Optic flow informs distance but not profitability for honeybees

Abstract

How do flying insects monitor foraging efficiency? Honeybees (Apis mellifera) use optic flow information as an odometer to estimate distance travelled, but here we tested whether optic flow informs estimation of foraging costs also. Bees were trained to feeders in flight tunnels such that bees experienced the greatest optic flow en route to the feeder closest to the hive. Analyses of dance communication showed that, as expected, bees indicated the close feeder as being further, but they also indicated this feeder as the more profitable, and preferentially visited this feeder when given a choice. We show that honeybee estimates of foraging cost are not reliant on optic flow information. Rather, bees can assess distance and profitability independently and signal these aspects as separate elements of their dances. The optic flow signal is sensitive to the nature of the environment travelled by the bee, and is therefore not a good index of flight energetic costs, but it provides a good indication of distance travelled for purpose of navigation and communication, as long as the dancer and recruit travel similar routes. This study suggests an adaptive dual processing system in honeybees for communicating and navigating distance flown and for evaluating its energetic costs.

Figure 1.

Top view of the experimental tunnels, and front view of the horizontal and vertical grids marking the entrance to the tunnels. The long tunnel (LT) was 40 cm wide and the short tunnel (ST) was 10 cm wide (not drawn to scale). See text for details.

Figure 2.

Mean waggle duration (±s.e.) of dances for the short (n = 38 bees) and long (n = 41 bees) tunnels (t-test, t = 4.47, d.f. = 77, p < 0.0001). For 29 bees, we had data from both ST and LT. Considering these data only, waggle duration was significantly longer for the ST than for the LT (paired t-test: t = 4.7, d.f. = 28, p < 0.0001).

Figure 3.

Proportion of bees that visited the ST more often than the LT in the choice phase. Of the 136 individual bees in the experiment, 10 visited both tunnels equally. Of the remaining 126 bees, 63% visited the ST more than the long one during the choice phase (***: χ² likelihood ratio = 8.2, d.f. = 1, p = 0.004, n = 126). Considering just the 113 bees that had visited both tunnels during either the no-choice or choice phase 61% preferred the ST during the choice phase (**: χ² = 5.6, d.f. = 1, p = 0.018, n = 113). Considering just the 108 that had visited both tunnels during the no-choice phase, 59% preferred the ST during the choice phase (*: χ² = 3.7, d.f. = 1, p = 0.054, n = 108).

Figure 4.

We measured mean total number of dance circuits in the first bout of uninterrupted dancing for 65 bees. Boxes represent median and interquartile range for the ST (n = 49) and LT (n = 53). Asterisk: Wilcoxon signed-rank-sum test: S = 2872, p = 0.0196. For 37 bees, we had data from both ST and LT. Considering just these data, total number of dance circuits in all dance bouts measured was greater for the ST than for the LT (paired t-test: t = 2.38, d.f. = 36, p = 0.0226; Wilcoxon signed-rank-sum test: S = 131, p = 0.0296).