The Attini tribe rely solely on the cultivation of Fungus Gardens for food. When an Attine Daughter Queen leaves her maternal home, she must carry within her mouth a Nucleus of Fungus to serve as the Starting Culture for her new Garden (Schultz and Brady 2008).
THE DEVELOPMENT OF AGRICULTURE
In a paper published in PNAS in 2008, Schultz and Brady provide detailed insights into the transition from simple agriculture to complex agriculture of the Attini tribe. The study suggests the Attini first developed agriculture approximately 50 million years ago in the forests of South America, coinciding with the early Eocene climatic optimum (50-55 mya). During this time there was a period of global warming and an extraordinary diversity of tropical plants occurring at middle and high latitudes in South America. The methods used in Attine agriculture have been divided into five distinct systems:
1. Lower Agriculture (practiced by the majority of the Attine)
2. Coral Fungus Agriculture (practiced by the “Pilosum Group”)
3. Yeast Agriculture (practiced by the “Rimosus Group”)
4. Generalized Higher Agriculture (practiced by the “Higher Attine”)
5. Leaf Cutting Agriculture (practiced by the Atta and Acromyrmex)
(Schultz and Brady 2008)
All five systems of agriculture utilize remarkably proficient planting, manuring, weeding, and sheltering techniques (Mueller and Rabeling 2008).
The original Attine agriculturalists collected withered plant bits and other debris on which to cultivate an unspecialized fungus that retained close genetic ties to free-living fungal populations (Mueller and Rabeling 2008). The “parasol mushrooms” grown using this method are, so far as is known, entirely capable of free-living existence without the help of the Attine growers. A paraphyletic grade of Escovopsis is known to infect the the paraphyletic fungal food sources used by the Lower Attine; but, like all Attine agriculturalists, they utilize an antibiotic produced by Actinomycete bacteria to control the parasite.
CORAL FUNGUS AGRICULTURE
The “Pilosum Group” of the Attini tribe began to cultivate coral fungi (Pterulaceae) between 10 and 20 million years ago. Recent research indicates that Coral Fungus Agricultural products are infected by a specialized grade of Escovopsis that is derived from an Escovopsis species that infects Lower Agricultural products. This species subsequently gave rise to a clade that switched hosts and began infecting the Higher Attine food sources (Schultz and Brady 2008).
Unlike typical Attine Mycelial Gardens, Yeast Gardens consist of small, irregularly shaped nodules of fungus growing in the yeast phase. Yeast Agriculture is confined to the “Rimosus Group” and originated sometime between 5 and 25 million years ago. The yeast grown are capable of a free-living, feral existence; however, they grow in the mycelial phase rather than the yeast phase. Indeed, these fungi are only known to grow in the yeast phase when attended by the Attine growers (or depending on conditions in artificial culture). The parasite Escovopsis is unknown to Yeast Agriculture (Schultz and Brady 2008).
The transition to higher agriculture and the subsequent origin of leaf cutting are arguably the two most ecologically significant developments in the history of the Attini tribe. The fungi grown by the Higher Attine suggest a significant degree of “domestication”, or modification for life with the Attine. These fungi do not appear capable of free-living existence separable from their growers. And only the fungi grown by the Higher Attine produce “gongylidia”, nutritious swollen hyphal tips that are harvested by the Higher Attine for food(Schultz and Brady 2008).
LEAF CUTTING AGRICULTURE
The development of Leaf Cutting Agriculture (rather than the debris collecting that is used in all the other systems) coincided with marked ecological transitions in South America (5-15 mya). The coincidence of grassland expansion with the development of Leaf Cutting Agriculture supports the hypothesis that early Leaf Cutters may have been Grass Cutting specialists with specializations in Broadleaf Cutting developing later. The most wide ranging Leaf Cutting Agriculturalists originated and expanded within the last 1 to 2 million years. Such a rapid acceleration in diversification and expansion of the Attini tribe underscores the belief that Leaf Cutting Agriculture represents one of the key innovations in Attine history (Mueller and Rabeling 2008).
Mueller, U., & Rabeling, C. (2008). A breakthrough innovation in animal evolution Proceedings of the National Academy of Sciences, 105 (14), 5287-5288 DOI: 10.1073/pnas.0801464105
Schultz, T., & Brady, S. (2008). Major evolutionary transitions in ant agriculture Proceedings of the National Academy of Sciences, 105 (14), 5435-5440 DOI: 10.1073/pnas.0711024105