Reward quality influences the development of learned olfactory biases in honeybees

Abstract

Plants produce flowers with complex visual and olfactory signals, but we know relatively little about the way that signals such as floral scents have evolved. One important factor that may direct the evolution of floral signals is a pollinator's ability to learn. When animals learn to associate two similar signals with different outcomes, biases in their responses to new signals can be formed. Here, we investigated whether or not pollinators develop learned biases towards floral scents that depend on nectar reward quality by training restrained honeybees to learn to associate two similar odour signals with different outcomes using a classical conditioning assay. Honeybees developed learned biases towards odours as a result of differential conditioning, and the extent to which an olfactory bias could be produced depended upon the difference in the quality of the nectar rewards experienced during conditioning. Our results suggest that differences in reward quality offered by flowers influence odour recognition by pollinators, which in turn could influence the evolution of floral scents in natural populations of co-flowering plants.

Figure 1

A comparison of the olfactory response function generated by associating two closely related odours with different outcomes (solid line, CS+/CS−, CS+=1.0 M sucrose and CS−=1.0 M salt, N=33). The response function formed simply by associating one odour with 1.0 M sucrose (dashed line, CS+, N=28) reveals that differential learning produces a bias towards novel olfactory stimuli (lreg, two-way interaction Χ₂²=7.03, p=0.030). For each response function, the ‘peak’ was significantly different from all other points along the gradient (comparisons performed within a gradient to the peak point only, one-tailed LSMC: p<0.05). The x-axis refers to the composition of the binary odour presented during the test; p_h refers to the proportion of 1-hexanol (p_h) present in the binary odour. The CS+ was always p_h=0.5 and the CS− was always p_h=0.7. The y-axis represents the probability of observing a conditioned response (proboscis extension) by honeybees towards a test odour.

Figure 2

The relative difference between outcomes experienced during differential conditioning affected the strength of a learned, olfactory bias. The graphs are comparisons of the olfactory response functions towards a gradient of binary odours produced during a test period after conditioning with two perceptually similar binary odours associated with different outcomes. In each graph, the dotted/dashed lines indicate the response functions from figure 1 (dotted line= CS+1.0 M sucrose and CS−1.0 M NaCl; dashed line=CS+1.0 M sucrose only); the axes are also the same as those in figure 1. The solid lines in each graph represent the response functions produced by differential conditioning with: (a) CS+ reinforced with 1.0 M sucrose containing 0.01 M proline and the CS− reinforced with 1.0 M salt (N=31); (b) the CS+ reinforced with 1.0 M sucrose and the CS− without reinforcement (CS−) (N=64); and (c) the CS+ reinforced with 1.0 M sucrose and the CS− reinforced with 0.3 M sucrose (N=59).