The cultivation of plants by ants is more widespread than previously realised, and has evolved on at least 15 separate occasions.
There are more than 200 species of ants in the Americas that farm fungi for food, but this trait evolved just once sometime between 45 million and 65 million years ago. Biologists regard the cultivation of fungi by ants as true agriculture long predating human agriculture because it meets four criteria: the ants plant the fungus, care for it, harvest it and depend on it for food.
By contrast, while thousands of ant species are known to have a wide variety of symbiotic relationships with plants, none were regarded as true agriculture. But in 2016, and at the University of Munich, Germany, discovered that an ant in Fiji cultivates several plants in a way that meets the four criteria for true agriculture.
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The ants (Philidris nagasau) collect the seeds of the plants and place them in cracks in the bark of trees. As the plants grow, they form round, hollow structures called domatia that the ants nest in.
The ants in these domatia, providing nutrients for the plant. In return, as well as shelter, the plant provides food in the form of nectar, which is protected by special caps that the ants can bite off but that prevent other animals getting the nectar.
This discovery prompted Chomicki and others to review the literature on ant-plant relationships to see if there are other examples of plant cultivation that have been overlooked. 鈥淭hey have never really been looked at in the framework of agriculture,鈥� says Chomicki, who is now at the University of Sheffield in the UK. 鈥淚t鈥檚 definitely widespread.鈥�
The team identified 37 examples of tree-living ants that cultivate plants that grow on trees, known as epiphytes. Epiphytes struggle to get enough nutrients, so they have a lot to gain by forming a relationship with ants, says at Durham University in the UK, one of the study authors.
By looking at the family trees of the ant species, the team was able to determine on how many occasions plant cultivation evolved and roughly when. Fifteen is a conservative estimate, says Campbell. All the systems evolved relatively recently, around 1 million to 3 million years ago, she says.
Because fungal farming has persisted much longer and the fungal-farming ants have diversified into more species, it seems that farming plants is less evolutionarily successful than聽farming fungi 鈥� it might evolve often, but probably also dies out frequently. While fungus farming evolved only once in ants, it also evolved in termites, six times in beetles and possibly in bees, too.
Whether the 37 examples of plant cultivation identified by the team count as true agriculture depends on the definitions used. Not all of the species get food from the plants, but they do rely on them for shelter, which is crucial for ants living in trees, says Campbell. So the team thinks the definition of true agriculture should include shelter as well as food.
It also isn鈥檛 clear in many cases the extent to which the ants are dependent on the plants, says Campbell.
鈥淚 absolutely agree with the authors that nutrition is only one of the reasons for agriculture, both human and non-human agriculture,鈥� says at the Smithsonian Institution in Washington DC. 鈥淚 also agree that farming of plants by ants is more widespread than previously recognised.鈥�
But at the University of Freiberg in Germany doesn鈥檛 think we should be looking at plant-ant relationships in these terms. 鈥溾�橝griculture鈥�, 鈥榗ultivation鈥� or 鈥榝arming鈥� are all anthropocentric, weakly defined terms,鈥� he says.
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