Symmetry breaking and pivotal individuals during the reunification of ant colonies

Abstract

Maintenance of a social group requires the ability to reach consensus when faced with divisive choices. Thus, when migrating colonies of the ant Temnothorax rugatulus split among multiple sites, they can later reunify on the basis of queen location or differences in site quality. In this study, we found that colonies can reunify even without obvious cues to break the symmetry between sites. To learn how they do so, we observed both symmetric reunifications (between identical nests) and asymmetric reunifications (between nests of unequal quality) by colonies of individually marked ants. Both reunification types were accomplished by a tiny minority that carried nestmates from the 'losing' to the 'winning' site. Reunification effort was highly skewed in asymmetric splits, where the majority of the work was done by the first ant to transport, which nearly always came from the winning site. This contrasted with symmetric splits, where the initiator did not play an outsize role and was just as likely to come from the losing site. Symmetric reunifications were also characterized by high transporter attrition, which may help to prevent deadlocks. Tandem runs were abundant in both types and were typically led by transporters as they returned to the losing site to fetch another nestmate. Few tandem followers joined the transport effort, suggesting that tandem runs do not serve to recruit transporters but may have another, as yet unidentified role. Our results underscore the potentially large contribution of highly active individuals to group behaviour, even in decentralized societies such as ant colonies.

Ants resort to majority concession to reach democratic consensus in the presence of a persistent minority

“…73,74 In particular, our study species features a caste of significantly larger soldier ants, and this may lead to the natural scenario described above. In such a scenario, as in our experiments, a colony reunification in the better nest at a later time (as described in other studies 38,[75][76][77] ) is impossible. Rather, our findings lead us to predict that under these circumstances, the majority of ants would concede the better choice and unite with the rest in an inferior nest.…”

The Power of Social Information in Ant-Colony House-Hunting: A Computational Modeling Approach

“…Experimental studies on the speed-accuracy trade-off showed that colonies often split in urgent emigrations, but they also noted that split colonies were eventually able to reunite (Franks et al, 2003a, 2009). Later studies (Doering and Pratt, 2019, 2016; Doering et al, 2020) showed that artificially divided colonies readily re-unite, using the same behavioral tools as in emigrations, but relying more on the efforts of a small group of active workers. These findings suggest that emigrations depend on a mixture of individual and colony-level decision making.…”

“…Although successful, models of this process have been limited to the simple challenge of choosing between two distinct and equidistant nests in a con-trolled laboratory environment. Real colonies face more complex scenarios, such as selecting among several sites of varying quality, avoiding splits when candidate nest sites are identical, and resolving colony splits when they occur (Doering and Pratt, 2019;Sasaki and Pratt, 2012). It also remains unclear how the colony maintains high performance with noisy and heterogeneous individuals, and how individuals modify their behavior to account for changes in context or colony state.…”

The Power of Population Effect in Temnothorax Ant House-Hunting: A Computational Modeling Approach

“…Consequently, most model predictions have been limited to the simple challenge of choosing between two distinct and equidistant nests in a controlled laboratory environment. Real colonies face more complex scenarios, such as selecting among several sites of varying quality, avoiding splits when candidate nest sites are identical, and resolving colony splits when they occur (Sasaki and Pratt, 2012;Doering and Pratt, 2019).…”