Short-term activity cycles in ants: generation of periodicity by worker interaction

Activity levels within ant colonies are monitored by using a solid-state automatically digitizing camera. The movement-activity levels of whole colonies and of isolated groups of workers are studied. Whole colonies of Leptothorax allardycei show rhythmic changes in movement-activity level. Fourier and autocorrelation analyses indicate that the activity levels of colonies are periodic, with an average period of 26 min. Single, isolated workers do not show the pattern of periodic changes in activity level. Single workers become active spontaneously, but at no particular interval. Pairs of workers, confined together, also do not show periodicity in activity level. One worker can stimulate another worker to become active, thus coupling their movement-activity patterns. As ants are placed in larger groups, the variation in the interval between activity peaks declines in a manner predicted by coupled-oscillator theory. It is argued that the colony can be regarded as a population of excitable subunits, and it is shown that the results of this paper can be explained in this light. The formal similarity between movement-activity patterns in ant colonies and the dynamics of epidemics is noted. It is further argued that periodicity in the movement-activity levels of the colony is probably an epiphenomenon rather than an adaptation. Selection at the colonial level probably acts on the spontaneity of activity and coupling strength to produce changes in the overall activity level of the colony. One of the patterns that can result is periodicity in the colony's activity level, but it is not the only possible outcome