Synchronized Activity Rhythms and Collective Motion in Ant Colonies

Abstract

The ability to synchronize both the movements and activity rhythms of many interacting individuals is a major component of the repertoire of collective behaviour in social species. Among the many social animals on Earth, ants have some of the most spectacular forms of synchronized behaviour because of their eusociality. One example of this impressive behavioral synchronization in ants is the so-called short-term activity cycle, where the workers inside a nest will move together in rhythmic pulses of activity that are separated by moments of collective rest and inactivity. There are many aspects of short-term activity cycles that are poorly understood. The first chapter of this thesis makes the case for why ants are a useful taxonomic group for studying social synchronization and introduces the phenomenon of ant short-term activity cycles. In chapter 2, I study intraspecific variation in short-term activity cycles and show that colonies’ queens, number of brood items, and number of workers all influence aspects of colony-level activity oscillations. In chapter 3, I show that colonies are capable of synchronizing despite noise in the behaviour of individual ants, and that colonies can modulate between multiple collective rhythms. In the same chapter, I also devised a model to study the role of noise more generally in excitable systems that have properties similar to those in ant colonies. Chapter 4 examined the effect of 24-hour light cycles on circadian activity, and I found that colonies kept in conditions with constant light or darkness had weaker circadian rhythms than colonies in an alternating light/dark regime. However, colonies’ short-term activity cycles were not affected by the external light regime. In chapter 5, I provide evidence that having synchronized rhythms of rest and activity provides a functional benefit for colonies: workers can, on average, more fully inspect the inside of the nest when activity is synchronized because inactive ants aggregate into piles that active ants have difficulty penetrating. In chapter 6, I recount a history of the research into synchronized and rhythmic activity patterns inside ant nests and consolidate the new results from the previous chapters with the existing literature to create an up-to-date review of what we currently know about short-term activity cycles. The work compiled here thereby sheds new light on an enigmatic form of synchronized behaviour in ants, one of the most ecologically important groups of animals on the planet.