The current and future state of animal coloration research

Abstract

Animal colour patterns are a model system for understanding evolution because they are unusually accessible for study and experimental manipulation. This is possible because their functions are readily identifiable. In this final paper of the symposium we provide a diagram of the processes affecting colour patterns and use this to summarize their functions and put the other papers in a broad context. This allows us to identify significant 'holes' in the field that only become obvious when we see the processes affecting colour patterns, and their interactions, as a whole. We make suggestions about new directions of research that will enhance our understanding of both the evolution of colour patterns and visual signalling but also illuminate how the evolution of multiple interacting traits works.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Keywords: animal colour patterns; colour pattern evolution; colour pattern functions.

Figure 1.

The functional (solid arrows) and evolutionary (dashed arrows) interactions among colour patterns, colour pattern–based behaviour, and the state of the receiver. The evolutionary link between fitness and reflectance is direct and the link to irradiance is through evolution of microhabitat choice. There are also evolutionary links to all of the other processes (sensory drive [1]). Note that this diagram makes no distinction between physical, neurophysiological or perceptual mechanisms and does not assume that they occur at the same level of organization or complexity. All these mechanisms occur during colour pattern function and hence all affect the mode and rate of colour pattern evolution. One could argue about the order of events between reception and decision-making, or even whether the processes occur in serial (as shown) or parallel. This diagram is intended more as a way of identifying understudied areas than an exact representation of the interactions. Understudied areas with respect to colour patterns are marked with asterisks.

Figure 2.

Signal detection. X represents the signal channel output value and f the frequency of occurrence of that value. X can be as simple as intensity or be a multivariate function of multiple properties of a colour pattern. X_a and X_b are mean values of two visual stimuli and N are the visual noise (background, channel and receptor noise). Note that N do not have to be equal as shown in this figure. X_b is the signal of interest and X_a is either the visual background or a second colour pattern, depending upon whether we consider detection or discrimination. The signal/noise ratio of detection or discrimination is (X_b − X_a)/N = S/N; the larger the S/N the easier the detection. X_t is a threshold value above which the detection system confirms detection and below which it accepts the null hypothesis of no detection. The use of X_t results in two forms of errors: I (false alarm) and II (missed detection), and systems should evolve to minimize both kinds of errors, depending upon their relative costs [32]. The detection criterion can be a threshold (X_t), but can also be an increasing (dashed line K) or decreasing function of X, as in mate choice.