Humans have mastered the art of taming animals for their benefit but can only we farm and tame? Not really. There is another set of species which tame other organisms, viz aphids. Ants are the only known organisms that farm other organisms for their benefit. This symbiotic relationship between ants and aphids may be mutualistic or exploitive depending on the prevailing circumstances. Ants have positive effects on some aphid species and negative effects on others. The highlighting feature of this interaction is that ants may sometimes protect aphids from predators and sometimes devour them. Whether this association is beneficial for aphids depends on the presence of predators.
The way ants control or farm aphids is quite sketchy. Joint research was done by Imperial College London, the Royal Holloway University of London and the University of Reading found that ants release a chemical from their feet which tranquillize and subdue aphids, thereby keeping them close and providing a ready source of food. Ants are also known to bite off the wings of aphids in order for them to not fly away. They secrete a kind of chemical onto the aphids which prevent the regrowth of wings. So yeah, this is pretty disturbing.
But then why do aphids still favour this association? The main benefit that ants get from tending aphids in the honeydew that aphids release. Honeydew is a rich carbohydrate source for the ants and serves as an energy source. This, in turn, reduces the waste products produced by aphids. Ants also remove dead aphids and exuviae, provide the transport to feeding sites, provide shelter and brood care. It is however hypothesised that ants feed on the aphids in spite of tending them in order to maintain carbohydrate-protein balance.
This interaction also is dependent on the availability of food for the ants. If floral nectaries are abundant, it is seen that ants do not prefer the honeydew produced by aphids. They instead predate the aphids. In this way plants which produce rich nectaries can protect themselves from harmful and exploiting aphids. Thereby, increasing their viability. It was observed in a study done by Joachim Offenberg that when ants were fed sugar, in this case, honey, their predatory behaviour increased whereas their tending behaviour and honeydew collection decreased. Supplementing the diet with prey induced no significant behavioural changes. An explanation for this might be that ants rely deeply on sugar for carrying out all its activities. On the contrary, only the queen and the larvae need significant amounts of protein. Also, ants are known to cannibalise their brood when food is scarce. Thus sugar is mainly needed for the functioning of the colony whereas protein is needed for colony growth.
“In this scenario, an ant colony's highest priority should be to secure a constant supply of sugar, while foraging for prey remains secondary. When prey is abundant, the colony should raise additional brood for a potential protein reserve in case of fluctuating prey supply” - Balancing between mutualism and exploitation: the symbiotic interaction between Lasius ants and aphids by Joachim Offenberg.
Billick, Hammer, Reithel, and Abbot. 2007. Ant–Aphid Interactions: Are Ants Friends, Enemies, or Both? Ann. Entomol. Soc. Am. 100(6): 887Ð892 (2007)
Offenberg . 2001. Balancing between mutualism and exploitation: the symbiotic interaction between Lasius ants and aphids. Behav Ecol Sociobiol (2001) 49:304–310
Oliver, Mashanova, Cook, Leather, Jansen. 2007. Ant semiochemicals limit apterous aphid dispersal' Proceedings of the Royal Society B: Biological Sciences