Abstract
Theory suggests that aposematism, specifically the learned avoidance of unprofitable prey via memorable color patterns, should result in selection for pattern uniformity. However, many examples to the contrary are seen in nature. Conversely, honest sexual signals are likely to exhibit greater variation because they reflect underlying variation in mate quality. Here we aim to characterize and quantify the mechanistic causes of color in Tectocoris diopthalmus to shed light on the costs of color production, and thus the potential information content of its color signals. We use Tectocoris diopthalmus because it is a weakly-defended stinkbug, and presents elements that have classically been studied in the context of aposematism (red coloring), and sexual selection (sexual dichromatism and iridescent coloring). Pigment analysis reveals that variation in orange coloration is due to the amount of erythropterin pigment, stored in intracellular granules. This pigment is common in Heteroptera, and as an endogenously produced excretory byproduct is unlikely to reflect mate quality or variation in unprofitability of the bug. Electron microscopy reveals the iridescent patches are caused by an epicuticular multilayer reflector, and the hue and patch size are directly related to the layer widths and extent of coverage of this layering. Furthermore, we identified melanin as an essential component of the multilayer reflector system; therefore, the quality of the iridescent patches may be affected by aspects of rearing environment and immunocompetence. We posit that T. diopthalmus has co-opted the melanic patches of a ‘typical’ red and black aposematic signal, transforming it into a complex and variable iridescent signal that may enhance its capacity to display individual quality.
Highlights
Aposematism is the phenomenon wherein defended prey advertise unprofitability through conspicuous signals
Identification of color production mechanisms We identified two major color production mechanisms in Tectocoris diopthalmus, a red-orange pigment and a multilayer reflector structure
The structure exists in the outermost exoskeletal layer, while the pigment is contained in intracellular granules in the epidermis
Summary
Aposematism is the phenomenon wherein defended prey advertise unprofitability through conspicuous signals. Often these take the form of bright ‘warning colors’, as it is theorized that conspicuous coloration aids predator recognition and memory [1]. Theory suggests that aposematic patterns should be under strong frequency-dependent selection by predators to be monomorphic [4] [5], and/or strong directional selection for conspicuousness [6] [7]. Recent theoretical treatments have addressed possible causes of intermediate aposematism and intraspecies variation in pattern, via such mechanisms as predator community structure [8], or variation in prey defense levels [9]. Recent empirical studies have demonstrated possible trade-offs between warning signal quality and physiological traits such as toxin excretion costs [11] and pathogen resistance [12], It is possible than the mechanisms of aposematic signals themselves, and related physiological traits, are contributing to variation
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