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

Follow us on Twitter at

Substack subscription form sign up
The material in this press release comes from the originating research organization. Content may be edited for style and length. Want more? Sign up for our daily email.

37 thoughts on “Moved by religion: Mexican cavefish develop resistance to toxin”

  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

  8. God knew that those fish would be exposed to those savage natives and created the fish before any of you were born. His wisdom is unending and all noeing. The earth was only created about 6000-10000 years ago according to real scientists who aren’t experimenting in school. I went to a great museum in the States on holiday explaining everything, I think you should all be required to go. Oh wait, but you can’t have Truth in school. This sort of article is democat propagenda. I can’t believe the Baptists at College Station allow this lies. You all need to accept the Lord Jesus into your heart, and you will be saved. You don’t understand what hell is like. WHy risk it?

    • holy wow.

      Most people I see shoving religion down peoples throat (like you) are control freaks who use religion to look down on with reasons that cannot be proven beyond just having faith.

      Works really well for lazy parents who want to instill the fear of God in their children to control them to be ‘good’, or else they will go to Hell.

      I feel sorry for ya… seems like this happened to you.

      BTW- you should be paying homage to our Sun. the sun brings light, energy for food and much more than it gets credit for.

      Out of 113 major religions, which one is right? none.

      But some people need the emotional solace of religion and they also like putting other people down who do not believe the same way as they do. This makes them feel better about themselves to look so far down on others.

      sad, really.

      So Jesus has conditional love? he only loves you if you are doing everything right? otherwise, you make one wrong move and you are gunna burn in Hell forever? This means only the perfect people get to escape Hell.

      Religion is a fear-based population control tool.

      Look up to the sky and describe what you see.

    • You’re joking, but on the Internet, any religious rant, no matter how ridiculous, will still be taken seriously by someone.

    • Mike. I can respec that you are trying to follow your faith, but you obviously don’t read your own Bible. It clearly states “with God All things are possible.” Who are you to tell God what is and is not correct in his own universe? If you are a true believer, you should understand that there CANNOT be a conflict between God and the science of the world that God created. Either we don’t understand what God is saying, OR we don’t fully understand the science.
      Evolution, as defined “a change in a species over time” has been scientifically shown to be true time and time again (the idea that man evolved from apes is still just a theory, last time I checked). Get a grip.

      • I have a grip thank you. Please visit this museum and you will find the answers to your questions about everything. How God made light, stars, time, your soul. I will pray for you in the maen time. And I’ll get my facebook prayer group power. How is that for using science for Good? To those who were being nice, thannk you for respecting our Beleifs. I’m just trying to help you too. When you discover these neat things in His plan, it is according to that plan.

        • I believe that anything man has a hand in is going to be tainted with falsehoods. I don’t need to prove modern science is wrong, because I know that God is right. It’s human nature to try and make sense of the world; I just think that it is arrogant and wrong to dismiss the evidence that we are given just to come up with other explanations so that it can fit in with our world view. I don’t object to you praying for me, as long as you only pray that I meet God’s will for my life and my understanding, as opposed to what you believe God’s will to be. And I will do the same.

  9. Holding a ceremony once each year and having done so for hundreds of years yet now it suddenly is no longer “sustainable”? What changed then, they’re releasing more of the stuff than they did previously? Do explain.

    Also yay for silly governmental greed displacing the natives’ practices. Hrmpf. What is a government there for anyway, hm?

  10. Kinda cool that they’ve found a unique situation where they can see the results of an “experiment” that’s been running for several hundred years.

  11. …huh???

    But…but…but I thought that evolution was all just a myth and a lie from Satan!

    /end sarcasm

    /begin facepalm

        • adaptation is evolution if it results in a behavioral or biological change that becomes common among survivors/adaptors. it sounds like it did.

      • You’re right that this is a clear case of adaptation. However you don’t have a good understanding of the word ‘evolution’. This is exactly what evolution means.

        Walking from the kitchen to the dining room is walking. Walking from New York to Los Angeles is walking. In evolution each generation is like a single step of walking. The only material difference between a walk to the living room and a walk to LA is how many step you take (and obviously more time must pass with each step you take).

        Nothing new or different happens when you walk from NY to LA.

        Nothing new or different happens over millions of years of evolution.

        Evolution MEANS adaption, and that’s all it means. Fifteen thousand years ago some wolves lived in the forest and some started living near people. The wolves living in the forest kept on adapting into better and better wolves. The wolves that lived with people simply adapted DIFFERENTLY. They adapted into dogs. A poodle and a modern wolf have a common ancestor, they merely adapted differently.

        Lions, tigers, panthers, leopards, lynx, pumas, cheetahs, cougars, jaguars and house cats all have a common feline ancestor about 25 million years ago. All those feline species started out as the same thing. They merely spread out to different places and adapted differently. More time merely mores more and more adaptation piled up.

        Cats and dogs and bears all have a common carnivore ancestor about 42 million years ago. They all started out as a single carnivore species. They merely moved to different places and lived in different ways and adapted differently. Lots and lots of adaption.

        The more time passes the more adaption there is. When a single population separates for any reason they will adapt differently. More time simply means more adaption, and when a population splits up for any reason then the differencing adaptions obviously makes them more and more different.

    • it doesn’t, it could support the argument that evolution is the method god chose to work his will. not my personal belief, but a plausible one, nonetheless.

    • Does chemistry prove there is no god? No, of course not.
      Does evolution prove there is no god? No, of course not.
      Does physics prove there is no god? No, of course not.

      Anyone who makes any of those claims is an idiot. It’s the evolution denialists who constantly spout that nonsense.

    • It’s impossible to prove or disprove the existence of a god who cannot be observed, and who supposedly can manipulate the universe at will.

      Which unfortunately means this argument will never go away…

Comments are closed.