Why and how to apply Weber's Law to coevolution and mimicry

Abstract

In mimicry systems, receivers discriminate between the stimuli of models and mimics. Weber's Law of proportional processing states that receiver discrimination is based on proportional, not absolute, differences between stimuli. Weber's Law operates in a variety of taxa and modalities, yet it has largely been ignored in the context of mimicry, despite its potential relevance to whether receivers can discriminate models from mimics. Specifically, Weber's Law implies that for a given difference in stimulus magnitude between a model and mimic, as stimulus magnitudes increase, the mimic will be less discriminable from their model. This implies that mimics should benefit when stimulus magnitudes are high, and that high stimulus magnitudes will reduce selection for mimetic fidelity. Whether models benefit from high stimulus magnitudes depends on whether mimicry is honest or deceptive. We present four testable predictions about evolutionary trajectories of models and mimics based on this logic. We then provide a framework for testing whether receiver discrimination adheres to Weber's Law and illustrate it using coevolutionary examples and case studies from avian brood parasitism. We conclude that, when studying mimicry systems, researchers should consider whether receiver perception conforms to Weber's Law, because it could drive stimulus evolution in counterintuitive directions.

Visual complexity of egg patterns predicts egg rejection according to Weber's law

“…Although we studied complexity because complex patterning remains understudied, Weber's law could also apply to other traits processed by hosts, for which it must be tested independently. For Weber's law to apply, traits must have certain properties such as having quantitative magnitudes on continuous or quasi-continuous scales [ 26 ]. This rules out traits such as colour differences between host and parasitic eggs, since although differences in colour between eggs can be calculated, an egg does not have a quantifiable (i.e.…”

“…Weber's law is known to be evolutionarily relevant in foraging and mate choice [ 27 , 56 , 57 ], but this is one of the first demonstrations of Weber's law applying to receiver discrimination in a coevolutionary system. In coevolutionary systems, receiver perception may influence the evolution of both models and mimics, and can inform expectations about evolutionary trajectories of multiple interacting species [ 26 ]. Weber's law states that discriminability declines as stimulus magnitudes increase, because for a given absolute stimulus difference, the relative stimulus difference is greater at a low-stimulus magnitude.…”

“…low magnitude of complexity) will be more easily discriminated than two complex phenotypes (i.e. high magnitude of complexity) [ 26 ].…”

“…hosts) benefit from increased discriminability in their stimuli. Therefore, under Weber's law, models (here, host eggs) should benefit from lower stimulus magnitudes since lower magnitude stimuli are easier to discriminate [ 26 ]. If complexity is a stimulus that is used by hosts in decision-making, therefore, Weber's law could provide a second mechanism selecting against the evolution of highly complex signatures in hosts ( figure 1 a ).…”

“…We carried out field experiments on host rejection of foreign eggs and used these to compute a biologically relevant complexity score based on higher-level attributes of pattern features quantified by a computer vision tool for analysing visual patterns, N ature P attern M atch [ 19 ]. We then tested whether Weber's law applies to how hosts process complexity, using the framework proposed in [ 26 ]. Finally, we tested for differences in pattern complexity between prinia and cuckoo finch eggs, to assess whether visual complexity provides useful information to hosts in detecting a parasitic egg, and to determine whether high complexity should be costly for hosts.…”