Daily activity time series of ants, Camponotus japonicus

Social insects often share tasks among individuals. In this study, we analyzed the foraging activity of ants (Camponotus japonicus) and recorded the daily passage event counts of individual workers between a nest chamber and a foraging arena using five monodomous colonies. We proposed two hypotheses on the time series of foraging frequency by individual worker ants:

(i) Regarding the time series of foraging frequency by individual worker ants, the foraging frequency on a certain day could be expressed by the product of the foraging frequency on the previous day and the exponential of a random number.
(ii) The random numbers are correlated between some pairs of worker ants.

The results for the five tested ant colonies showed that the probability of total daily passage counts (the sum of an individual's passage count) were characterized by a log-normal distribution. The worker ants behaved differently in terms of active days and foraging frequency. However, for >54% of the worker ants, the probability of the daily passage count was characterized by a log-normal distribution, and these worker ants performed >72% of the tasks in each colony. Furthermore, for >73% of the worker ants, the time development of passage count was confirmed to be mathematically modelled; the logarithmic first difference between the passage counts on a certain day and those on the previous day was a random normal variable. These results support hypothesis (i). Additionally, the random numbers, that were equivalent to the logarithmic first difference, were correlated for some pairs of worker ants. These results support hypothesis (ii).

Methods
Using an RFID tag system (SK-Electronics CO., Ltd., Japan), we obtained the count data for five colonies of ants, Camponotus japonicus, collected from the Higashi-Hiroshima campus of Hiroshima University. The colonies A, B, and C contained a queen ant, but the others, D and E, were without one. For each colony, almost 150 workers attached the RFID tag on their thoraxes with resin glue (Kiyohara UVR, KIYOHARA & Co., Ltd.) without exposure to CO2. The observation started one week after the workers attached RFID tags to avoid observing the adjustment state of workers. The RFID reader was installed on the tube's roof, connecting each colony's nest chamber and the foraging arena.