Collective decision-making when quantity is more important than quality: Lessons from a kidnapping social parasite.

Identifying the general principles that shape mechanisms of collective decision-making requires studies that span a diversity of ecological contexts. However, collective decision-making has only been explored in a handful of systems. Here, I investigate the ecologically mediated costs and benefits of collective decisions by socially parasitic kidnapping ants Temnothorax americanus over where to launch raids to steal host brood. I first investigate their sampling strategies and preferences with choice tests. Using more realistic spatial scales, I confirm the findings of others that colonies use a sequential choice strategy, and do not compare options simultaneously. I then ask which ecological conditions could favour the evolution of this strategy by testing the following hypotheses from optimal foraging and mate choice theories: (a) raiding decisions are time constrained or (b) search payoffs are low due to resource uniformity. Spatial distribution and phenological data on nest contents support the time constraints hypothesis. Host nests contain an optimal ratio of brood and workers for a brief period relative to discovery rates. Colonies therefore benefit from raiding most nests they find in this period rather than deliberating over the best choice, favouring host quantity over quality. The decision strategy for raids uncovered here contrasts with best-of-n collective decision-making found in other systems. These findings demonstrate that ecological constraints on information acquisition can alter how collectives process information.