To quiver or to shiver: increased melanization benefits thermoregulation, but reduces warning signal efficacy in the wood tiger moth

Abstract

Melanin production is often considered costly, yet beneficial for thermoregulation. Studies of variation in melanization and the opposing selective forces that underlie its variability contribute greatly to understanding natural selection. We investigated whether melanization benefits are traded off with predation risk to promote observed local and geographical variation in the warning signal of adult male wood tiger moths (Parasemia plantaginis). Warning signal variation is predicted to reduce survival in aposematic species. However, in P. plantaginis, male hindwings are either yellow or white in Europe, and show continuous variation in melanized markings that cover 20 to 90 per cent of the hindwing. We found that the amount of melanization increased from 40 to 59 per cent between Estonia (588 N) and north Finland (678 N), suggesting melanization carries thermoregulatory benefits. Our thermal measurements showed that more melanic individuals warmed up more quickly on average than less melanic individuals, which probably benefits flight in cold temperatures. With extensive field experiments in central Finland and the Alpine region, we found that more melanic individuals suffered increased predation. Together, our data suggest that warning signal efficiency is constrained by thermoregulatory benefits. Differences in relative costs and benefits of melanin probably help to maintain the geographical warning signal differences.

Influences of geographic differentiation in the forewing warning signal of the wood tiger moth in Alaska

“…Trade-offs occur when the expression of one trait is linked to a decrease in the expression of another trait (Roff 2002). If the nature of a relationship between warning signal components and physiological requirements changes across the geographic distribution of an organism, then it is possible that a shift in warning signal appearance will also ensue (Thompson 1984b, Brakefield 1985, Hegna et al 2013a. Two forces other than predation that are thought to be influential on visual warning signal traits through trade-offs are the need for thermoregulation (Speed & Ruxton 2007, Lindstedt et al 2009) and sexual selection , Cummings & Crothers 2013.…”

“…The potential that the many migratory species of birds that visit northerly areas widely generalize their avoidance of aposematic taxa by general cues also exists (i.e., avoiding anything with bright conspicuous colors or patterns). Some work on alpine borer beetles in Europe has shown evidence of positive FDS (Borer et al 2010), but work on the wood tiger moth has not (Nokelainen 2013, Hegna et al 2013a). …”

“…When I looked at correlations between phenotypic traits and geographic distance while controlling for genetic distance, results also supported traits evolving in a way that was non-neutral. Hindwing melanization can potentially be beneficial for thermoregulation (Hegna et al 2013a) and lower temperatures at some of the higher elevation sites in Japan may drive such differences, apart from predators. Differences in elevation between the two populations on Honshu that differ the most in appearance, though, are not that great (200-300m).…”

“…The diversity has been recognized across the distribution (Coolidge 1910, Inoue 1961, Okano & Katayama 1976, Koçak 1989, Chenglai 2000, Dubatolov & Zahiri 2005, Leraut 2006, Powell & Opler 2009), but no concerted effort to study the variation as a whole has been made. Male hindwing color is polymorphic over much of its distribution (yellow or white) with variable melanized markings that can obscure the color completely in some areas and limit warning signal efficacy (Hegna et al 2013a). Both yellow and white male hindwing colors function as warning signals .…”

Gloger’s Rule in Plants: The Species and Ecosystem Levels

“…A common exception is of dark (melanic) animal morphs in certain temperate animals selected for because their dark color allows for better warming. [11][12][13][14] Being darker at higher latitudes for warming was also proposed to be important for various plant taxa. For instance, in Collinsia parviflora and Mimulus guttatus plants, growing in the Flat Top Islands, British Columbia, plants with upper-epidermis anthocyanin pigmentation are found more frequently in cold, exposed habitats.…”