, experts stress that it is still too early to determine whether the deformities seen in Sierra Morelos are a local and limited phenomenon or whether, on the contrary, they could be the tip of the iceberg of a broader problem affecting other humid areas of the State of Mexico or even neighboring regions.
DNA analysis with Comet Assay
To go beyond photographs and field observations, the team has launched a genetic study based on the technique known as the Comet Assay , which has been applied to frogs since August 2026. This procedure allows the evaluation of the state of DNA in blood cells and the detection of genotoxic damage, that is, alterations in the genetic material.
The protocol involves obtaining small blood samples directly at the sampling site , without needing to transport the animals to the laboratory. These samples are then processed under controlled conditions to observe whether the DNA strands remain compact or, conversely, become fragmented, forming a kind of "tail" when subjected to an electric field, reminiscent of the silhouette of a comet.
The main objective of the study is to compare the DNA of frogs with abnormalities to that of apparently healthy specimens living in the same area. If significant differences are detected in the degree of genetic damage between the two groups, it would reinforce the idea that visible malformations are only part of a deeper problem that also affects the organism internally.
According to the researchers, the Comet Assay is a useful tool for detecting the traces of pollutants or radiation on genetic material. While it cannot identify the specific substance or factor causing the damage on its own, it does reveal whether cells have suffered damage that could be related to certain types of pollutants present in the environment.
The laboratory work is planned to last several months, with the aim of obtaining a sufficient number of samples from both deformed individuals and frogs without external signs of abnormalities. In this way, the researchers hope to draw more solid conclusions about whether or not there is a pattern of genetic damage associated with the deformities.
Possible causes: pollution, radiation, or mutations
For now, the possible causes behind the deformed frogs in Sierra Morelos remain hypotheses under investigation . Among the open lines of inquiry are the potential contamination of water bodies and soil by heavy metals, pesticides, or other chemical compounds , as well as exposure to high levels of ultraviolet radiation or the presence of mutations spreading within the populations.
The contamination hypothesis gains strength because the park is surrounded by urban areas and agricultural zones , where the use of pesticides and other substances could reach, through runoff or seepage, the pools, streams, and wetlands where amphibians live. In many studies conducted in other regions, the presence of certain pesticides has been linked to malformations in frogs and other aquatic organisms.
Another possibility considered is that the anomalies are linked to ultraviolet radiation , especially during sensitive developmental stages such as the tadpole phase. Changes in vegetation cover, climate variations, or alterations in water quality could amplify the effects of radiation on embryos and juveniles, causing DNA damage that ultimately manifests as physical malformations.
It is also possible that we are dealing with genetic mutations that are becoming fixed and replicating within the population. In this scenario, an initial event—whether or not caused by external agents—could have given rise to individuals carrying hereditary alterations, which would eventually spread throughout the population if they do not pose an immediate lethal disadvantage.
The researchers insist, however, that it is still too early to point to a single culprit . Further analyses of water, soil, and possibly tissues, as well as genetic comparisons between different individuals and species, will be necessary to arrive at a more complete explanation of what is happening in this protected ecosystem.
A warning about the health of the ecosystem
Beyond the impact on the frogs themselves, the phenomenon observed in Sierra Morelos is interpreted as a possible warning sign about the state of the environment in the area. Because they are so sensitive to changes in their surroundings, amphibians are often among the first groups of fauna to show signs of alarm when something is out of balance in the ecosystem.
If the abnormalities detected in these frogs are ultimately linked to chemical pollutants, radiation, or persistent environmental disturbances , the problem would not be limited to a few dozen deformed individuals. It could pose risks to other species of fauna and flora that share the habitat, and even to people who live or work in the vicinity of the park.
For now, experts emphasize that the frequency of frogs with malformations is not yet considered an emergency situation , but it is unusual enough to warrant close monitoring. Fieldwork will continue in the coming months, with further sampling aimed at identifying more affected specimens and determining whether the proportion of individuals with anomalies is increasing, decreasing, or remaining stable.
The investigation will continue for at least another six months , according to those in charge. During that time, they hope not only to obtain results from the Comet Test and other analyses, but also to better define the possible connections between the detected deformities and the environmental factors present in Sierra Morelos.
This case, which has sparked curiosity and concern among the scientific community and local citizens, joins other documented episodes in different regions, where the appearance of frogs with recurring malformations has served as a wake-up call about the human influence on ecosystems and the need to closely monitor the quality of the environment.
As studies in Sierra Morelos progress, the results could provide valuable clues for better understanding how pollution, radiation, and genetic mutations interact in amphibian populations living in protected areas subject to external pressures. What are currently unknowns and suspicions could become a solid foundation for conservation decisions, improving the control of potentially hazardous substances and strengthening the protection of ecosystems that, as these deformed frogs demonstrate, are not immune to the changes occurring around them.