February 20, 2012 | 5
They found themselves, like any first creatures, lost. Without means, they were unable to survive by anything other than what was in the immediate surroundings. They ate what grew. They planted nothing. They never left home. There were many dire moments, until they found the animals. The first time would have been accidental. A young one caught an animal and rode it out somewhere, the way a storybook character might ride a boat down the river and out to sea.
With time though, they learned more tricks. They waited where the animals came to feed. They found them where they slept. Soon they were riding them all the time, clinging to their dark bodies as they darted here and there into the unknown. Good luck took them to more food. Bad luck killed them. Time, birth and death made good luck more common.
Over years, they reined their new beasts in until, as is the case today, the steeds go out and gather food and bring it back. The fungi grow and wait. They have become fat kings whose success can be measured by the number of their beasts. And they are not few. These protagonists, each one a fungal herders, have evolved multiple times. They are exotic, and yet in some contexts, far closer to home than you might believe.
1-The Tree Eaters—We can start with the tree eaters. The problem facing tree-eating fungi, like any gatherers, is not the amount of food. The problem is finding the food, being where it is at the right moment. This problem is made worse by the absence of legs. A fungus can grow toward food, of course, or toss its spores up into the wind. But one can grow only so fast and the wind is fickle and mean-spirited. The trick, if you want to know, is to find an animal that will carry you to the next dead thing. It needs to be quick and it is best if it is going where you would like to go. As a fungus, you want to arrive before the godforsaken bacteria can begin to divide. Bacteria can turn a good piece of wood terrible faster than you would imagine, at least from the perspective of fungus.
The ambrosia fungi have evolved the ability to get beetles carry them from one piece of dead wood to another. Many different fungi ride many different beetles. Outside your house there is a veritable mid-air highway of fungal horseman heading out to new lands. But the ambrosia fungi have bent the beetles to their needs more than have other fungi. The beetles, in turn, cautious of their riders demanding riders, have evolved saddle-like pouches in which to carry the fungus and feed them during the ride. And once the beetles have reached their destination, they put the fungus in a safe place (with few other fungi and bacteria to compete with) and get to work reproducing, in order to make more beetles, in order to carry the demanding fungus to even more dead trees. By riding beetles, ambrosia fungi have colonized most of the world, in some cases traveling from one continent by taking advantage of the beetles who, in turn, take advantage of us. We, accidentally, move these beetles around in dead wood and in doing so extend their fungus’s domain1.
These fungi, these beetle tamers, make use of no fewer than five different lineages of beetles, each of which has evolved special attributes to assure the fungi’s success.
[Image 1. The ambrosia fungus’s beetles (Coptodryas pubifer) caring for them in the fungal nest. Photo by Jiri Hulcr.]
2-The Leaf-Eaters—Leaf-eating fungi face a problem different from those who eat dead trees. Leaf-feeding fungi have food everywhere, but growing the long distances necessary to colonize it is costly, especially in dry lands. It requires miles of dangerous ground to be crossed. Life would be so much easier, if the leaves were gathered, brought to a single place. Fungi love a leaf pile. In New England, fungi are rewarded each fall by busy parents and children who gather leaves. Elsewhere though, these same clans of fungi have tamed termites.
[Image 2. The fungi that tamed termites. Photo by Piotr Naskrecki.]
Across Africa, fungi live in giant nests built for them by termites. They have for at least twenty million years. The termites travel hundreds of meters and sometimes kilometers to bring the fungi leaves, which they also bite into small, easy to digest pieces deposited in the form of fecal pellets on the fungus2. If the fungi who tend beetles are like nomads with their goats, the fungi who farm termites are more like Midwestern dairy farmers, who send their cows out to pasture and then milk them at the barn. The termites don’t produce milk, but they do produce pre-digested leaves, which to the fungus are, if not visually appealing (the fungi do not have eyes), wonderful. These fungi have become enormously successful and termites build palatial homes for them all over Africa3.
[Image 3: Leaf-eating fungi and the nest termites have built for them, photo from... http://www.williamyuezhang.com/2011/11/termites.html].
3-The Green Eaters—More successful even than the beetle riders or the dead leaf eaters are the eaters of living leaves. Instead of farming termites, these fungi farm ants. These fungi began to take advantage of ants more than forty million years ago. Once, these fungi relied on insect parts and other dead animals gathered by their ants. Some still do. But for others, their demands became greater and more elaborate. For these latter fungi, the ants now gather, on their behalf, bits of flowers and leaves. It is a dangerous job, but, to the fungus, the ants are expendable. The leaves are brought back with care and then fed to the fungus. The ants also produce compounds that help to kill other pathogenic fungi around the fungus (and may even farm antibiotic producing bacteria that help in this same job). The fungus, in other words, makes the ants do weeding4. This fungus has spread throughout the Americas. On the backs of ants, it has been very successful. In some cases, the nests of this fungus can include millions of ants, all working on the fungus’s behalf. Many different species of ants are now farmed by the descendents of the first fungus to be farmed by ants, each with its own small (or large) farm. The ants like many domesticated beasts, have become totally dependent on their fungi and cannot live without them. In order to colonize new habitats, the fungi rides the ants to new places, in a specialized pocket, in their tiny, mouths.
[Image 4. The nest of the leaf-fungus in which ants (Atta cephalotes) tend to its needs. Photo by Alex Wild.]
4-Egg Fungi—In an early piece in this series, I have already described the fungi that mimic termite eggs. These fungi have convinced termites to care for them and bring them to food. These fungi offer little in return, but succeed because their termites seem to have never really figured things out. These fungi have not yet taken over their termites, not totally, and yet they offer a measure of just how simply animals can be tricked. In the end, this is a key piece of the story of fungi and animals. In order to farm animals, fungi must offer them rewards, whether real or perceived (It is better, in the case of the fungi, if they are only perceived.). Rewards keep the animals doing the long hours of work their fungi require.
5-The Fungi that Tamed Humans—One clan of fungi has evolved the ability to control humans. Once these fungi were relatively uncommon. They floated in the air and landed where they could, to eat what they might. But then at least some of them figured out the weakness of humans, alcohol. Humans would work in order to get alcohol. In this way, the humans were better for the fungus than were the beetles, termites or ants. The humans carried fungus from place to place. They had ceremonies in which they celebrated their fungus. They poured libation to the fungus. They also fed them, endlessly, no matter the cost. The termites gathered dead leaves. The ants gathered live leaves. But it was only the humans who went out and planted fields just to make food to feed their fungus (While many animals farm, we appear to be the only species to farm plants) whereas the fungi rewarded beetles, termites and ants with valuable food (essentially, part of their body) the humans asked for nothing. All they claimed was waste, an alcohol. The humans asked for even less than the termites tricked by the pretend termite eggs. All the humans ever wanted was what the fungus would never ever need. These humans are, of course, you and me and this fungus is our yeast6.
We tend to view the evolutionary stories of animals and fungi from the perspective of the animals. We are animals. We relate to the animals. But the animals have been the ones more constrained by the origins of agriculture than have been the fungi. As Uhlrich Mueller and colleagues put it in a recent review of agriculture in insects,
“Interestingly, there are no known cases of reversal from agricultural to nonagricultural life in any of the nine agricultural insect lineages…, suggesting that the transition to [farming fungus] is a drastic and possibly irreversible change that greatly constrains subsequent evolution.”
One might extend this statement to humans. No major societies seem to have abandoned the farming of yeast. But the fungi have, in many cases, abandoned their animals. They use, but do not always require the species of animals they interact with. Does this suggest the animals have been farmed to a greater extent than the fungi? Maybe. Maybe not. In the stories of animals and fungi, it is never very clear who is winning or who is “farming” who. This is the nature of evolutionary “partnerships” between species. With very few exceptions, there are no true partnerships, there are just relationships in which the best interests of species coincide more or coincide less. When both species benefit, it is a mutualism. When one benefits but the other bears no cost, it is a commensalism. When one benefits and the other loses, it is parasitism. If humans and yeast both benefit from the production of beer, wine, and the like, they will both go on, the yeast “farming” the humans and, from the human perspective, the humans farming the yeast. But it is in the best interest of the humans to cheat the yeast, just as it has long been in the best interest of the fungi to cheat the humans.
In the first article in this series, I considered the possibility that human agriculture began because we needed crops in order to brew beer, which is to say, in order to feed our yeast. Maybe it was reasonable to begin farming in order to feed our yeast, because making beer helped us to survive. But don’t think for a second that the yeast wasn’t trying to cheat us the whole time. It didn’t do it consciously of course. Yeast have no brain, but yeast evolve quickly, perhaps as quickly as human culture can change and so the non-exclusive possibility is that, if we did begin to farm in the first place, in part to make beer, that we did it because the yeast tricked us into doing so, taking advantage of our the weakness of our minds to alcohol. Termites are unable to tell a ball of fungus from their own children. We are unable to tell what is good for us apart from what feels good to us. Drinking beer, for example, is good for the fungus, yeast, many beers after the societal good is gone. For now, the yeast seems to have gotten more out of society than we have gathered from it and so in the final chapter of this series, I will consider the story of humanity from the perspective of the more successful species, the yeast.
Continue reading (tomorrow)
Table of evolutionary contents: Here you can skip ahead or backward to the other chapters in the story of the other species in our daily lives, whether they bethe cow, the chicken, the hamster, bacteria (on Lady Gaga, on feet, in bathrooms,as influenced by antimicrobial wipes, as probiotics, in the appendix), pigeons and urban gardens, house sparrows (to be published next week, stay tuned), predators,diseases, dust mites, basement dwellers, lice, field mice, viruses, yeast, the fungus that produces penicillin, bedbugs, houseflies, or something more.
Or for the big picture of how Rob thinks these stories come together to make us who and who we are, check out The Wild Life of Our Bodies. Rob Dunn is a writer and evolutionary biologist in the Department of Biology at North Carolina State University. Find him on twitter at robrdunn. Find him in person somewhere in Europe with his family while they are all on sabbatical.
1-Check out a great story and slideshow of/about these beetles with more photos by Jiri Hulcr.
2-One is tempted to see this as a sort of protest against the fungus, but it is not. The feces is rich in the nutrients the fungus needs.
3—More than three hundred species of termites of the Macrotermitinae are farmed by fungi of the genus Termitomyces. Also, here I want to take a moment to apologize. In contrast to what I have said elsewhere, termite biologists are likeable, interesting people. I swear. I really mean it. I do. I hope I’m not protesting too much.
4-Which is incomplete. The ants weed pathogens of the fungi, but do not tend to weed out different cultivars of fungi, such that in a given nest two or more fungal species (or at least “cultivars”) might be fighting for the rights to the ants.
5-Yeasts, although we don’t tend to think of them as such, are single-celled fungi. They are the rot in your beer and wine, but also in your bread and many other foods.
6-Interestingly, one of the differences between agriculture between fungi and insects and that between our crops and us is that we do not tend to sequester our crops in our cities away from their competition. This is true for our crops like wheat, but, interesting, it is not true of our fungus. We do sequester yeast. With the exception of minor uses, like some homemade breads, yeast is nearly always kept in doors and attempts are made, even within that environment, to exclude other species. We are even more like the ants, beetles and termites than we sometimes seem.