In search of an honest butterfly: Sexually selected wing coloration and reproductive traits from wild populations of the Cabbage White Butterfly

Abstract
Sexual selection is central to many theories on mate selection and individual behavior. Relatively little is known, however, about the impacts that human-induced rapid environmental change are having on secondary sexually selected characteristics. Honest signals function as an indicator of mate quality when there are differences in nutrient acquisition and are thus potentially sensitive to anthropogenically altered nutrient inputs. We used the cabbage white butterfly, Pieris rapae (L.) (Lepidoptera: Pieridae), to investigate differences in color and testes size in a system that is often exposed to agricultural landscapes with nitrogen addition. We collected individuals from four sites in California and Nevada to investigate variation in key traits and the possibility that any relationship between wing color and a reproductive trait (testes size) could vary among locations in the focal butterfly. Coloration variables and testes size were positively albeit weakly associated across sites, consistent with the hypothesis that females could use nitrogen-based coloration in the cabbage white as an indicator for a male mating trait that has the potential to confer elevated mating success in progeny. However, variation in testes size and in the relationship between testes size and wing color suggest complexities that need exploration, including the possibility that the signal is not of equal value in all populations. Thus these results advance our understanding of complex relationships among environmental change and sexual selection in the wild.

Methods
Individual butterflies were collected from four different wild population sites in northern Nevada and northern California (“Site”). Wing reflectance data for male individuals were calculated from raw spectra data and summarized in the data set provided. These variables (R50, LR50, and BR50) were calculated as described in Morehouse, N. I., & Rutowski, R. L. (2010), Tigreros (2013) and Espeset et al. (2019). Wing lengths were measured using imageJ software, and testes weights were dissected from male individuals and weighed. Spermatophores were dissected and counted from females. Data was used in multiple linear models using R v.3.5.1.

Morehouse, N. I., & Rutowski, R. L. (2010). In the eyes of the beholders: Female choice and avian predation risk associated with an exaggerated male butterfly color. American Naturalist, 176(6), 768–784. https://doi.org/10.1086/657043

Tigreros, N. (2013). Linking nutrition and sexual selection across life stages in a model butterfly system. Functional Ecology, 27(1), 145–154. https://doi.org/10.1111/1365-2435.12006

Espeset, A., Kobiela, M. E., Sikkink, K. L., Pan, T., Roy, C., & Snell-Rood, E. C. (2019). Anthropogenic increases in nutrients alter sexual selection dynamics: A case study in butterflies. Behavioral Ecology, 30(3), 598–608. https://doi.org/10.1093/beheco/arz004