Agriculture

They might seem simple and insignificant, but like humans, ants have discovered the benefits, and costs of agriculture. In the ant world there are species which farm livestock, protecting them from predators and milking them for rich nectar, and others which cultivate tiny underground fields of fungus, pruning it and using chemicals to prevent disease and pests.

Crops and Livestock

Humans developed farming around 10,000 years ago, but the ants have been at it much longer. In its simplest form, ant farming consists of simply pruning the surrounding forest. Ants of one species found in the Amazonian rainforest have been found to remove unwanted plant species when they appear in its foraging area. Although simple, this ‘weeding’ behaviour can be devastating, with ants clearing huge sections of forest of any species which is not beneficial to them.

Another, better known, species of ant can clear a forest for an entirely different form of farming. The leaf-cutter ants collect leaves and other foliage, sometimes clearing huge sections of forest in the process. Foraging leaf-cutter ants return to their nest carrying huge chunks of leaf many times larger than their own body size. But after all this labour, when they return to the nest, they don’t eat the leaves they have collected. Instead, they use them to fertilise their underground crop. They are fungus-farmers, and they not only provide fertiliser for their garden, they have also been found to use pesticides – cleaning it regularly with antibiotics to preventing infection. When mature, the fungus is used to feed their growing offspring. The relationship between fungus and ant has become so intimate that the fungus can no longer survive on its own, and vice-versa. When a leaf-cutter queen founds a new colony, she carries a tiny piece of fungus with her to seed her new garden. Without this her colony would be doomed. Fungus is so precious to these ants that if a colony loses its fungus crop to disease or predation, they will try to steal a fresh crop from their neighbours, risking their lives in the process.

Ants have not only mastered agriculture, but there are also species who tend livestock. Several species of bug are tended to by ant societies, mostly in the aphid and mealybug families. Aphids and mealybugs feed directly on plant sap, a nutrient poor food source meaning that they have to consume extremely high volumes. This results in aphids consuming far more sugar than they require in their diet, and the excess is excreted as a sugary syrup called honeydew. Adult ants who tend to aphid herds milk the aphids for their honeydew, and in some species ants have domesticated the aphids to such an extent that they are able to request honeydew on demand.

Around a quarter of aphid species are tended by ants, and there are significant benefits to these aphid herds. Ants protect their livestock from predators, direct them to the best food sources, and on rare occasions even help to rear young aphids.The ants are so beneficial that in some cases different aphid species are forced to compete for ant farmers. Ants have the pick of the most profitable aphid species, and have been shown to select those species who produce the best quality honeydew.

While humans only began tending to livestock relatively recently, the association between ants and aphids dates back to the early Oligocene, 30 million years ago. The long evolutionary history of ants farming aphids has enabled physical as well as behaviour adaptations, in both farmer and livestock, which make the interaction more profitable for both parties. For example, some aphid species have evolved a setal basket, which enables them to store honeydew for short periods of time until an ant requires it. Although for most species of aphid, ant farmers is not essential, in some cases the aphids have become totally dependent on their captors. As with the fungus-farming ants, in some aphid-ant associations, the mutualism is so specific that ant queens have to carry mealybugs with them to ensure success when founding a new nest.

Although extremely rare, recent research suggests that some ants may have taken one step further, not only milking their aphid livestock but even consuming it for meat. Melissotarsus ants of Madagascar have been found to raid their aphid farms from time to time, feeding directly on aphid meat.

The remarkable associations observed between ants, fungus and aphids demonstrate that humans are not the only species capable of co-opting other species to provide food. Like humans, ant species have developed agriculture and livestock both for milk and meat. There is even evidence for some domestication occurring, and in some cases these interactions are so important that the two parties cannot survive without each other. The tens of millions of years of coevolution between the farmers and their crops and livestock put the mere 10,000 years of human farming to shame.

Articles in this Series:

Part One: Civilisation
Part Two: Agriculture – Crops and Livestock
Part Three: Crime and Punishment

Want to Know More?

• Buckley and Gullan (1991) More Aggressive Ant Species Provide Better Protection for Soft Scales and Mealybugs Biotropica 23; 282 – 286
• Fischer, Hoffmann and Volkl (2001) Competition for mutualists in an ant-homopteran interaction mediated by hierarchies of ant attendance OIKOS 92; 531 – 541
• Johnson et al (2001) Acropyga and Azteca Ants with Scale Insects: 20 million years of intimate symbiosis American Museum of Natural History 3335; 1 – 18
• Kishimoto-Yamada, Itioka and Kawai (2007) Biological characterization of the obligate symbiosis between Acropyga sauteri Forel and Eumyrmococcus smithii Silverstri on Okinawa Island, southern Japan Journal of Natural History 39; 3501 – 3524
• Offenberg (2001) Balancing between Mutualism and exploitation: the symbiotic interaction between Lasius ants and aphids Behavioural Ecology and Sociobiology 49; 304 – 310
• Scheider and LaPolla (2011) Systematics of the mealybug tribe Xenococcini (Hemiptera: Coccoidea: Pseudococcidae), with a discussion of tropohbiotic associations with Acropyga Roger ants Systematic Entomology 36; 57 – 82
• Scheider (unpublished data) / Bob Holmes (2011) Zoologger: The first non-human meat farmers” New Scientist 30/06/11
• Stadler and Dixon (2005) Ecology and Evolution of Aphid-Ant Interactions Annual Review of Ecology, Evolution and Systematics 36’ 345 – 372