Speed versus accuracy in decision-making ants: expediting politics and policy implementation

Abstract

Compromises between speed and accuracy are seemingly inevitable in decision-making when accuracy depends on time-consuming information gathering. In collective decision-making, such compromises are especially likely because information is shared to determine corporate policy. This political process will also take time. Speed-accuracy trade-offs occur among house-hunting rock ants, Temnothorax albipennis. A key aspect of their decision-making is quorum sensing in a potential new nest. Finding a sufficient number of nest-mates, i.e. a quorum threshold (QT), in a potential nest site indicates that many ants find it suitable. Quorum sensing collates information. However, the QT is also used as a switch, from recruitment of nest-mates to their new home by slow tandem running, to recruitment by carrying, which is three times faster. Although tandem running is slow, it effectively enables one successful ant to lead and teach another the route between the nests. Tandem running creates positive feedback; more and more ants are shown the way, as tandem followers become, in turn, tandem leaders. The resulting corps of trained ants can then quickly carry their nest-mates; but carried ants do not learn the route. Therefore, the QT seems to set both the amount of information gathered and the speed of the emigration. Low QTs might cause more errors and a slower emigration--the worst possible outcome. This possible paradox of quick decisions leading to slow implementation might be resolved if the ants could deploy another positive-feedback recruitment process when they have used a low QT. Reverse tandem runs occur after carrying has begun and lead ants back from the new nest to the old one. Here we show experimentally that reverse tandem runs can bring lost scouts into an active role in emigrations and can help to maintain high-speed emigrations. Thus, in rock ants, although quick decision-making and rapid implementation of choices are initially in opposition, a third recruitment method can restore rapid implementation after a snap decision. This work reveals a principle of widespread importance: the dynamics of collective decision-making (i.e. the politics) and the dynamics of policy implementation are sometimes intertwined, and only by analysing the mechanisms of both can we understand certain forms of adaptive organization.

Figure 1

Experimental arenas used to control dispersion of the ants. When the original nest has been destroyed, the second arena with the new nest site can be connected with an acetate bridge.

Figure 2

Experiment 1. FTRs disrupted in the treatment but not in the control. Mean numbers of adults in the (a) old and (b) new nests for colonies from the control (grey line) or treatment (black line) group as a function of time since the old nest was destroyed. The error bars represent the standard error to the mean. FTRs expedite emigrations.

Figure 3

Experiment 2. In the treatment, scouts were released in the arena with the new nest. In the control, scouts were released in the arena with the old nest. Mean numbers of individuals in the (a) old and (b) new nests for colonies from the control (grey line) or treatment (black line) group as a function of time since the old nest was destroyed. The error bars represent standard errors. In the treatment group, the ants used many reverse tandem runs. In the control group, the ants used many FTRs. RTRs can expedite emigrations.

Figure 4

Experiment 1. Box-plots of the number of successful (a) forward and (b) reverse tandem runs in control and treatment groups (10 colonies in each group). Horizontal lines within boxes are medians, boxes show inter-quartile ranges and whiskers show entire range (excluding the outliers represented by circles). The squares indicate means. ^***p<0.001.

Figure 5

Experiment 2. Box-plots of the number of successful (a) forward and (b) reverse tandem runs in control and treatment groups (10 colonies in each group). Interpretation of box-plots and symbols as in the legend of figure 4. ^**p<0.01, ^*p<0.05.

Figure 6

Experiment 2. The relationship between colony size and the number of active ants that left the old nest in the period between 5 and 20 min after the old nest had been opened. The relationship is best described by active scouts=5.95+0.3795 (colony size) (r²=0.77, p<0.001). The central line is the fitted regression line; also shown are the 95% confidence limits for this line and for the dataset. One outlier has been removed.