Conspecifics as informers and competitors: an experimental study in foraging bumble-bees

Abstract

Conspecifics are usually considered competitors negatively affecting food intake rates. However, their presence can also inform about resource quality by providing inadvertent social information. Few studies have investigated whether foragers perceive conspecifics as informers or competitors. Here, we experimentally tested whether variation in the density of demonstrators ('none', 'low' and 'high'), whose location indicated flower profitability, affected decision-making of bumble-bees Bombus terrestris. Bumble-bees foraged on either 'simple' (two colours) or 'complex' (four colours) artificial floral communities. We found that conspecifics at low density may be used as sources of information in first flower choices, whereas they appeared as competitors over the whole foraging sequence. Low conspecific densities improved foragers' first-visit success rate in the simple environment, and decreased time to first landing, especially in the complex environment. High conspecific densities did not affect these behavioural parameters, but reduced flower constancy in both floral communities, which may alter the efficiency of pollinating visits. These results suggest that the balance of the costs and benefits of conspecific presence varies with foraging experience, floral community and density. Spatio-temporal scales could thus be an important determinant of social information use. This behavioural flexibility should allow bumble-bees to better exploit their environment.

Figure 1.

Examples of the spatial arrangements of flower patches for the (a) simple and (b) complex floral communities. Each community contained 40 flowers organized in eight patches of five similar flowers. ‘colony’ indicates the location of a beehive. The ‘simple’ floral community was composed of 20 high-quality (30% w/w sucrose solution) dark-blue (B) and 20 low-quality (10% w/w sucrose solution) light-blue (b) flowers. The ‘complex’ floral community was composed of 10 high-quality dark blue (B), 10 high-quality orange (O), 10 low-quality light blue (b) and 10 low-quality yellow (Y) flowers.

Figure 2.

Kaplan–Meier survival curves of the time to first landing for the (a) simple and (b) complex floral communities, depending on the density of demonstrators: ‘none’, ‘low’ and ‘high’. These discrete, stepped survivorship curves represent the cumulative proportion of flying bees before the first landing (‘non-landed bees’) over time. Solid lines indicate the distributions of bumble-bees foraging at none, dashed lines at low and dotted lines at high density of demonstrators. The two successive sequences were not individually represented (n = 10 individuals per experimental treatment).

Figure 3.

First-visit success rate of focal bumble-bees for the (a) simple and (b) complex floral communities, depending on the density of demonstrators: none, low and high. Values are the estimated mean ± s.e. predicted by the selected statistical model (generalized linear mixed effects model with repeated measures, lmer function, link = logit). The statistical model fitted empirical binary data corresponding to the vertical dashed lines: first visit to a high-quality flower at the top and first visit to a low-quality flower at the bottom of the plot. Grey vertical dashed lines represent the numbers of individual first visits for the first sequence and black ones for the second sequence (n = 10 individuals per experimental treatment).

Figure 4.

Flower constancy (mean ± s.e.) of focal bumble-bees on the (a) simple and (b) complex floral communities, depending on the density of demonstrators: none, low and high. The two successive sequences were not individually represented (n = 10 individuals per experimental treatment).