Small white or small ultraviolet butterfly?

Graced with a few black spots and a shade of yellow in their underwings, the plain wings of white butterflies contrast with the colourful and rich patterns of other butterflies. Unfortunately, this is a reflection of the limitations of our visual system. Male and female Small White butterflies, Pieris rapae, have different upper wing colours, especially in the ultraviolet (UV) spectrum: males absorb UV strongly but reflect most visible light, so they look a brighter white than females to us, but have a dark tone in the UV spectrum, which we cannot see.

A Small White bilateral gynandromorph, with visible female coloration on the left and male coloration on the right (top). Below, a false-colored representation of how these wing colors might be perceived by small whites, which are able to see UV light (photo courtesy of Nathan Morehouse)

Wing colour patterns in pierid butterflies, the family of the Small White, depend on beads made up of pigments called pterins, which are deposited on wing scales, and in the Small White are UV-absorbing. Males vary widely in their brightness and UV reflectance. Males able to produce more beads have darker UV patterns and appear brighter to our eyes. The amount of pterins a male can produce depend of the diet they had as caterpillars. Pterins need a lot of nitrogen to produce - they are the most nitrogen-rich pigment in animals - and the caterpillars can be nitrogen limited, so a brighter male is an indication of a good caterpillar diet. A brighter male also is also expected to be more desirable to a female, as during mating, male white butterflies transfer nutrients to the females in the form of infertile sperm, a nuptial gift which will increase the females life expectancy and therefore will be able to lay more eggs. A male with a higher quality diet can afford to be brighter and to produce bigger and more nutritious nuptial gifts.
 These butterflies are also able to see UV patterns, they have a rich repertoire of visual receptors in the short wavelength, including three types of photoreceptors that peak at UV, violet and blue. Crucially, being aware of their ability both to emit and detect UV allows us to understand their mating behaviour. Nathan Morehouse  and Ronald Rutowski carried out some elegant experiments to test the hypothesis that females choose males with more contrasting UV patterns. They carried out two experiments, in both of them they used lab-reared virgin females, which they presented with some males in a large cage. In the first experiment they used males that were from wild origin, and therefore had a natural range of colour patterns, in the second experiment they used laboratory reared males, but dipped the males' wings in a chemical that partially removes the beads, this way they created a range of variation that mimicked the natural one, but removing other possible confounding effects such as age, nutritional state, effects of encounters with predators or mating history. They removed the mated males to carry out measurements and they allow each female to mate three times. Then they also measured males that hadn't been mated. Females are able to exercise mate choice as they can adopt the "mate refusal posture" by which they lower their wings and raise their abdomen as shown in the photo below when they don't want to mate. Males showcase their wings during their ritualised courtship flight

Courting Small White butterflies. The female, underneath the male is rejecting him by lifting her abdomen and lowering her wings.

Their experimental results (below) together with further modeling of the female's visual system indicated that females, as expected, prefer to mate with brighter males. Sexual selection therefore favours brigher males.

Spectral reflectance of male and female Pieris rapae in study 1 (A) and study 2 (B). Average female phenotype (dashed lines), average mated male phenotype (solid lines), average unmated male phenotype (dashed and dotted lines), and minimum and maximum male values (dotted lines) are displayed. Histograms to the right are for all males, with the proportion of mated males (solid region) and unmated males (open region) indicated in each bin (from Morehouse & Rutowski 2010).

 However, not everything is rosy (or ultraviolet if you wish) for these males, as the same colour patterns that make males more attractive to females, make them more visible to their main predators, birds, which are also able to see in the UV range. This study shows how sexual selection and natural selection through predators can work in an opposite direction, and reminds us that beauty is in the eye of the beholder.

References

Morehouse NI, & Rutowski RL (2010). In the eyes of the beholders: Female choice and avian predation risk associated with an exaggerated male butterfly color. The American naturalist, 176 (6), 768-84 PMID: 20942644