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This has been puzzling me for a long time now. There is an evolutionary predilection for living things which have evolved strong defensive measures (that may not themselves be visible) to give distinct visual or audible warnings advertising these measures.

Wasps, hornets, snakes, and bees are all brightly coloured. Snakes may even have rattles to advertise their venom. These things by in large don't want to have to use their poison in a defensive capacity. Getting revenge on a predator who has mortally wounded you is a Pyrrhic victory.

These warnings are so potent that many animals without such measures will mimic these warnings in order to piggy pack on a general evolutionary bias towards an instinctive avoidance of anything that warns of being poisonous.

But take the death cap mushroom.

death cap mushroom

This innocuous fungus looks identical to completely harmless mushrooms. It has no foul smell. In fact, evidence shows that it actually has a pleasant taste! That picture above actually looks pretty tasty in my opinion.

This fungus has gone to the trouble of developing one of the most potent poisons in nature (humans have a seriously robust digestive systems by animal standards, and yet a single death cap can easily kill an adult human).

So why so secretive about its arsenal?

I know this same question about evolving poisons that are not well advertised could be applied to a large number of animals, insects, and particularly fungi, but I think the death cap is the most extreme example in both the lethality of the poison considered, and combined lack of advertising.

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  • $\begingroup$ I collect and eat wild mushrooms and that Amanita looks very frightening to me. $\endgroup$
    – blacksmith37
    Aug 10, 2019 at 15:21
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    $\begingroup$ Two questions need to be asked, 1) Was the mushroom's poison evolved specifically to serve as a poison, or does it serve some other function and just happens to poison humans? (Given the long-delayed effect, this seems possible.) 2) Was the poison evolved for humans, or for some other species (insects, perhaps), which can detect warnings like odors or UV markings that humans can't? Or a third possibility: the mushrooms are fruiting bodies. A critter that eats them gets dusted with spores, goes somewhere else, and dies, providing a food supply for the spores. $\endgroup$
    – jamesqf
    Aug 10, 2019 at 18:08
  • $\begingroup$ Related biology.stackexchange.com/questions/35532/… $\endgroup$
    – John
    Aug 15, 2019 at 18:48

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Most of this type of evolutionary question are generally worthless, IMHO. You can rationalize anything post hoc and then go away with the misapprehension that you have been involved in science.

To add a few of those possible post hoc explanations (which took longer to type out than think up):

1) The poison is only accidentally a poison. It serves some other function in the mushroom's metabolism. There are numerous possible examples of this. For instance, the stalks of rhubarb are edible, while the leaves are poison; potatos are edible unless they've turned green from exposure to sunlight...

2) The poison might not be directed at humans. The mushroom might have some warning that would be obvious to other species, for instance a smell or markings visible in UV (such as many flowers have). We have lots of examples of poisonous plants that we grow because they're attractive to us: datura, lily of the valley, foxglove, oleander, delphinium...

3) If I'm not mistaken, mushroom poisoning takes rather a long time to kill, which argues against it being a defense. The poison might have evolved as an aid to reproduction. Mushrooms are the fruiting body of the fungus. If an animal eats it, it consumes the spores. If it dies hours or days later, it has transported those spores to a distant location, and the corpse provides nutrition for the new fungi.

However…

Doesn’t the all-white toadstool look distinctive and memorable to you? It doesn’t look remotely like the field mushrooms or other edible mushrooms I would collect. What edible fungus would you confuse it with?

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    $\begingroup$ The death cap is infamous for looking like safe mushrooms. For instance mushroomguru.co.za/uploads/5/5/2/1/5521125/4187215_orig.png This is also totally safe, albeit not as tasty. wildfooduk.com/wp-content/uploads/2017/06/White-false-11.jpg All evolutionary questions are post hoc, unless you have a couple of million years to spare to actively observe change. $\endgroup$
    – Stumbler
    Aug 10, 2019 at 21:55
  • $\begingroup$ @Stumbler — One of your links is broken, but I am only speaking from experience and my post hoc reasoning may be faulty. All evolutionary questions may be post hoc, but that doesn’t mean they have the same level of uncertainty. Speculating about why something is the way it is is in a different league to calculating the time of the last common ancestor of mice and rats from their DNA sequences, for example. $\endgroup$
    – David
    Aug 10, 2019 at 22:07
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    $\begingroup$ I'd be okay with this sort of "we can't really know" answer if you included some of the different likely competing explanations. Jamesqf's comment is a decent starting point that hits most of the basics (might not be primarily a poison, might not target humans, might not be necessarily selected to prevent predation, etc). $\endgroup$
    – Bryan Krause ♦
    Aug 10, 2019 at 22:17
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    $\begingroup$ Bryan — Well my main point was really that the “Exterminating Angel” whiteness appeared distinctive to me and my wife (who knows more about fungi than me). I suppose it depends on where you live. But I invite @jamesqf to edit my question with his comment to improve it. That’s supposed to be acceptable, here. $\endgroup$
    – David
    Aug 10, 2019 at 22:23

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