Similar modes of communication can be applied to confer very different messages based on specific ecological contexts. And depending on the message – either ‘stay away’ or ‘come hither’ – the effectiveness of the communication signals can be crucial for survival and/or reproduction and are, therefore, subject to selection and evolution. The hindwings of the pipevine swallowtail butterfly, Battus philenor, have brilliant blue iridescent patches on both the dorsal (upper) and ventral (lower) surfaces. Both sexes display the larger ventral patches early and late in the day when the butterflies are at rest, and evidence suggests these patches function as predator deterring aposematic (warning colouration) signals, advertising the distastefulness of these butterflies. The dorsal patches are smaller and are far more intense in the males, suggesting that they are sexual signals, flashed by males at females in flight around midday. Because of the different ecological contexts of these iridescent communication signals one would also expect differences in the evolutionary processes that established these iridescent wing patches.
Ron Rutowski and colleagues, from Arizona State University analysed the reflectance spectra of wild and lab-reared butterflies' iridescent patches to test the differences in signals directed towards predators or potential mates. They made five predictions. (1) Ventral iridescence has greater intensity than dorsal iridescence because of lower light levels when warning off predators. (2) Ventral iridescence has longer wavelength hues because of more reddish light illuminating these patches in the early morning and late afternoon. (3) Ventral iridescence has greater angles of visibility than dorsal iridescence since the approach directions of predators are more variable than the in-flight directions when females view courting males. (4) Ventral iridescent colours in males are less variable than dorsal iridescence as the ventral patches are selected by predation and the dorsal patches by sexual selection. (5) Ventral iridescence (an anti-predator signal) is less dependent on rearing conditions than dorsal iridescence (a sexual signal).
Rutowski and colleagues tested their predictions using reflectance spectrophotometry and found that their data supported predictions 1 and 3. The ventral iridescence in both sexes, directed towards predators during low-light hours, was almost twice as intense and could be viewed over a wider range of angles than the dorsal iridescence, which is specifically directed toward females when the light is brighter.
By contrast, predictions 2 and 4 were not supported. The ventral and dorsal hues did not differ, and the dorsal iridescence in males was not more variable than the ventral iridescence – contrary to the expectation that the sexual signals may vary to allow females to choose the best mate. In fact, the ventral and dorsal colour parameters were closely correlated, possibly because the developmental processes driving iridescent wing-scale production are coupled in both patches, constraining colour differentiation and variability.
Finally, evidence supporting the fifth prediction was weak. Compared with wild caught specimens, lab rearing generally improved the animals' condition (larger body masses and longer wings); however, the lab conditions were possibly too constant to effect differences in lab animals' colour condition. Surprisingly, though, the dorsal patches in wild caught males were more intense than in the lab reared animals. Food quality could explain this but the authors warn that these results emphasise the need to broaden the scope of investigation of iridescent signals. For example, the dorsal patches may not be sexual signals signifying male quality but could aid in species recognition, as Battus philenor shares its habitat with several other species that mimic its iridescent colour patterns.
