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What Is the Symbiotic Relationship Between Aphids and Ants?

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Symbiosis, also known as mutualism, is a long-standing relationship between members of different species, usually for the benefit of both. When human livestock producers tend farm animals, both species get something out of it, making their relationship symbiotic. The same principle is believed to apply to ants, who "farm" aphids using surprisingly similar methods. But the real mystery at the root of this phenomenon is how ants are capable of such complex behavior at all.

Just How Brainy Are Ants, Anyway?

Ant intelligence fascinates Antoine Wystrach, lead author of a study on ants' navigational abilities published August 2013 in the journal "Proceedings of the Royal Society B." Apparently, despite having tiny brains, ant behavior is as complex as that of any big-brained mammal, according to Wystrach. Human brains "build a unified map of the world" capable of relating all stored information to other information, but ants' brains appear to have varied and specific modules dedicated to specific tasks, Wystrach says. Another study, published in February 2013 in "Current Biology," revealed evidence of a new level of behavioral complexity. Scientists know that ants communicate through chemical signals called pheromones, but some ants and pupae, the stage between larva and adulthood, also generate sound to communicate -- otherwise known as talking.

Aphids and Why Ants Like Them

Thousands of aphid species exist throughout the world. These bugs are so small that when they extend their wings, they can float for hundreds of miles on the wind. Green is the most common aphid color but they can also be red, brown or black. Female aphids don't need males to reproduce but without them, can only produce daughters genetically identical to their mother -- clones, in other words -- so populations can increase very rapidly. Farmers hate aphids because they do so much crop damage. They pierce the vascular systems of plants to feed on their juices, extracting protein but excreting sugar, which their bodies don't need. This residue is called honeydew and many other insects, including ants, lap it up.

To Ants, Aphids Are Livestock

When an aphid-hunting ant comes upon a herd of his quarry, he returns to the nest, leaving a trail of "recruitment" pheromones for worker ants to follow to the source. From then on, the aphids are effectively enslaved by the ants. First, they're drugged with a tranquilizing chemical excreted from the ants' feet, slowing down their speed of movement by about a third. To make doubly sure the aphids don't get any ideas about taking off, ants may also bite off their wings or secrete chemicals that impair wing development. To "milk" the aphids, ants massage their abdomens with their antennae, stimulating the release of honeydew. And, if the ants want more dietary protein, they eat the aphids.

Aphid Husbandry

Ants control the reproductive capacities of aphids to boost numbers of the most useful while phasing out the least productive, mainly by eating them. As a study involving yellow meadow ants in northern Europe showed, they do this by exploiting the female aphid's ability to clone herself. The ant colonies studied were inside a 4.5 square mile area containing three different species of aphids being farmed by ants in underground chambers of their nests. Even in mounds where more than one species of aphid was being farmed, 95 percent of all chambers contained aphids produced from a single clone. "In a parallel with human farming methods, this most likely gives colonies the possibility to actively manage the diversity and abundance of their livestock," said lead author Aniek Ivens of the 2012 study published in "BMC Evolutionary Biology."

What Aphids Get Out of the Deal

Not all ants farm aphids, and not all aphids serve ants. Though we can't know how aphids feel about their arrangement with ants, experts think that the bugs willingly trade their freedom for the protection ants offer from parasites and predators such as ladybugs. When ants dine on baby aphids and adults who have outlived their usefulness, it's a form of population control, Ivens suggests. The Encyclopedia of Entomology notes that aphids not only don't flee from ants but actually compete for their attention. And if ants didn't gobble up all the honeydew secreted by aphids, it would attract a toxic fungus, thereby fouling the aphids' own nests, the encyclopedia says.