A common, conspicuous, even gaudy mushroom in our area is known as Amanita muscaria (commonly called fly agaric). Its spores are wind-dispersed, and it is widespread in the northern hemisphere, where it is mycorrhizal with the roots of many different kinds of trees. Somehow, this mushroom has arrived in New Zealand (first recorded there in 1937), Australia and South America, where it’s so versatile that it can form these nutrient-exchanging associations with trees not normally found in the northern hemisphere.
What we call Amanita muscaria may have originated somewhere in Beringia. But it may well not be a single species but rather a collection of several different but closely related species. Analysis of the DNA from specimens collected in North America, for example, have revealed eight separate lineages, each one specializing on particular habitats and locations. For instance, two of them are found on Santa Clara Island in California, and one in the oak-pine forests of southeastern U. S. Here in Alaska, there seem to be three types.
The mushroom cap is usually red, but yellow caps are also known from many lineages. As a young mushroom develops, it breaks through an enclosing membrane that leaves small whitish “warts” strewn over the top of the cap. These so-called warts can be washed off by heavy rain, and they may fall off old mushrooms, removing one of the clues that usually help to make easy the identification of this mushroom. There are many species of Amanita, which differ greatly in toxicity and palatability; making the right ID matters. Amanita muscaria is said to be edible by humans if properly prepared to reduce toxicity (boiling in several waters, etc.), but raw ones are a different story: for instance, there’s a record of a dog dying with a stomach full of fresh Amanita muscaria.
Amanita muscaria is well-known for its hallucinogenic and trance-inducing effects in humans, being used since ancient times in various rituals in Siberia and elsewhere. The principal psychoactive, neurotoxic chemicals are ibotenic acid and the related muscimol, which are located mainly just inside the skin of the cap on a young mushroom. The amount of these toxic chemicals decreases with age, and it varies both seasonally and regionally.
Why does Amanita muscaria produce these toxins? Probably as protection from the depredations of various animals: Flies and beetles are the main insects that eat fungi; the toxins are fatal to a variety of fungus-eating flies. Protecting young and maturing mushrooms allows the fungus to produce its spores and complete its reproductive cycle, but protection for aging mushrooms has little effect on reproductive success and evolutionary fitness. But the lethal effect can vary from place to place: not only does the amount of toxin vary among places, but also local populations of particular insect species sometimes gradually adapt to coping with the toxins. Even closely related insects are likely to differ in their vulnerability to the toxins; those that typically eat this fungus are more resistant that those that don’t.
Around here, many of the flies seen on this mushroom are in the taxonomic family Dryomyzidae, which generally feed on decaying plant material and fungi. Local naturalists have found numerous fly larvae living in the mushroom caps, but their identity is still unknown. On some specimens, local naturalists have identified dead adult flies called fungus gnats in the taxonomic family Mycetophilidae. Larvae of some (but not all) fungus gnat species eat fungus, but the cause of death for these adults is of course not known. Small brown slugs are commonly found, scraping away at the surface of the cap. I have found no reports of any physiological effects on the slugs. We often see springtails on the caps, too, although they may be eating mostly bacteria and micro-algae or the slime that sometimes occurs on the caps. Check out the video at https://vimeo.com/361673622.
Vertebrates eat Amanita muscaria too. I’ve seen a photo of a red fox biting one of these mushrooms. Around here, red squirrels harvest whole mushrooms of this (and other) species (see video), caching them in fairly dry places where they don’t rot quickly. Naturalists have observed tooth marks in the caps, which could have been left by mice or voles that sampled the fungus. Sometimes a sizable fraction of the cap is missing, but who took it? Do bears eat it? Birds have been seen pecking at the cap, but they were probably snatching insects.
The effect of these hallucinogens on vertebrates is not known, apparently. One mycologist joked that perhaps a squirrel that is unusually vigorous in scolding an intruding human is “high” from eating this mushroom. But I have found no hard data on the toxic effects.
At least some species of Amanita produce toxins called amanitins. These toxins interfere with RNA, which carries instructions from DNA for protein synthesis, so they are potentially toxic to all living organisms except bacteria and other one-celled organisms that lack a cell nucleus. However, certain forms of amanitins are toxic to mammals but others apparently are less so. Amanitins can also be toxic to insects, but some insects are more sensitive than others. Amanita muscaria is reported to have low concentrations of amanitins, at least in some times and places. Clearly, it would be useful to have samples from many times and places, as well as assessments of sensitivity in a variety of organisms. The effects of amanitins may be farther reaching: I’ve read that some fruit flies that are tolerant of amanitins can breed in Amanita muscaria because the toxin poisons nematodes that would otherwise parasitize the flies.
This mushroom also can accumulate vanadium and mercury from the soil, sometimes in relatively large amounts, no doubt varying greatly, depending on local soils. It is not recorded if animals that might eat a mushroom full of such elements would be damaged, but it seems likely, since humans can be poisoned by ingestion of mercury. Vanadium can be toxic also. It would be interesting to know if Amanita muscaria can also accumulate other potentially toxic elements, such as arsenic (some mushrooms do).
With all the to-do about toxicity, I wondered about the nutritional value of this mushroom. Mushrooms in general contain a lot of water. Carbohydrate content of a wide sample of dried mushrooms varied from 25% to 82%; protein content varied from 19 to 35%; fats ranged from 1 to 8% (dry weight basis). I do not know where Amanita muscaria falls within that spectrum. But animals that eat it must make a tradeoff between nutritional value and potential toxicity … how much toxin do you risk in order to eat? Wildlife biologists often suggest that many animals eat a varied diet, in order to avoid too many toxins from any one source and, at the same time, obtain enough nutrition.
As so often happens, there are more questions than answers!
• Mary F. Willson is a retired professor of ecology. “On The Trails” is a weekly column that appears every Wednesday.