Planet Earth

Researchers go neutrino-hunting and penguin-watching at the South Pole.

They're organized and ruthless, brilliant engineers and cold killers. They're ants, and this is their world.

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Photo Credits: All text by Andrew Moseman; Image: Alex Wild

If ants' vast array of abilities gives you pause, consider this: In many ways, they're the one animal outside the primate family that acts an awful lot like humans. And, like us, they've conquered nearly the entire planet.

Ants are native to every continent except Antarctica, but Argentine ants weren't content to live out their lives in South America. By hitching rides on planes, trucks, and ships, this group has spread across the world; they're seen here ripping a larger ant to bits.

Some scientists consider Argentine ants to be one giant super-colony spread across the globe. They will tear other species to shreds, but when ants from a colony in Argentina meet their brethren in Japan, the long-lost cousins get along swimmingly.

Photo Credits: Image: Alexandra Achenbach/ Ludwig-Maximilians University

Ants can be vicious, organized, and terrifying. But while some will attack any threat that comes in their path--people included--much of their cruelest behavior is reserved for other species of ants.

Take slavery: Protomognathus americanus ants raid the colonies of smaller ants called Temnothorax, steals their children, carries them back to the invaders' colony, and forces them into a life of servitude caring for the young.

Temnothorax ants aren't the only species that can be enslaved by a stronger competitor, but they are the only ones seen launching a slave rebellion. Temnothroax prisoners have been observed slaughtering some of the Protomognathus ants they'd been charged with babysitting.

Photo Credits: Image: iStockphoto

Some ants have another secret talent: They're trapeze artists. In certain species, if the foraging group needs to cross a gap to another tree branch, the ants will build a bridge out of their own bodies to allow their coworkers to step across.

This feat has been performed in the lab. The University of Bristol's Nigel Franks set up a piece of wood for Panamanian army ants to traverse, but he drilled holes in it to impede their progress. Individual ants spread their bodies over the holes to bridge the divide. Meanwhile, other ants tested the holes to determine whose body was the right size to act as a filler.

Photo Credits: Image: Alain Dejean and Jerome Orivel

Certain ants are soldiers, and others are farmers; leaf cutter ants famously haul large chunks of plant material to their colony, upon which they grow a fungus they like to eat. Then there's the Allomerus decemarticulatus, which are both.

Allomerus ants cultivate a fungus as well, but not to eat. Instead, they take plant hairs and other materials and use the fungus as an adhesive to build a trap on a tree limb. The structure contains small holes where the ants lie in wait, invisible. When a larger insect flies into the trap, they burst forth and ambush the prey, killing it with repeated stings before they carve it up and carry it away.

Photo Credits: Image: flickr/ViaMoi [http://www.flickr.com/photos/viamoi/]

Ant species can be cruel and fierce, but they can be benevolent, too--provided there's something in it for them.

One type of ant enjoys a symbiotic relationship with milkweed plants and tiny aphids. The milkweed gives off a sap that the aphids eat; then they secrete honeydew, a delicious snack for the ants. For their part, the ants run a protection service, fighting off predators that would prey on the other two.

Photo Credits: Image: Proceedings of the National Academy of Sciences USA

Many of us see somebody moving around, and assume that the person is alove. Not so with Argentine ants. If they think a fellow ant is dead, they'll carry it off to the trash heap, even if the poor creature is still moving.

The reason for this behavior is that during their lifetime, Argentine ants emit a chemical signal to signify that they are alive. Biologist Dong-Hwan Choe recently discovered that if he inhibited that chemical, the colony's undertakers would decide that the ant was dead and would proceed to dump it out back, no matter how much the still-living victim protested.

Photo Credits: Image: Nigel Franks & Tom Richardson

It is not unusual for one animal to follow another to a food source, but the back-and-forth way this happens for Temnothorax ants led some researchers to call the behavior the first formal teaching ever seen in animals.

The teacher leads its student on a "tandem run," showing it a new route--but the follower gets to determine the speed. It looks around to learn the new terrain, and only when it's ready to move on will it tap the hind legs or abdomen of the guide.

The teacher would reach food four times faster if it left the student behind, but ants have learned the same lesson that all human teachers know: Teaching requires great patience.

Photo Credits: Image: USDA/Sanford Porter

Last year DISCOVER brought you the gallery "Zombie Animals and the Parasites That Control Them," but we missed one: Fire ants have a South American nemesis, the phorid fly, that takes over a fire ant's mind. After the female lays its eggs inside the ant's body, the larvae move to the ant's head and force it to move away from the nest.

After a while, the larvae hatch, thereby decapitating the ant.

Because the flies inflict this fate only on invasive fire ants and not other ant species, Texas officials have begun releasing phorids in the hopes of slowing down the fire ant spread.

Photo Credits: Image: Alex Wild

In DISCOVER's gallery of cannibal animals, we listed species like the praying mantis that engage in sexual cannibalism, with the female eating her mate. But Mycocepurus smithii ants don't need males in the first place. Nary a male of this species has ever been found. Instead of reproducing sexually, the species reproduces only when the queen clones herself with a muscular reproductive organ that scientists don't yet understand.

A University of Arizona research team first studied this ant species because of its ability to farm not just fungi, like leafcutter ants do, but other crops as well. But when the researchers couldn't find any males, they performed DNA analysis and found all the ants to be clones of their queen. Asexual reproduction by females is terribly rare, and the team doesn't know how long ago this species evolved their peculiar behavior.

Photo Credits: Image: Alex Wild

More than 200 species are classified as "army ants" because of their odd and aggressive behavior; they exist primarily as nomads rather than building nests, their queens cannot fly, and they famously attack their prey in an incredible swarm.

But how this behavior became so popular remains a mystery. Some researchers held that the behavior evolved separately on different continents. But in 2003, Cornell University researcher Sean Brady published a paper concluding that "army ant syndrome" evolved exclusively on the supercontinent Gondwana, and spread its ferocity around the world from there.

If true, this hypothesis means that army ants have been terrorizing the world for more than 100 million years.

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