Warning Signal Efficacy: Assessing the Effects of Color, Iridescence, and Time of Day in the Field

Abstract

Warning coloration deters predators from attacking distasteful or toxic prey. Signal features that influence warning color effectiveness are not well understood, and in particular, we know very little about how effective short-wavelength and iridescent colors are as warning color elements in nature and how warning signal effectiveness changes throughout the day. We tested the effect of these factors on predation risk in nature using specimens of the distasteful pipevine swallowtail butterfly, Battus philenor. B. philenor adults display both iridescent blue and diffusely reflecting orange components in their warning signal. We painted B. philenor wings to create five different model types: all-black, only-iridescent-blue, onlyorange, iridescent-blue-and-orange (intact signal), and matte-blue-andorange. We placed 25 models in each of 14 replicate field sites for 72 h and checked for attacks three times each day. Model type affected the likelihood of attack; only-orange models were, the only model attacked significantly less than the all-black model. Iridescence did not enhance or decrease warning signal effectiveness in our experiment because matteblue-and-orange models were attacked at the same rate as iridescentblue-and-orange models. Time of day did not differentially affect model type. Video recordings of attacks revealed that insectivorous birds were responsible. The results of this experiment, when taken with previous work, indicate that the response to blue warning coloration is likely dependent on predator experience and context, but that iridescence per se does not affect warning signals in a natural context.

Differential Bird Responses to Colour Morphs of an Aposematic Leaf Beetle may Affect Variation in Morph Frequencies in Polymorphic Prey Populations

“…The combination of red and black elements is a textbook example of aposematic coloration (Endler and Mappes 2004), but the monotonous metallic colouration was as effective as light red-and-black pattern in our experiments in terms of predator avoidance learning and signal memorability. A metallic lustre is generally interpreted as a warning signal (Borer et al 2010), but studies of bird responses to iridescence are scarce and the findings are contradictory: some, including our study, have demonstrated that iridescence itself enhances avoidance learning, (Fabricant et al 2014;Waldron et al 2017) while others have reported no effects of prey iridescence on bird attack rates (Pegram et al 2015). This discrepancy may indicate that bird responses to iridescent coloration of prey are idiosyncratic.…”

Learning of salient prey traits explains Batesian mimicry evolution

“…More crosses, including backcrosses to disentangle genetic effects on melanism, orange and blue coloration, together with additional experiments with bird predators, would be needed to better estimate the relative importance of black, orange, and blue for the mimetic function. Experiments both in the laboratory and the field on the possible importance of orange and blue coloration for aposematism in B. philenor have shown that orange and blue both contribute to aposematism (Pegram et al 2013) or that orange possibly contributes more (Pegram et al 2015). Considering our experiments, melanism appears to be the dominating component of the B. philenor warning signal.…”

“…) or that orange possibly contributes more (Pegram et al. ). Considering our experiments, melanism appears to be the dominating component of the B. philenor warning signal.…”

“…Both of the major bird color vision systems, UVS and VS, are likely to be represented among insectivorous birds that may prey on B. philenor and its mimics, considering the bird species found to attack B. philenor in the field (Pegram et al. ). The vision system could influence the sensitivity to the blue coloration.…”

“…Blue tits are sensitive to ultraviolet light and have a Ultraviolet sensitive (UVS) color vision system (Ödeen and Håstad 2003;Endler and Mielke 2005;Hart and Hunt 2007;Lind et al 2014). Both of the major bird color vision systems, UVS and VS, are likely to be represented among insectivorous birds that may prey on B. philenor and its mimics, considering the bird species found to attack B. philenor in the field (Pegram et al 2015). The vision system could influence the sensitivity to the blue coloration.…”