Leaf-cutting ants (Formicidae, Attini) prune their fungus to increase and direct its productivity

1. The mutualistic fungus of leaf-cutting ants produces ant rewards in the form of nutritive bundles of hyphae called staphylae, We studied how worker activities affect staphyla production by the fungus garden of Atta sexdens. 2. Ant-free fungus garden that was exposed to workers for 3 h produced 1.3 times more staphylae 2 days later than garden that remained ant-free. 3. Simulating the possible mechanical and chemical reasons for this effect showed that damaging hyphae with a mounted needle increased the subsequent standing crop of staphylae after 3 days by 1.2 times. Removing staphylae had no effect on subsequent standing crops, but total crops (which included the staphylae previously removed) were greater than on control garden. Crude head extracts and worker faeces had no visible effect on staphyla production. 4. Surfaces of the fungus garden that were easily accessible to workers produced significantly more staphylae than inaccessible surfaces. 5. The pruning activity of the ants and the response of the fungus can be interpreted both as an evolved behavioural adaptation by the ants to maximize the production of the staphylae they eat, and as a physiological adaptation by the fungus to produce nutritive staphylae for the ants only in those areas where and when the ants are active.