Data from: Weber's law, the magnitude effect and discrimination of sugar concentrations in nectar-feeding animals

Weber’s law quantifies the perception of difference between stimuli. For
instance, it can explain why we are less likely to detect the removal of
three nuts from a bowl if the bowl is full than if it is nearly empty.
This is an example of the magnitude effect – the phenomenon that the
subjective perception of a linear difference between a pair of stimuli
progressively diminishes when the average magnitude of the stimuli
increases. Although discrimination performances of both human and animal
subjects in various sensory modalities exhibit the magnitude effect,
results sometimes systematically deviate from the quantitative predictions
based on Weber’s law. An attempt to reformulate the law to better fit data
from acoustic discrimination tasks has been dubbed the “near-miss to
Weber’s law”. Here, we tested the gustatory discrimination performance of
nectar-feeding bats (Glossophaga soricina), in order to investigate
whether the original version of Weber’s law accurately predicts choice
behavior in a two-alternative forced choice task. As expected, bats either
preferred the sweeter of the two options or showed no preference. In 4 out
of 6 bats the near-miss to Weber’s law provided a better fit and Weber’s
law underestimated the magnitude effect. In order to test the generality
of this observation in nectar-feeders, we reviewed previously published
data on bats, hummingbirds, honeybees, and bumblebees. In all groups of
animals the near-miss to Weber’s law provided better fits than Weber’s
law. Furthermore, whereas the magnitude effect was stronger than predicted
by Weber’s law in vertebrates, it was weaker than predicted in insects.
Thus nectar-feeding vertebrates and insects seem to differ in how their
choice behavior changes as sugar concentration is increased. We discuss
the ecological and evolutionary implications of the observed patterns of
sugar concentration discrimination.