Why do house-hunting ants recruit in both directions?

Abstract

To perform tasks, organisms often use multiple procedures. Explaining the breadth of such behavioural repertoires is not always straightforward. During house hunting, colonies of Temnothorax albipennis ants use a range of behaviours to organise their emigrations. In particular, the ants use tandem running to recruit naïve ants to potential nest sites. Initially, they use forward tandem runs (FTRs) in which one leader takes a single follower along the route from the old nest to the new one. Later, they use reverse tandem runs (RTRs) in the opposite direction. Tandem runs are used to teach active ants the route between the nests, so that they can be involved quickly in nest evaluation and subsequent recruitment. When a quorum of decision-makers at the new nest is reached, they switch to carrying nestmates. This is three times faster than tandem running. As a rule, having more FTRs early should thus mean faster emigrations, thereby reducing the colony's vulnerability. So why do ants use RTRs, which are both slow and late? It would seem quicker and simpler for the ants to use more FTRs (and higher quorums) to have enough knowledgeable ants to do all the carrying. In this study, we present the first testable theoretical explanation for the role of RTRs. We set out to find the theoretically fastest emigration strategy for a set of emigration conditions. We conclude that RTRs can have a positive effect on emigration speed if FTRs are limited. In these cases, low quorums together with lots of reverse tandem running give the fastest emigration.

Fig. 1

Diagrams of the two different models for which reverse tandem runs are hypothesised to increase speed. FTR Recruitment through forward tandem running; RTR recruitment through reverse tandem running. The parameters are explained in Table 1

Fig. 2

Optimal fractions of post-quorum time spent on reverse tandem runs (top figures), and optimal quorum thresholds (bottom figures) for models 1 (left two columns) and 2 (right two columns) for varying fractions of active ants F and scouting probabilities μ and for two values of recruitment latencies k. Other parameter values used are given in Table 1. See text for simulation details. For both values of k, reverse tandem runs are part of the optimal emigration strategy when scouting probability is high or when fraction of active ants is low. Lowering k enhances the use of reverse tandem running

Fig. 3

Numbers of forward and reverse tandem runs, number of carried ants and emigration time, computed for each of the optimal strategies for models 1 (top row) and 2 (bottom row), illustrated in Fig. 2. See that figure for details and parameter choices. Here we have only illustrated k = 0.001

Fig. 4

Examples of temporal dynamics for models 1 and 2. At μ = 0.05, F = 0.1447, we have taken parameters optimal for models 1 and 2, respectively. Notice that, in model 1, the number of recruiters rises to about 35, but in model 2, there are no less than 100 recruiters at the end of the emigration, indicating that recruitment from the carried class (model 2) may give rise to very unrealistic emigration dynamics