Ants and aphids have a symbiotic (or mutually beneficial) relationship. The aphids provide the ants with a food-source - the sugar-rich honeydew they excrete when eating plants - and, in return, the ants protect the aphids from ladybirds and other insects that prey on them.
To ensure a constant supply of honeydew, some ant species cultivate large numbers of aphids, and prevent them from straying too far from the colony by biting and damaging, or even completely removing, their wings. The ants also secrete a chemical from their mandibles which inhibits wing development in juvenile aphids.
Ants communicate with each other using a large repertoire of chemical signals, which are actively secreted onto surfaces from exocrine glands on the legs. These signals can recruit nest-mates to food sources, and are also used to mark a colony's territory. Ants secrete chemicals passively too - as an ant moves, hydrocarbons are shed from the cuticle (the waterproof outer lining of the exoskeleton), leaving a chemical trail.
Ants use behavioral signals called semiochemicals to manipulate aphids' nervous systems. (Ant's own behaviour can be manipulated too, by parasitic fungi.) Earlier work had shown that the presence of ants can somehow tranquilize aphids and limit their motor functions, but whether or not this required direct contact between the ants and aphids was unclear.
Using digital video cameras to measure their walking speeds, Tom Oliver of Imperial College London, and colleagues from Royal Holloway and the University of Reading have now shown that aphids move much more slowly on paper that had previously been walked on by ants than on plain paper. They believe that the chemicals laid down in the ants' footprints are used to maintain an aphid "farm" near the ant colony.
Maintaining a populous aphid farm in a small area is obviously beneficial to the ants, as it would provide them with large quantities of honeydew. However, the relationship between the two species is complex, and it seems that the ants' manipulation of the aphids' behaviour is exploitative.
Normally, aphids wander off to new locations when conditions become crowded, to establish new populations nearby. And although ant-attended aphid populations are bigger and live longer than those not attended by ants, the ants prevent the aphid dispersal that is necessary to maintain a stable meta-population, and makes the aphids more vulnerable to parasites.
Oliver and his colleagues suggest that the aphids might use the chemical footprints to stay inside the area within which they are protected. They also note that their findings migth be useful in developing methods for controlling the dispersal of blackfly, which are considered as pests and sometimes carry diseases.
Oliver, T. H., et al. (2007). Ant semiochemicals limit apterous aphid dispersal. Proc. R. Soc. B. doi: 10.1098/rspb.2007.1251. [Abstract]
I knew ants kept aphids but I didn't know the details you cited in your post. What an amazing example of evolution. It makes me wonder about other examples of cooperation in animals. I wonder if there is any truth to the myth of crows leading wolves to prey?
(Thanks for emailing me the links to bee information. I really had no idea the honey bee had memory before your post. I learn something new here on your blog every week.)
Perhaps it isn't so much cooperation as an example of ant animal husbandry - ie. the ants are keeping an aphid herd. If the chemical that is laid down by ants automatically reduces the speed at which aphids walk it would be more like a fence for cattle.
The article says that aphids naturally want to move out of high population areas to make new colonies but don't because of the ant chemical. Instead of the aphid recognizing the chemical signal as a protective barrier, I'd bet it affects them more of a fence which they can't move past.
It is probably a stretch to call this cooperation. I think animal husbandry is more accurate, though as the article states the aphids do get the benefit of protection from predators from it. I suppose the same thing could be said about our own herd animals.
Still, it's amazing that the ant evolved the ability to manipulate their herds with chemical cues secreted from their own bodies. We haven't gotten that far ourselves yet.
Can you help me by answering this question: Why does ants like honey better than sugar or maybe it is the other way around. reply back soon!
Nabila: I can't answer your question I'm afraid.
I agree that this behavior can hardly be called cooperation, I would call it "human" because we herd our cattle... but now, I'm confused... ants have been doing this kind of things for a really long time. What's next?... Do they also have bull fighting? (just kidding) I'm truly amazed. I'm glad I found this blog.