Fungal Farming in Leafcutter Ants
Enter The World of the Attine Ants
Leafcutter ants constitue 41 species of ant across the genera Atta and Acromyrmex, and represent the most socially complex division of the fungal farming ants, which encompases some 200 species of the family Formicidae called the attine ants (2), sometimes referred to as the attini tribe. Indeed, their unique relationship with a symbiotic fungus has been compared to the agricultural practices of humans!
Colonies consist of up to eight million individuals, with significant polymorphisms based around specialized roles, or castes, within the nest. Their underground nests can range from 30 to 600 meters, with smaller rounds radiating from a central mound that can itself be 30 meters in diameter. All leafcutter species are endemic to Southern and Central America and parts of the Southern United States, and are responsible for the consumption of 20 percent of the fresh leaf biomatter of the Neotropics (2).
Leafcutter ants are not leaf-eater ants. Their herbivorous efforts go into feeding fungal cultivars of the Lepiotaceae family scrupulously maintained within the nest. The fungal hyphae secrete digestive enzymes into the fresh leaf substrate, transforming the cellulose in the leaves into an accessible form. Swelling with sugars and protein, the hyphal tips, called 'staphylae' or 'gongylidia', are then consumed by the ant and their larvae (2).
Mutualism and Symbiosis
Mutualism is typically understood to describe the interaction between two organisms or species such that both experience a benefit that correlates to overall fitness or survivorship. It is one type of symbiosis, long-term interactions between species, which constitutes a broad category also including commensalism and parasitism. Though symbiotic relationships are typically understood as occurring between two species, increasing evidence suggests that "it is important to recognize and study symbioses as potential complex networks of interacting species, rather than binary relationships" (9). In instance of mutualism explored here, the term symbiont is often used by scientists of the field to refer to the fungal cultivar, although both species are dependent on their shared interactions.
Guiding Questions and Emergent Biology
In relationships in which both species benefit, it can be misleading to lable one species as the initiator of the relationship or suggest that one is more dependent on the relationship for survival, and hence more interested in sustaining it. This website explores the operation and development of ant-fungus mutualism, including its hypothesized co-evolution, through the lens of Niko Tinbergen's four questions of animal behavior, adapted from Suzy Renn's Animal Behavior Website:
Throughout the site, there are numerous opportunities to consider whether the ants are really farming the basidiomycete cultivars, or if the fungi are farming the ants. However, rather than seeking to arbitrarily side with one or the other, many scientists today advocate a move beyond the limitations of dualistic thinking and consider the matrix of ant-fungus interactions and specializations developed within leafcutter colonies as constituting a "superorganism", that is a collection of individuals that are each "as specialized in their tasks as the cells of the body and that jointly perform the task of keeping the nest alive" (2). In the following pages, we explore the functions of different elements of this mutualistic system, their development on evolutionary and proximate timescales, and the benefits that make this symbiosis mutualistic.