Down on the Fungus Farm
Many millions of years ago, long before the existence of Man (Homo sapiens), a group of American ants started to grow fungi for food. And they still do it, and do not eat anything else. These ants do exactly the same things as the farmer does with his crops and animals. The fungal gardens are weeded and groomed, fertilized and sprayed with antibiotics to kill bacteria, and after 5 to 7 weeks cuttings are taken and replanted for new gardens elsewhere in the nest, while the old gardens are abandoned. All these activities take place underground. Most importantly, the ants bring food in the form of plant material for the fungi. It is fascinating to realize that humans now are just reinventing methods which have been in use literally for ages by minute critters.
The ants are members of the tribe Attini, or attine ants, a group of 13 genera and circa 200 species, only occurring in the Neotropics. The leaf cutting ants (genera Atta and Acromyrmex) - the so-called higher attine ants - bring freshly cut leaves into the nest. Their mandibles are shears with which the leaves are cut, and smaller worker ants often ride on the leaves on their way to the nest to protect the material against fly attacks. Just because of the sheer size of their colonies these ants cause a lot of damage to crops. Five to eight million animals can live in one colony, using per day as much plant material as a fullgrown cow. The cut leaves are scissored into smaller and smaller pieces by smaller and smaller worker ants, chewed on and then given to the fungi. There are also lower attine ants who dont bring fresh leaves to the fungi, but take everything edible from the forest floor.
All these ants cultivate Lepiota-like species. Actually several groups of species are involved, allied to Leucoagaricus rubrotinctus, and Leucocoprinus birnbaumii (Lepiota lutea). However, the ants are only interested in the vegetative state of the fungus, and dont want it to fruit, so they actually suppress the formation of mushrooms. This makes it difficult to know which species of fungus is used by the ants. Molecular methods have made it possible to compare the different species, place them in groups, and relate them to species which do produce fruiting bodies. Mushrooms occur when the nest is deserted for whatever reason. The number of times this has been reported is small, but they still can do it!
One particular group of fungi does not form hyphae at all, but exists in yeastform: simple cells, no hyphae, no structures. The fungi cultivated by the leaf cutters form grape-like swellings on the hyphae, and these are gathered by the ants and brought to the larvae. The other ants let their larvae loose on the hyphae to graze. The normal way the fungus is transferred from one nest to the next is by the founding queen. She puts a bit of fungus in a pocket inside her mouth, before starting on her wedding flight. And she keeps it there during the mating, and the digging of the hole for the new nest! This wad of fungal material will sustain the new colony.
Neighbouring ant colonies of different species may grow the same fungal species. If your food supply dies off, you go to the neighbour to take some!
Not only can the garden be raided by other fungus growers, there are even pirate ants, who cannot cultivate the fungus themselves, but just want to live a life of luxury for a while before moving on to the next bed of fungi.
One of the big questions is: How did it all start, 50 millions or more years ago? Was there a single founding episode? Was it accidental, in other words, were there already (mycorrhizal) fungi growing in the nest? Did the fungi start out growing on the ants garbage heaps? Or were bits of spores and hyphae found during food trips and carried around in the ants mouth pockets, and subsequently grown in the nest?
Another big question is whether there is any advantage in the whole process for the fungi. Is their role just a passive one (they do resemble pigs in industrial farming dont they?), or can they manipulate the ants behaviour and turn the situation into a positive one for themselves? Can the fungi influence the biology of the ants, for instance, by regulating the number of female ants born? There are definitely indications that the fungi do have something to say in what the ants do and dont do. If the ants collect plants the fungi dont like, they are clearly told not to do that again. The reprimand is possibly a chemical signal that in one way or another is transferred from the fungus to the workers in the nest to the foragers who cover great distances in the outside world.
Research on the chemicals used by the ants and the fungi can play a role in decreasing the crop damage caused by the leaf cutters. New discoveries in the way the fungi and the ants live together can give more insight into the process of evolution. Anyway, there is much to learn from these exciting mutualisms!
Further reading: about ants, and the interactions between ants and fungi:
- Hölldobler, B. & E.O. Wilson 1990, The Ants. Harvard University Press, Cambridge MA.
- Hölldobler, B. & E.O. Wilson, 1994. Journey to the Ants. A story of scientific exploration. Harvard University Press, Cambridge MA.
- Mueller, U.G., 2002. Ant versus fungus versus mutualism: Ant-cultivar conflict and the deconstruction of the attine ant-fungus symbiosis. The American Naturalist 160, Supplement: S67-S98. 2002.
- North, R.D., C.W. Jackson & P.E. Howse, 1997. Evolutionary aspects of ant-fungus interactions in leaf-cutting ants. Trends in Ecology and Evolution 12: 386-389.