Effect of the land area elevation on the collective choice in ants

Abstract

Collective decisions regarding food source exploitation in social insects are influenced by a range of parameters, from source quality to individual preference and social information sharing. Those regarding the elevation of the physical trail towards a food source have been neglected. In this work, we investigated the effect of ascending and descending paths from the nest to a food source on collective choice in two ant species Lasius niger and Myrmica rubra. Our hypothesis that returning loaded with food from the high source is more energy efficient was validated by choice experiments: when the sources are simultaneously introduced the high food source is preferentially exploited by both species. The flexibility of colony response was then tested by introducing the preferred source (high) incidentally, after recruitment towards the down food source began. Despite the well-known lack of flexibility of L. niger, both species showed the ability to reallocate their foraging workforce towards the highest food source. The collective choice and the flexibility are based on the difference between the u-turn rates when foragers are facing the ascending or descending branch. We discuss these results in terms of species-specifics characteristics and ecological context.

Figure 1

Cumulative number of foragers at the two food sources for 2 h in each condition. Mean and standard deviation; numbers above bars = number of replicates. DD = Down-Down, control condition, two descending paths to food sources. HH = High-High, control condition, two ascending paths to food sources. HD = High-Down, experimental condition, left path ascending to food source and right path descending to food source. DH = Down-High, experimental condition, left path descending to food source and right path ascending to food source. D → H = Down → High, experimental condition, only down food source available at the end of left path during the first 20 min of experiment before incidentally introduction of the high food source at the end of the right branch.

Figure 2

Strength of choice towards a food source. Percentage of experiments against percentage of total foragers present at the high food source for each experimental condition and species. For the D → H condition, as in Table 1, we separated the results into two phases, before and after the introduction of the second food source.

Figure 3

Discovery of and time spent at food sources. Mean and standard deviation; numbers above bars = N.

Figure 4

HD experimental setup. The nest (1) and its unique entry (2) facing the first branch of 15 cm leading to the T-maze with ascending and descending branches (HD setup shown). A syrup feeder (1 M sucrose) was placed on a platform at the end of either the ascending or descending branch of the maze. The number of ants was counted at each food source, and U-turns were measured at point (2) of the ascending branch and (4) for the descending one.