I asked him if there is reason to believe that psilocybin is a defense chemical for the mushroom. Defense against pests and diseases is the most common function of the so-called secondary metabolites produced in plants. Curiously, many plant toxins don’t directly kill pests, but often act as psychostimulants as well as poisons, which is why we use many of them as drugs to alter consciousness. Why wouldn’t plants just kill their predators outright? Perhaps because that would quickly select for resistance, whereas messing with its neurotransmitter networks can distract the predator or, better still, lead it to engage in risky behaviors likely to shorten its life. Think of an inebriated insect behaving in a way that attracts the attention of a hungry bird.
But Beug pointed out that if psilocybin were a defense chemical, “my former student Paul Stamets would have jumped on it long ago and found a use for it as an antifungal, antibacterial, or insecticide.” In fact Beug has tested fungi for psilocybin and psilocin levels and found that they occur only in minute quantities in the mycelium—the part of the organism most likely to be well defended. “Instead the chemicals are in the fruiting bodies, sometimes at over 2 percent by dry weight!”—a stupendous quantity, and in a part of the organism it is not a priority to defend.
Even if psilocybin in mushrooms began as “an accident of a metabolic pathway,” the fact that it wasn’t discarded during the course of the species’ evolution suggests it must have offered some benefit. “My best guess,” Beug says, “is that the mushrooms that produced the most psilocybin got selectively eaten and so their spores got more widely disseminated.”
Eaten by whom, or what? And why? Beug says that many animals are known to eat psilocybin mushrooms, including horses, cattle, and dogs. Some, like cows, appear unaffected, but many animals appear to enjoy an occasional change in consciousness, too. Beug is in charge of gathering mushroom-poisoning reports for the North American Mycological Association and over the years has seen accounts of horses tripping in their paddocks and dogs that “zero in on psilocybes and appear to be hallucinating.” Several primate species (aside from our own) are also known to enjoy psychedelic mushrooms. Presumably animals with a taste for altered states of consciousness have helped spread psilocybin far and wide.