Are the keystone individuals the achilles' heel of the ant colonies?

Abstract

In ant colonies different task response thresholds lead to a non-homogeneous distribution of the labor within workers. Some workers, called keystone individuals, perform a determinate task in a number of times greater than the others. These individuals are highly influential by interfering with the activity level of nestmates. Thus, although promoting great benefits, keystone individuals can become points of susceptibility of the colony and then be considered as the Achilles' Heel, whose absence or error can put in risk the colony success. Using foraging as a model, we investigated how Acromyrmex subterraneus workers with a high level of activity in the leaf transport influence (i) the degree of activity of the other workers and (ii) the colony foraging efficiency. Also, we verified if the task response threshold of keystone workers is short term persistent. By individually marking workers from five colonies, we registered the frequency of leaf discs transport in five social environments in which we manipulated the presence and absence of the keystone individuals. We verified that leaf delivery rate (LDR) of the three categories was similar in initial social environment. However on absence of the keystone individuals, less active workers had similar efficiency/foraging trip to initial social environment. However, after the keystone individuals return, the presented lower LDR in comparison with before the manipulation and than from less active workers. When keystone individuals were absent, less active one increased their foraging effort and non-active individuals started to working. On the other hand when, considering the category efficiency, we registered higher efficiency for keystone categories. However, they were not the Achilles' heel of the colony in A. subterraneus, since the leaves supply was maintained through their replacement by other workers, who became more efficient and also by new active foragers.