Cooperation between plants and ants reaches levels of sophistication that are hard to imagine. An example is the association between these insects and numerous tree species belonging to the genus Acacia, native to Africa and Latin America. Some acacias produce special fruiting bodies to feed the ants and supply them with spaces, created inside specific structures of the tree, where the ants live and breed their larvae. But that is not all: as on one of those home shopping channels where the presenter never ceases to add products to entice you to buy, the acacias offer, in addition to food and shelter, free drinks in the form of the most welcome extrafloral nectar. In return, the ants are in charge of defense against any plant or animal that could somehow damage the plant on which they are hosted. And they do so very effectively. Not only do they keep far from the tree every other insect that has the unfortunate idea of approaching it, but they also attack with great vehemence even animals whose size is billions of times greater than theirs. So, it is not uncommon to see ants bite herbivores the size of an elephant or a giraffe until they desist.
The active defense implemented by the ants, however, is not limited to warding off animals, irrespective of their size; it goes far beyond that. Each plant that dares to come out of the ground within a radius of several feet from the host plant is mercilessly chopped off. So, in the middle of the Amazon rain forest, it is not unusual to see perfectly circular areas devoid of any vegetation developing around an acacia – an unexplained phenomenon to local people, who call these areas “the Devil’s gardens”. In short, this arrangement between ants and plants seems to be a splendid form of collaboration for both parties; at first sight, it is a classic example of mutualistic symbiosis. However, things are not exactly as they seem, and recently many studies have presented a more disturbing picture: under the guise of an idyllic, mutually beneficial relationship, there seems to hide, instead, a vile story of manipulation and deception, which sees the acacias in the unpopular role of the bad guy.
The extrafloral nectar the plant produces, as we have seen, is a sugary liquid that is very energetic; everyone knows that nothing attracts insects more than sugar, so for years it was believed that this was the secret of the appeal of these secretions. However, the nectar does not contain just sugars; it also contains hundreds of other chemical compounds, including alkaloids and nonprotein amino acids. These substances play an important role of control on the animals’ nervous system, regulating their neuronal excitability and therefore their behavior. One, for example, is the main inhibitory neurotransmitter in both vertebrates and invertebrates, including ants. Thus, alterations in ants’ constitution due to the consumption of extrafloral nectar can significantly modify their behavior. In addition, the alkaloids contained in the nectar – including caffeine, nicotine and many others – not only affect ants’ cognitive capacity (as well as that of other pollinating insects that consume nectar) but they also create addictions. What has been discovered recently is that acacias, like many other species of myrmecophilus plants, are capable of modulating the production of these substances within the extrafloral nectar so as to modify ants’ behavior. Not only that: like experienced dealers, acacias first attract the ants, luring them with the sweet nectar rich in alkaloids, and then, once the ants are addicted, they control their behavior, for example increasing their aggressiveness or their mobility on the plant – all this by modulating the amount and quality of the neuroactive substances present in the nectar. Not bad for beings that we continue to perceive as helpless and passive but that, precisely because they are rooted to the ground, have turned their ability to manipulate animals through chemistry into a true form of art.
(THE REVOLUTIONARY GENIUS OF PLANTS by Stefano Mancuso, translation by Vanessa Di Stefano, Atria Books)