Fungus garden nutritional needs drive foraging decisions in the social symbiosis of the desert leafcutter ant Acromyrmex versicolor

Organismal nutritional needs vary across time and context, with consequent impacts on foraging decisions. In collectively foraging social groups, this includes variation in nutritional needs among group members. In social symbioses, such as between ants and their farmed fungus in a leafcutter ant colony, nutritional decisions are being made for multiple species as well as multiple group members, and the optimal nutrient balance may vary accordingly. In this study, we examined how nutritional decisions are organized in this symbiotic mutualism by measuring the impact of changes in fungal volume and brood number on relative and absolute collection of protein and carbohydrates by colonies of the desert leafcutter ant Acromyrmex versicolor . Our results show that there was no change in relative collection of protein and carbohydrates or in total nutrient intake following an increase or decrease in the proportion of brood to adult ants. Decreasing the relative abundance of fungus, however, resulted in an increase in absolute nutrient collection, while relative nutrient collection remained constant. These results demonstrate that the fungus plays a distinct and central role in driving foraging decisions in this symbiosis, and that its nutritional needs shape the social organization of the colony in the context of foraging.