Tuesday, August 4, 2009

Why ants are awesome: Cooperation, zombification

Be afraid. Be very afraid. Because ants have already taken over the world.

To get a sense of their global dominance, consider the following facts. Ants monopolize 75% of the total insect biomass and 15% of the total terrestrial animal biomass. Their colonies can become huge; one colony of Argentine ants (Linepithema humile) in California spans 560 miles, while another colony from the same species along the Mediterranean spans 3,700 miles. What's worse, ants from the super-colonies from America and Europe already act like members of an international mafioso; when continent-hopping ants of this species encounter each other on foreign soil, they cooperate instead of fighting.

Be very, very afraid.

Ants have achieved such ascendancy in the environment through sociality on a staggering scale. Individual ants are specialized to the extent that the majority of individuals in any given colony are sterile, the reproductive functions being given over to the queen of a colony. On the face of it, such a strategy seems evolutionarily unsustainable, since each individual daughter worker seems to have a strong incentive to quit cooperating with her sisters and become a baby-making factory of her own. However, ant scientists (myrmecologists) have discovered a strange quirk of ant genetics: each daughter ant is more related to her sisters than her mother.

This occurs because ant sex is determined by the number of chromosomes any given ant receives. Ants formed by the union of a sperm and an egg (diploids) become females, while ants formed by eggs alone (haploids) become males. Because female ants have two chromosomes, half of which are contributed to producing a new ant, each daughter ant shares 50% of her genes with her mother. However, daughter ants always receive all of the single chromosome possessed by her father; this means that she shares 75% of her genes with her sisters. Thus, daughter ants have an incentive to keep their mother safely and continuously churning out fellow sister ants.

This peculiar system of sex determination has made possible a wide variety of cooperative behaviors, which has made ants the poster-child of the burgeoning field of sociobiology. Together, sociobiologists and myrmecologists (along with scientists in a number of other disciplines) have catalogued a dizzying array of interesting behaviors, including:

Agriculture. This, of course, is one of ants' most renowned cooperative behaviors, made famous in pretty much every nature documentary about the Amazon ever. Farming ants cultivate a variety of mutualistic funguses, which they feed and protect in exchange for being, well, ant food. One particular species of farmer ant, Atta colombica, has also been observed weeding and grooming its fungus gardens of parasitic fungi. The ants also cultivate a variety of bacteria that acts as a sort of antibiotic for the parasitic fungus. Sounds like a pretty sweet deal, as long as you don't mind being, well, ant food.

Fortunately, ant farmers still have yet to invent the plow
Animal husbandry. This is another well-known ant ability. Although individual species of ants have evolved to herd a variety of animals, one of the most famous herding relationships is that forged with aphids. The aphid's typical diet, plant sap, is low in needed nitrogenous compounds, which means that aphids need to consume great quantities of sap to meet their nutritional requirements. The excess sap is excreted out the anus in the form of sugary "honeydew". Some species of ants have evolved to protect aphids in large farms in return for this sweet anus residue. In order to keep their aphid-cattle docile, the ants excrete tranquilizer from their feet. This also enables the ants to keep the aphids closer together than the aphids normally prefer. It also makes it easier for the ants to kill the aphids when they occasionally decide to eat them.

Mmmm, aphid anus-juice
Slavery. This is an interesting one. Certain species of ants raid the colonies of other species for larvae. They then raise the larvae as their own, and through intricate chemical deception, fool the slave ants into working for a foreign colony. However, slave ants sometimes get their just desserts; some species have evolved a means of detecting the chemical deception and revolt on their former masters, killing large numbers of larvae before they succumb.

If I haven't already convinced you of ant supremacy, there is one more reason to fear the coming ant apocalypse: the existence of ant zombies.

Apparently, the Ophiocordyceps unilateralis fungus specializes in hijacking ant bodies, then forcing the ants to climb to specific locations on the leaves of rainforest trees and causing the ants to develop a "death-grip" on the undersides of those leaves. The fungus then extrudes its tendrils through the ant head, eventually fully taking over the ant's body. When it comes time to reproduce, the fungus' spores spread out in a "killing field" that zombifies any unfortunate ants that come within 1 meter of the mature fungus.

What I'm trying to tell you all in this post is that we have an animal that constitutes over 15% of the terrestrial animal biomass that is cooperative, has farming, agriculture, and slavery, and that has a ready avenue of zombification. I don't know about you, but when the coming zombie infestation sweeps the earth, I plan to be prepared with gifts of aphid brains and honey.

Consider yourselves warned.


  1. first our octopus overlords, and now ants?? I don't know who to idolize first!

  2. "this means that she shares 75% of her genes with her daughters" Do you mean "with her sisters"?

  3. I already have a healthy fear of ants. I was experiencing a lot of static in my land line and frequent drops of my DSL connection last year. A guy from the phone company came to my house, checked out the wiring, and declared that ants had made a nest inside the box that connected my apartment to the main telephone line. The ants ate through the wires. The technician informed me that he replaced the damaged wires and sprayed the box with ant poison. From this, I learned two things: Ants are trying to destroy the internet, and this is a common enough problem that the repair guys carry ant poison with them at all times. Since then I've switched to a WiMAX internet provider, but I'm sure ants will eventually figure out how to conquer wireless technology as well.

  4. Ants have apparently been persistently anti-science for some time. According to Hölldobler and Wilson (1990), one colony occupied the entirety of the Biological Laboratories at Harvard University. The university only initiated a serious extermination campaign when the ants were observed filching radioactive materials from petri dishes and carrying them into the walls. At least the extermination efforts were successful - I'm sure it was only a matter of time until the Harvard colony's incipient nuclear program made miniature craters of the US' institutions of higher learning.