Moved by religion: Mexican cavefish develop resistance to toxin

COLLEGE STATION, Nov. 4, 2010 — A centuries-old religious ceremony of an indigenous people in southern Mexico has led to small evolutionary changes in a local species of fish, according to researchers from Texas A&M University.

Since before the arrival of Christopher Columbus to the New World, the Zoque people of southern Mexico would venture each year during the Easter season deep into the sulfuric cave Cueva del Azufre to implore their deities for a bountiful rain season. As part of the annual ritual, they release into the cave’s waters a distinctive, leaf-bound paste made of lime and the ground-up root of the barbasco plant, a natural fish toxin. Believing the cave’s fish to be gifts from their gods, they scoop up their poisoned prey to feed upon until their crops are ready to harvest.

However, a team of researchers led by Dr. Michael Tobler, an evolutionary ecologist at Oklahoma State University, and Dr. Gil Rosenthal, a biology professor at Texas A&M, has discovered that some of these fish have managed not only to develop a resistance to the plant’s powerful toxin, but also to pass on their tolerant genes to their offspring, enabling them to survive in the face of otherwise certain death for their non-evolved brethren.

Their findings recently were published in the online journal “Biology Letters.”

Tobler has been studying the small, cave-dwelling fish species known as the Atlantic molly or Poecilia mexicana and its uncanny ability to survive in the toxic sulfur environment of Cueva del Azufre since 2004. He earned his Ph.D. from the University of Zurich in 2008 and spent the next two years as a postdoctoral research associate at Texas A&M, studying under Rosenthal and Dr. Kirk Winemiller, a professor in wildlife and fisheries science, as part of a two-year, $79,000 Swiss National Science Foundation Postdoctoral Fellowship.

After learning about the Zoque people’s sacred ritual and witnessing the event firsthand in 2007, Tobler and Rosenthal decided to investigate the effects of this peculiar ceremony on the mollies and their habitat. Ironically, it was the last ceremony ever held, as the Zoques ended the practice that year due to political pressure from the government, which sought to preserve the cave as a hotbed for tourism and potential revenue.

“We wanted to do a lab experiment where we exposed fish from different parts of the creek to barbasco,” Tobler says. “Some of these fish had been more exposed than others.”

In March 2010, the team collected molly specimens from two different areas of the cave annually exposed to the barbasco toxin as well as from two different areas upstream, further away from the Zoque’s ritual. With both groups of fish in a single tank, they then introduced the barbasco root to determine how both groups would react.

They found that the mollies annually exposed to the barbasco indeed were more resistant than the fish further upstream — to the extent that they were able to swim in the noxious water nearly 50 percent longer. Tobler and Rosenthal’s group concluded that human beings had, over time, not only affected molly population dynamics, but also inadvertently kick-started the evolutionary process of natural selection as well. Mollies able to tolerate the poisonous conditions survived and passed those traits to their offspring, resigning those that perished to their fate of serving as a ceremonial feast for the Zoque.

“The cool thing is that this ceremony has gone on a long time and that the fish responded to it evolutionarily,” Tobler says. “Lots of species couldn’t live with these changes. It highlights how nature is affected by human activity.”

Rosenthal contends that the idea of imposing evolutionary divergence on a species at an extremely localized spatial scale is not a new concept. In fact, he says, it’s been happening since the beginning of mankind and that the idea of the “noble savage” is passé.

“We tend to have this wonderful Pocahontas idea that before Europeans came in, everything was pristine and in harmony with nature and that all of the changes in our environment have been post-industrialization,” he explains. “No. People have been changing the environment forever.”

Moreover, Rosenthal says, once a species has become genetically adapted to human presence, it is not very easy to suddenly reverse.

Their ritual since banned, the Zoques still perform a mock ceremony each Easter season. Tobler, however, would like to see the Zoque’s original ceremony resume, but in a way that is sustainable to nature as well as other cave inhabitants. The key, he and Rosenthal believe, is to find a balance between human activity and their environment. In the case of the Zoques, it may mean a few limitations on barbasco usage for their ritual, such as releasing the toxin only 50-to-60 meters into the cave rather than 100 meters.

Pending further resolution, Tobler will continue his research with the mollies at Oklahoma State, where they are housed in a special tank built to safely imitate their sulfuric living conditions in Cueva del Azufre.

“We need to understand what the impact really is on these fish rather than eliminate the ceremony completely,” Tobler says. “We want to hopefully find a balance between the cultural practices of these people and the ecosystem.”

To read their detailed paper, visit

For more information on Tobler’s research with Atlantic mollies, visit

Contact: Chris Jarvis, (979) 845-7246 or [email protected], Dr. Michael Tobler, (405) 744-6815 or [email protected] or Dr. Gil Rosenthal, (979) 845-3614 or [email protected]

For more news about Texas A&M University, go to

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  1. What I missed in the «biology letters» paper was a detailed analysis of the genomic differences found between fish more resistant and those less resistant to the effects of the toxin(s). Hope the authors will return to this matter, which could have more general implications for the manner in which vertebrates deal with toxic substances…..


    • there’s probably little genomic difference, perhaps small differences in the regulation of enzymes that break down this toxin

  2. whether @Mike was posting Flame-bait or not, it gave me an outlet to say something on a topic that usually cannot be discussed live.

    Yep, I took the bait and got to explain and think about the sub-topic.

    Just to re-iterate… Every person would not be here without the power of the Sun. religions have taken away credit from the Sun and personified the sun. and then they tell you you better believe in THEIR VERSION or go to hell.


    “Let there be light!” – Sol, the solar sun that powers Earth

  3. So to test this theory, they took a bunch of otherwise happy, healthy fish, and exposed them to massive amounts of the toxin and watched them die. Nice.

    Why does this matter? Leave the poor fish alone.

    • erm..You didnt read the article did you? Thats not at all what happend. What happend was a religious ceremony went on for centuries. A religous ceremony. This ceremony involved mixing a substance into the water, that was toxic to fish. The fish who live there now no longer die from this ritual; those who live there now are mostly immune to the toxin

      Scientists did nothing but examine the results. This was caused by centuries of magic and mysticism combining to poison our ecology. This was the work of religion. Talking about a theoretical model as if it were what was happening is generally looked down on on the anti-scince side, isnt it? In any case, the majority of tests will be done using cultures from the system, not in the system itself. Its just how you would get optimal results. Its cherry picked quote that is an exggeration of a hoped for system to test it in. These scientists dont actually want to poison the water. If you can’t see that, go to somewhere that isnt science related.

      • Read it again. Quote below. None of the fish in the experiment conducted in that tank survived, I can guarantee you that.

        “In March 2010, the team collected molly specimens from two different areas of the cave annually exposed to the barbasco toxin as well as from two different areas upstream, further away from the Zoque’s ritual. With both groups of fish in a single tank, they then introduced the barbasco root to determine how both groups would react. They found that the mollies annually exposed to the barbasco indeed were more resistant than the fish further upstream — to the extent that they were able to swim in the noxious water nearly 50 percent longer.”

        • Are you a vegan? If not, your hypocrisy is disgusting. Humans kill animals all the time for all kind of reasons, but usually to eat them.

          More animals die each year than there are humans alive on the planet, and THIS is what draws your ire?

          At least these fish died for the advancement of the sum of human knowledge.

          A far better fate than being killed due to the stupidity and ignorance of locals.

          • I’m vegan. I do not support the killing of animals for any reason, whether to eat them, experiment on them, or just for sick fun. What human knowledge was advanced exactly? What practical purpose does this cruel “experiment” serve?

          • @joe a *vegan*? So you don’t want insulin for diabetics? Or pig organs that can be used to replace a failing human organ? You don’t want ANY scientific or medical research? And the fish died because *fish die* its not because of the conditions. All the scientist did was put them all together in a single tank to see how they reacted to the toxin that is found *naturally in their enviornment* All it did was move fish from one stationary spot in a river, and another stationary spot in a tank.

            If youd like to give up your kids, we can increase medical research by 10 fold. But most people consider that crossing a line, so we have to compromise with ubiquitous animals.

        • Ummm, if you read the original article it says ALL fish survived and were returned to the caves. Fish loose motion control way before they die.

  4. Evolution does not prove that there is no god but it certainly suggests it.

    When the theory of evolution first came out every religion I know of condemned it as complete BS. If god is responsible for evolution he never did manage to communicate that to his loyal followers making them all look like fools. If there is a god he doesn’t mind making his followers look like ignorant fools. Evolution and the resistance to it has caused a huge problem for all religions. If religion can’t get the basic facts right what is it good for? Does Galileo ring a bell? How many centuries did it take for the church to admit Galileo was right and they were wrong? What evolution does prove is that religions know nothing about how we got here and the world around us.

  5. @mike

    Everyone point at Mike and laugh for falling for Pascal’s wager.

    Mike, you’ve heard about Islam and don’t see it as true. You’re going to Islamic hell. Why risk it?

    Everyone point at Mike and laugh for thinking the universe is less than 10k years old.

    Mike, when you look up at the night sky, try to find the Andromeda galaxy. It’s visible with the naked eye. It’s 2.5 MILLION light years away. Hmmm, how can you see it?

  6. LOL – reminds me of the time my wife forgot about the garden hose when topping off our goldfish pond. Several dead fish the next day, but the ones which survived surely had a higher tolerance for tap water.

    I mused that if we went at it slow enough and long enough, you could probably end up with fish which could live in battery acid!

  7. The Australian Aborigines of Arnhem Land in Australia have something very similar in terms of catching fish. They pound up the bark of a certain tree and wade it through the water. The sap from the bark then de-oxygenates the water causing the fish to go belly up and float to the top while still alive. They then just pluck out the fish by hand