Religious ritual has altered the evolutionary path of Mexican cavefish

For centuries, a group of Native Mexicans have poisoned and harvested cavefish as part of an annual ceremony. Now the cavefish have gotten wise, as they have evolved increased resistance to the deadly toxins in order to adapt and survive.

The Zoque people are an indigenous group in southern Mexico. For centuries, they have traveled deep into a sulfuric cave known as Cueva del Azufre every year around April. They do this in order to ask their gods for a strong rain season that will allow them to produce bountiful harvests. As part of the ritual, they pour into the cave's waters an unusual paste made of leaves, lime, and the root of the barbasco plant.

That last part is bad news for the cavefish that live in the waters, as barbasco is toxic to fish. As such, the paste often kills vast swathes of cavefish, which is another very good reason for the Zoque to travel into the cave - they can scoop up the dead fish and live off them until their crops are ready. The ceremony ended in 2007 after government pressure forced the Zoque to cease their centuries-old practice due to concerns about the cave's ecological balance and its tourism potential.

A team of evolutionary ecologists were present at that last ceremony, and they decided to study the longterm effects of the barbasco on the cavefish, known as Atlantic mollies. Earlier this year, they collected specimens from areas where the Zoque had tended to release the paste, as well as additional fish from regions that the Zoque had generally left alone. They then placed the fish in a single task and introduced barbasco.

The results were stunning. The mollies that had a history of annual barbasco exposure were able to last 50 percent longer in the toxic water than their counterparts. The ecologists conclude that the Zoque had altered the course of evolution for the mollies, as natural selection in their preferred parts of the cave had favored those with heightened resisted to barbasco.

Researcher Dr. Michael Tobler explains how unusual this finding is:

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

Meanwhile, his colleague Dr. Gil Rosenthal thinks it's a good reminder that all humans throughout history have affected what is sometimes thought of as a "natural" order:

"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. No. People have been changing the environment forever."

And, while the change wasn't as dramatic and probably wasn't evolutionary, the mollies also changed the Zoque. The annual harvesting of the cavefish provided a vital food source, and the success of the Zoque in that particular region likely depended in part upon this ritual. Without the mollies, they might not have been as well nourished, which would have reduced their overall health, or they might have had to move elsewhere to areas with more consistently available food.

As a general rule, all relationships between predator and prey are symbiotic - although, if humans and mollies are any indication, it's the prey that are expected to do the heavy duty changing.

[Biology Letters; up top is an image of a Zoque ceremony in the cave.]