How a few help all: living pothole plugs speed prey delivery in the army ant Eciton burchellii

We investigated an extreme behavioural specialization in the army ant Eciton burchellii. The spectacular group raids of these ants, which can contain up to 200 000 workers, always remain connected to the nest by a ‘principal trail’ of forager traffic. Remarkably, some workers use their bodies to plug potholes in the natural surfaces that the principal trail travels over, to provide a partly living surface for their nestmates to use. We found that this highly specialized behaviour results in a clear net benefit to the colony. Our experiments show that foragers do not discriminate against surfaces that force all individuals to run as slowly as the smallest workers, but that this prompts some ants to plug neighbouring low-quality surfaces. Individuals size-match to the hole they plug and cooperate to plug larger holes. The resulting modified surface allows prey-laden foragers to attain maximum speed. Overall, this results in a strong positive relationship between ant size and speed and an increase in the mean speed of prey-laden traffic. Moreover, calculations suggest that under a range of realistic scenarios, plugging behaviour results in a clear increase in daily prey intake. Broadly, our study provides rare quantitative evidence that extreme specialization by a minority can significantly improve the performance of a majority, and benefit the group as a whole. It also suggests, however, that these benefits are a consequence of the unusual and derived foraging strategy of E. burchellii. This highlights the importance of considering ecology and evolutionary history in the study of social organization in animal societies.