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

Beetle iridescence induces an avoidance response in naïve avian predators

It has recently been found that iridescence, a taxonomically widespread form of animal coloration defined by a change in hue with viewing angle, can act as a highly effective form of camouflage. However, little is known about whether iridescence can confer a survival benefit to prey postdetection and, if so, which optical properties of iridescent prey are important for this putative protective function. Here, we tested the effects of both iridescence and surface gloss (i.e. specular reflection) on the attack behaviour of prey-naïve avian predators. Using real and artificial jewel beetle, Sternocera aequisignata, wing cases, we found that iridescence provides initial protection against avian predation by significantly reducing the willingness to attack. Importantly, we found that the main factor explaining this aversion is iridescence, not multiple colours per se, with surface gloss also having an independent effect. Our results are important because they demonstrate that even when prey are presented up close and against a mismatching background, iridescence may confer a survival benefit by inducing hesitation or even, as sometimes observed, an aversion response in attacking birds. Furthermore, this means that even postdetection, prey do not necessarily need to have secondary defences such as sharp spines or toxins for iridescence to have a protective effect. Taken together, our results suggest that reduced avian predation could facilitate the initial evolution of iridescence in many species of insects and that it is the defining feature of iridescence, its colour changeability, that is important for this effect.

Variation in predator response to short-wavelength warning coloration

Warning coloration deters predators from attacking unpalatable prey, and is often characterized by long-wavelength colors, such as orange and red. However, warning colors in nature are more diverse and include short-wavelength colors, like blue. Blue has evolved as a primary defense in some animals but is not common. One hypothesis for the maintenance of this diversity is interspecific variation in predator responses to signals. We tested this hypothesis with galliform birds: Gambel's quail (Callipepla gambelii) and two domestic chicken breeds (Gallus gallus domesticus; Plymouth Rocks, Cochin Bantams). We measured innate avoidance and learning responses to only blue prey, only orange prey, and orange-and-blue prey, where the blue was iridescent to represent the natural coloration of the pipevine swallowtail butterfly (Battus philenor). We predicted birds would have similar responses to orange, but vary in response to blue. Upon first encounter, Cochin Bantams did not attack blue and Gambel's quail readily attacked, indicating innate avoidance by Cochin Bantams. Plymouth Rocks had no innate aversion to any color, lower attack latencies and attacked most prey items. Cochin Bantams and Gambel's quail both learned orange and orange-and-blue quicker than blue. Our results support the hypothesis that interspecific variation in predator response could maintain warning color diversity.

Signal or cue: the role of structural colors in flower pollination

Angle dependent colors, such as iridescence, are produced by structures present on flower petals changing their visual appearance. These colors have been proposed to act as signals for plant–insect communication. However, there is a paucity of behavioral data to allow for interpretations of how to classify these colors either as a signal or a cue when considering the natural conditions under which pollination occurs. We sampled flowers from 6 plant species across various viewpoints looking for changes in the visual appearance of the petals. Spectral characteristics were measured with different instruments to simulate both the spectral and spatial characteristics of honeybee’s vision. We show the presence of color patches produced by angle dependent effects on the petals and the calyx of various species; however, the appearance of the angle dependent color patches significantly varies with viewpoint and would only be resolved by the insect eye at close distances. Behavior experiments with honeybees revealed that pollinators did not use angle dependent colors to drive behavior when presented with novel flower presentations. Results show that angle dependent colors do not comply with the requirements of a signal for plant–pollinator communication since the information transmitted by these colors would be unreliable for potential, free-flying pollination vectors. We thus classify angle dependent colors produced by micro- and ultra-structures as being a cue (a feature which has not evolved for communication), and observe no evidence supporting claims of these angle dependent colors having evolved as visual signal.

Learning of salient prey traits explains Batesian mimicry evolution

Batesian mimicry evolution involves an initial major mutation that produces a rough resemblance to the model, followed by smaller improving changes. To examine the learning psychology of this process, we applied established ideas about mimicry in Papilio polyxenes asterius of the model Battus philenor. We performed experiments with wild birds as predators and butterfly wings as semiartificial prey. Wings of hybrids of P. p. asterius and Papilio machaon were used to approximate the first mutant, with melanism as the hypothesized first mimetic trait. Based on previous results about learning psychology and imperfect mimicry, we predicted that: melanism should have high salience (i.e., being noticeable and prominent), meaning that predators readily discriminate a melanistic mutant from appearances similar to P. machaon; the difference between the first mutant and the model should have intermediate salience to allow further improvement of mimicry; and the final difference in appearance between P. p. asterius and B. philenor should have very low salience, causing improvement to level off. Our results supported both the traditional hypothesis and all our predictions about relative salience. We conclude that there is good agreement between long-held ideas about how Batesian mimicry evolves and recent insights from learning psychology about the role of salience in mimicry evolution.

Cholesteric liquid crystals in living matter

Liquid crystals play an important role in biology because the combination of order and mobility is a basic requirement for self-organisation and structure formation in living systems. Cholesteric liquid crystals are omnipresent in living matter under both in vivo and in vitro conditions and address the major types of molecules essential to life. In the animal and plant kingdoms, the cholesteric structure is a recurring design, suggesting a convergent evolution to an optimised left-handed helix. Herein, we review the recent advances in the cholesteric organisation of DNA, chromatin, chitin, cellulose, collagen, viruses, silk and cholesterol ester deposition in atherosclerosis. Cholesteric structures can be found in bacteriophages, archaea, eukaryotes, bacterial nucleoids, chromosomes of unicellular algae, sperm nuclei of many vertebrates, cuticles of crustaceans and insects, bone, tendon, cornea, fish scales and scutes, cuttlebone and squid pens, plant cell walls, virus suspensions, silk produced by spiders and silkworms, and arterial wall lesions. This article specifically aims at describing the consequences of the cholesteric geometry in living matter, which are far from being fully defined and understood, and discusses various perspectives. The roles and functions of biological cholesteric liquid crystals include maximisation of packing efficiency, morphogenesis, mechanical stability, optical information, radiation protection and evolution pressure.