Zoque villagers staging the introduction of Barbasco (here positioned on a leaf) as done during the religious ceremony. The baskets traditionally are used to scoop anaesthetised ﬁsh from the water (Image: Mona Lisa Productions)

Each year, members of the Zoque people of southern Mexico gather in the Cueva del Azufre – a dark, sulphurous cave, home to the cave molly fish. They bring with them the mashed root of the Barbasco plant, a powerful anaesthetic they use to stun the fish as part of a ritual to ask their gods for rain.

Now it turns out that this centuries-old religious ritual has given evolution a helping hand. Michi Tobler of Oklahoma State University in Stillwater and his colleagues had been studying the cave molly for many years, when they became interested in how the toxin might affect the fish.

They collected fish from water where the Barbasco toxin was added every year, and others from water upstream, then mimicked the ritual by adding Barbasco paste to both tanks. They were surprised to a find differences between the two populations. Fish from the sulphur cave resisted the anaesthetic for longer, suggesting that over time, evolution has selected for fish that can cope with the toxin.


“The study indicates that the fish have adapted to the local Zoque traditions,” says Tobler, who describes the effect as “an intimate bond between nature and local culture.” It is yet more evidence showing how human activities can affect the evolutionary trajectory of species, he says.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2010.0663

When this article was first posted, we misspelled Michi Tobler’s name and also got his university affiliation wrong.