Foraging for the fungus: why do Acromyrmex subterraneus (Formicidae) queens need to forage during the nest foundation phase?

1. Semiclaustral ant queens must pay the price of foraging to guarantee their survival and also that of their brood through the nest foundation phase. For leaf‐cutting ant species, the challenge is even greater, as they also need to cultivate their symbiont fungus. 2. We hypothesised that suppressing foraging activity could reflect in larger offspring and higher survival, as queens would be entirely dedicated to rearing the brood and initial fungus. 3. By providing easy access to leaves or completely denying it, we evaluated how foraging runs interfere in Acromyrmex subterraneus queens' survival, productivity, and variation of body mass components. 4. Three foraging conditions were simulated: Starved – no leaves, Spoiled – leaves inside the nest chamber, and Walker – leaves 3 m away from the nest chamber. We registered the number of dead queens, offspring, weight of the fungus garden, fresh and lean dry mass of the queens, as well as their lipid and water contents. 5. Survival of Starved queens, which did not oviposit, was lower than that of Spoiled and Walker queens, although weight loss, water content, and lean dry mass were similar. While lipid content reduction was similar for Spoiled and Walker queens, the former had more larvae and pupae, and a heavier fungus garden than the latter, suggesting a greater rearing effort of Spoiled queens toward them both. 6. The simulated foraging conditions provided novel data for a semiclaustral leaf‐cutting ant species that evidences that fungus garden cultivation is definitively the main driving force for the colony founding success. Suppressing steps of the queens' foraging, by offering leaves without a foraging run or not offering them any, we evaluate how it affects their survival, productivity, and body mass components. Starved queens had lower survival rate and neither offspring or fungus, while Walker queens had less offspring and a lighter fungus than Spoiled, probably due to a greater rearing effort. The amazing symbiose with the fungus allows Acromyrmex to diverge from the behavioural pattern of other semiclaustral species, with fungus garden cultivation as the main factor for nest foundation success.