Collective exploitation of a temporally unpredictable food source: mushroom harvesting by the ant Euprenolepis procera

• Mushroom occurrence was not completely unpredictable.
• In given patches ephemeral mushrooms regrew periodically.
• Ant foraging behaviour was well adapted to the periodic growth pattern of mushrooms.
• Foraging trails were used periodically, i.e. they could be deactivated and reactivated.
• This represents a collective strategy for utilizing ephemeral food sources.

Foraging on spatiotemporally unpredictable food sources is widespread among animals. Locating unpredictable food is, however, expected to pose specific challenges to an animal's foraging strategy. Primarily exploiting wild-growing mushrooms, the ant Euprenolepis procera represents a remarkable example of an animal that relies on ephemeral resources. In the present study we investigated how the ant's foraging system has adapted to this challenging diet. We tested feeding preferences of E. procera, monitored spatiotemporal growth patterns of mushrooms and analysed the ant's foraging trail system. The growth patterns of mushrooms turned out to be more predictable than commonly believed, since they reappeared frequently (up to 15 times in 1 month) in close vicinity of former growth locations. Generally, such spatially fixed and periodically regrowing resources can be utilized effectively by revisiting them. While many animals rely on idiosyncratic spatial memory to accomplish this task, we found that E. procera established a wide-ranging, stable trail network that served as an external colony memory. Subsections of the trail network were used only periodically. Inactive trail sections persisted and served as the external colony memory allowing the ants to check growing sites periodically and eventually harvest regrowing mushrooms collectively. We conclude that this unusual foraging behaviour, which features permanence (route fidelity over time) as well as flexibility (activation/deactivation of trail sections), is well adapted to the growth pattern of mushrooms, underlining the high potential of collective behaviour for an efficient exploitation of ephemeral food sources.