Individual variation in exploratory behaviour improves speed and accuracy of collective nest selection by Argentine ants

Abstract

Collective behaviours are influenced by the behavioural composition of the group. For example, a collective behaviour may emerge from the average behaviour of the group's constituents, or be driven by a few key individuals that catalyse the behaviour of others in the group. When ant colonies collectively relocate to a new nest site, there is an inherent trade-off between the speed and accuracy of their decision of where to move due to the time it takes to gather information. Thus, variation among workers in exploratory behaviour, which allows gathering information about potential new nest sites, may impact the ability of a colony to move quickly into a suitable new nest. The invasive Argentine ant, Linepithema humile, expands its range locally through the dispersal and establishment of propagules: groups of ants and queens. We examine whether the success of these groups in rapidly finding a suitable nest site is affected by their behavioural composition. We compared nest choice speed and accuracy among groups of all-exploratory, all-nonexploratory and half-exploratory-half-nonexploratory individuals. We show that exploratory individuals improve both the speed and accuracy of collective nest choice, and that exploratory individuals have additive, not synergistic, effects on nest site selection. By integrating an examination of behaviour into the study of invasive species we shed light on the mechanisms that impact the progression of invasion.

Keywords: Linepithema humile; behavioural syndrome; collective behaviour; dispersal; group composition; individual variation; personality; range expansion; relocation.

Figure 1

Determining exploration of individuals. (a) The eight-arm maze used to determine worker exploratory behaviour. The ant being tested is circled in red. (b) Distribution of the number of spices visited in preliminary trials. Ants that visited one spice or no spices (white bars) were defined as nonexploratory, and ants that visited two or more spices (grey bars) were defined as exploratory.

Figure 2

T-maze used to examine group nest choice. Dashed lines indicate the locations where we measured crossings between the junction and the various compartments.

Figure 3

Investigative behaviour of all-exploratory, half-exploratory–half-nonexploratory and all-nonexploratory groups of ants. Different letters indicate significant differences using a post hoc Tukey test (P<0.05). Box plots: boxes indicate the lower and upper quartiles; horizontal lines within boxes indicate the median, whiskers extend to the 1.5 interquartile range from the box, and open circles indicate outliers.

Figure 4

(a) Number of visits from the T junction to covered (dark boxes) or uncovered (open boxes) alternative nest sites by groups of all-exploratory, half-exploratory–half-nonexploratory and all-nonexploratory ants. Box plots as in Fig. 3. *P <0.05 (paired Wilcoxon signed-ranks test). (b–d) Representative examples of the trajectories of (b) all-exploratory, (c) half-and-half and (d) all-nonexploratory groups. Images show the density of the trajectories along the T-maze throughout the trial; brightest lines indicate the highest density of trajectories for that particular trial.

Figure 5

Investigation speed of all-exploratory, half-exploratory–half-nonexploratory and all-nonexploratory groups of ants. Different letters indicate significant differences using a post hoc Tukey test (P <0.05). Box plots as in Fig. 3.