Across biological systems, from plant defense to social insect behavior, organisms must allocate limited defenses to protect assets of value. Theory predicts that those choices should be shaped by the asset’s value, defensibility and risk of attack. In the turtle ants (Cephalotes), a genus of arboreal and polydomous ants, the assets in question are scarce nesting cavities, and the defenses are soldier ants with a specialized shield-like head used to block those nest entrances. In field and laboratory experiments, colonies have been shown to flexibly and dynamically allocate soldiers across different nests, choosing nests to defend and adjusting levels of defense based on value, defensibility and risk. How are these seemingly adaptive colony-level patterns produced by the choices of individual ants with only limited information? To answer this question, we presented laboratory colonies of Cephalotes varians with an array of new nests, varying in defensibility, and tracked the movements and decisions of individually marked workers and soldiers over the course of seven days. Each colony was subjected to two different risk treatments: one with heightened risk (5 individual ants of another arboreal species were added to the arena) and one without.
Results/Conclusions
We found that workers showed clear bursts of investigation to specific nests, suggesting that they communicate with one another about which nests to explore. Surprisingly, we observed a strong preference for nests with oversized entrance holes, which require many more soldiers to defend effectively. This was true even under heightened threat, although workers actually investigated more nests (including those with small, easily defensible entrances) under those conditions. Our data also suggest that colonies may use one of two different strategies, depending on colony size. In smaller colonies, individual ants seem to investigate several different nests and make decisions about which nests to occupy and defend based on their own experience. In larger colonies, however, individual ants often investigated only a single nest, suggesting that colony-level allocation patterns across nests may be an emergent result of communication – direct or indirect – between individual ants. Understanding how these ant colonies are able to effectively balance defensive resources across nests according to need may give us insight into other distributed defense systems, such as the animal immune system.