Noise resistant synchronization and collective rhythm switching in a model of animal group locomotion

Abstract

Biology is suffused with rhythmic behaviour, and interacting biological oscillators often synchronize their rhythms with one another. Colonies of some ant species are able to synchronize their activity to fall into coherent bursts, but models of this phenomenon have neglected the potential effects of intrinsic noise and interspecific differences in individual-level behaviour. We investigated the individual and collective activity patterns of two Leptothorax ant species. We show that in one species ( Leptothorax sp. W), ants converge onto rhythmic cycles of synchronized collective activity with a period of about 20 min. A second species ( Leptothorax crassipilis ) exhibits more complex collective dynamics, where dominant collective cycle periods range from 16 min to 2.8 h. Recordings that last 35 h reveal that, in both species, the same colony can exhibit multiple oscillation frequencies. We observe that workers of both species can be stimulated by nest-mates to become active after a refractory resting period, but the durations of refractory periods differ between the species and can be highly variable. We model the emergence of synchronized rhythms using an agent-based model informed by our empirical data. This simple model successfully generates synchronized group oscillations despite the addition of noise to ants' refractory periods. We also find that adding noise reduces the likelihood that the model will spontaneously switch between distinct collective cycle frequencies.

Synchronized locomotion can improve spatial accessibility inside ant colonies

“…For example, if an ant leaves the nest to forage between frames or if an ant climbs on top of another ant, then the total number of ants across a given frame pair will not be identical. Despite these unavoidable limitations, this automated method of tracking collective activity through image binarization can reliably detect when bursts of activity are occurring in a Leptothorax nest, and it has been applied in many previous studies [18][19][20][21]24,33]. In fact, the period estimates of colonies using this method are in very close agreement to those obtained using the more accurate (but also more invasive, time-consuming and costly [34]) method of tracking individually tagged ants [23].…”

“…All recordings were started in the afternoon between 14.30 and 16.00. We obtained colony-level activity time series of the approximate proportion of ants in a colony that were moving over time (electronic supplementary material, video S1) using the same methods employed in several previous studies [18][19][20]24]. Our tracking algorithm extracts frames from a video recording every 30 s and then creates binary images by using adaptive thresholding to segment ants inside the nest from the comparatively lighter background.…”

“…Wavelet analysis allows for the detection of periodic components at multiple points in a time series even if some non-stationarity is present [35]. It is often applied to biological rhythms [35][36][37], including the ant collective activity oscillations being studied here [19,20,23]. Briefly, we used the continuous wavelet transform function in MATLAB (cwt) to calculate the wavelet magnitudes of different oscillation frequencies at every data point in a time series (for additional details on this method, see the work of Doering and colleagues [19,20]).…”

“…It is often applied to biological rhythms [35][36][37], including the ant collective activity oscillations being studied here [19,20,23]. Briefly, we used the continuous wavelet transform function in MATLAB (cwt) to calculate the wavelet magnitudes of different oscillation frequencies at every data point in a time series (for additional details on this method, see the work of Doering and colleagues [19,20]). The oscillation frequency associated with the highest wavelet magnitude (i.e.…”

“…Temnothorax, Leptothorax, Myrmica, Solenopsis) [15][16][17]. These synchronized activity cycles have been most intensively studied in the twin genera Temnothorax and Leptothorax [15][16][17][18][19][20][21]. Leptothorax species form small colonies typically made up of only a few hundred workers [22].…”