The psychophysics of sugar concentration discrimination and contrast evaluation in bumblebees

Abstract

The capacity to discriminate between choice options is crucial for a decision-maker to avoid unprofitable options. The physical properties of rewards are presumed to be represented on context-dependent, nonlinear cognitive scales that may systematically influence reward expectation and thus choice behavior. In this study, we investigated the discrimination performance of free-flying bumblebee workers (Bombus impatiens) in a choice between sucrose solutions with different concentrations. We conducted two-alternative free choice experiments on two B. impatiens colonies containing some electronically tagged bumblebees foraging at an array of computer-automated artificial flowers that recorded individual choices. We mimicked natural foraging conditions by allowing uncertainty in the probability of reward delivery while maintaining certainty in reward concentration. We used a Bayesian approach to fit psychometric functions, relating the strength of preference for the higher concentration option to the relative intensity of the presented stimuli. Psychometric analysis was performed on visitation data from individually marked bumblebees and pooled data from unmarked individuals. Bumblebees preferred the more concentrated sugar solutions at high stimulus intensities and showed no preference at low stimulus intensities. The obtained psychometric function is consistent with reward evaluation based on perceived concentration contrast between choices. We found no evidence that bumblebees reduce reward expectations upon experiencing non-rewarded visits. We compare psychometric function parameters between the bumblebee B. impatiens and the flower bat Glossophaga commissarisi and discuss the relevance of psychophysics for pollinator-exerted selection pressures on plants.

Weber’s Law, the Magnitude Effect and Discrimination of Sugar Concentrations in Nectar-Feeding Animals

“…In order to test whether the near-miss to Weber’s law provides a better fit to observed data than Weber’s law we individually fitted psychometric functions as Weibull sigmoid curves using the following equation [15], [23], [24]:where x is the larger and a is the smaller of the sugar concentrations of the test and standard feeders, m is the threshold, s is the slope at the threshold, π l is the lapse rate, and β is the exponent from Equation 4. In all models x and a were the independent variables, discrimination performance was the dependent variable, and m , s and π l were estimated parameters.…”

“…Here we analyzed the pooled data from all 23 bats. It has been demonstrated that this type of data pooling only causes an underestimation of the slope parameter, but does not affect the threshold and lapse rate [24]. The same pooling was done in the remaining analyzed studies as well, even if individual data were available, for better comparison between data sets.…”

“…Knowledge of gustatory information processing [7], [19], [20] is important for understanding the formation of economical food preferences [21]–[24] and may have implications for our understanding of the co-evolution of nectar rewards and animals’ discrimination abilities. Diverse groups of nectar-feeding animals such as bees [24]–[26], birds [27]–[29] and bats [23], [30], [31] show a general pattern of preference for sweeter sugar solutions and more precise discrimination at low concentrations.…”

“…Diverse groups of nectar-feeding animals such as bees [24]–[26], birds [27]–[29] and bats [23], [30], [31] show a general pattern of preference for sweeter sugar solutions and more precise discrimination at low concentrations. (Discrimination of nectar volume rewards also follows this pattern, so that choice proportions in 2AFC tests can be fitted against the relative intensity of the stimuli [22], [32], [33].)…”

Getting to the start line: how bumblebees and honeybees are visually guided towards their first floral contact

“…A metallic identification tag is glued to the thorax (see Fig. 2) and detected by a reader placed at strategic locations (Streit et al, 2003;Sumner et al, 2007;Ohashi et al, 2010;Stelzer and Chittka, 2010;Decourtye et al, 2011;Silcox et al, 2011;Nachev et al, 2012;Katzenberger et al, 2013). We have recently adapted the technology to detect flower-naïve bumblebees exploring unrewarding flowers (Orbán and Plowright, 2013).…”