The authors test Endler’s sensory drive hypothesis. They predict that in order to enhance conspicuousness of an important visual signal used in male-male competition, the coloration of male damselfly species will contrast most with the habitat during their peak activity period. The activity of 6 sympatric species of Enallagma was measured and irradiance spectra were formed to characterize ambient light at the hours of peak activity. To depict the coloration of males, reflectance spectra were taken from the meso- and metapleural surface of the thorax and abdominal segments of each individual. Reflectance of primary background vegetation on the pond surface and shore were recorded as well. Subsequently, the authors utilized these measurements to formulate mean values for chromatic and achromatic contrast under 2 different models of odonate vision: a trichromatic model and a tetrachromatic model.

They found that the 3 species most active during midday had peak reflectances between 430nm and 470nm (‘the blue species’). Similarly the 3 species most active near dusk, reached their max. reflectances near 700nm (‘long wavelength species’). All but one ‘long-wavelength species’ showed higher color contrast during their peak activity time under the tetrachromatic model of vision. All the ‘blue species’ showed higher contrast during their most active time regardless of the vision model used. Moreover, all but one species of damselfly studied experienced greatest achromatic contrast during their respective periods of highest activity. Thus, all but one species of damselfly demonstrated a drastic deviation of body color from background/ambient light conditions. Furthermore, the eyes of male ‘long-wavelength species’ have red, orange and yellow screening pigments that serve to amplify transmission of long-wavelengths while the males of ‘blue species’ lack these screening pigments This evidence further supports the idea that the visual systems of ‘blue species’ and ‘long wavelength species’ are specially tuned to their specific light environments.

What we learned in class about effective visual signaling is the basis of Endler’s theory of sensory drive. We discussed that the design and function of a signal relies heavily on that signal’s visual background; this background includes light form the sun, reflected light from nearby signals, and reflected light from the sky. Accordingly, for a signal to stick out and thus reach the receiver effectively, it should maximize contrast with this background. We learned that perception of contrast depends not only on the contrast of hue (the particular wavelength that is selectively reflected) but also on that of brightness which describes the intensity of a specific hue (contrast can also be enhanced by visual patterns or movement). This justifies the authors in measuring both chromatic and achromatic contrast of male coloration. Since the intensity and spectral composition of ambient light changes throughout the day, this means that maximal visual contrast is influenced by temporal variation of the habitat as well. Thus, the model of visual signal design helps explain the use of two different color signals in sympatric species of damselflies whose peak activity periods have divergent light environments.