Beyond busy workers: exploring the sensitivity of inactive ants to environmental cues

Ant colonies are often considered to be highly efficient societies skilled at cooperating and sharing workload among workers. Yet, several studies have revealed low colony activity levels and a subgroup of specialized inactive individuals, raising questions about their role. This study investigates whether these inactive ants differ in their sensitivity to environmental cues compared to other worker groups. We monitored the behavior of individually tagged workers from Myrmica rubra ant colonies, categorizing them as foragers, nurses, domestics, or inactives. Approximately one-third of the workers exhibited high levels of inactivity, forming a distinct group that remains stable over several days. Subsequently, individual tests were conducted on these ants to compare their responses to intranidal or extranidal location cues, and to task-related stimuli such as larvae or prey. In individual tests, foragers displayed distinct responses compared to internal workers. They exhibited greater responsiveness to prey, but were less inclined to care for larvae, stay in colony-marked or shaded areas, or approach nestmates. Among internal workers, except for the inactive ants, responses to various stimuli did not significantly differ. Notably, inactives had a stronger attraction to colony odor and were more likely to carry larvae when compared to foragers. Our results suggest that the inactivity of some individuals is not due to a lack of sensitivity to surrounding stimuli. Furthermore, their high propensity to interact with larvae supports the hypothesis of a “reserve caste” function, where these workers become active during periods of increased colony workload.