Effects of temperature on monarch caterpillar pigment variation in nature

Abstract Insect colour patterns serve a wide range of ecological functions and the biotic and abiotic factors mediating colour variation in nature have been well characterized. However, the majority of studies in this field have focused on adult insects (particularly butterflies). Almost nothing is known about the factors that mediate intraspecific colour variation in juveniles in nature, even though they are often as conspicuously coloured as their adult counterparts. Here we show that temperature predicts a small but significant amount of monarch (Danaus plexippus) caterpillar pigment variation in nature. Over a 650,000 km2 region in Canada and the USA, caterpillars found in warmer locations or lower latitudes had thinner black stripes than those found in colder locations or higher latitudes. Caterpillars have also become less black over the last five years, a result consistent with observed short-term increases in summer temperature in this region. Our study demonstrates that the relationship between temperature and monarch caterpillar pigmentation seen in laboratory settings is also apparent in nature, although with considerable variation. Our study also highlights the utility of online biodiversity repositories such as iNaturalist for characterizing colour variation in nature.