“The Redder the Better” . . . Sometimes
Nature is a wonder of color: flowering plants are the kaleidoscopes of many landscapes, and animals ranging from flatworms to parrots display a rainbow of shades and hues. Color is often appreciated for its aesthetic value alone, but in reality it often carries great functional significance. An animal’s hues and patterns may help it to hide from predators, attract mates, or signal that it wields a toxin or sting that potential attackers would be wise to avoid. Two recent studies, however, have uncovered novel aspects of pigmentation in widely dissimilar species: monarch butterflies (Davis et al. 2012) and wild boars (Galván et al. 2012). In each case, red coloration was found to have profound significance for the animal’s fitness, although in strikingly disparate ways.
It has long been known the orange and black pattern on monarch wings acts as a warning that these butterflies are unpalatable and even toxic. There is, however, a degree of variation in the “orangeness” of individual monarchs, raising questions as to the physiological an/or ecological significance of their pigmentation intensity.
Populations of migrating monarchs--which are active in the fall--have been shown to display more intense, saturated orange pigmentation on their wings than non-migratory summer populations, as shown in an earlier study conducted by Andrew Davis of the University of Georgia (Davis 2009). This raises the question of whether the coloration has to do with the migration itself, seasonal differences in environmental conditions, or myriad other potential ecological and/or physiological factors. Recently, Davis led a research team in a new set of experiments to determine whether orange coloration affects flight performance by monarch butterflies.
Monarchs with a range of wing hues were tested using a “flight mill” apparatus. The butterflies were tethered to a carbon rod, which was in turn attached to a pivot point, with a flag on the counterbalance to the rod. A photogate next to the apparatus recorded the passage of the flag as the butterflies flew in circles, allowing a computer to note the distance, speed, and duration of time that each butterfly flew. You can see a link to a video of this operation in action here.
For each of the 121 butterflies that were put through these rigors, the researchers measured wing area, aspect ratio, degree of melanism, and a “hue score” measuring the orange/redness of the wings. A lower hue score indicated a darker, redder coloration. The researchers then conducted statistical analyses to determine whether those morphological factors affected flight performance. All of the butterflies were raised in the same lab conditions, to prevent confounding environmental differences that could influence development, and the analyses controlled for the effects of sex and other traits. This turned out to be important, as the results showed that females flew significantly farther distances than males (an average of 4.57 km versus 3.59 km).
The verdict? Butterflies with the darkest wings flew significantly farther and for longer durations than those with lighter colored wings. None of the other morphological variance seemed to matter—only wing hue. This is not, of course, to say that color itself caused the difference in performance. The authors speculated that a common developmental pathway may influence flight endurance traits and pigmentation. It was also noteworthy that only red color was associated with flight performance—melanism, or the degree of black patterns on the wings, was not. The authors note that the lab conditions did not represent the temperature extreme that wild butterflies might face, and that in ‘real world’ migrations the thermoregulatory function of black banding may have an effect on performance.
So for butterflies, “the redder the better,” as noted in the clever title of the PLoS ONE study. But it appears red carries more ominous implications for wild boars. A new study led by Ismael Galván, of the Museo Nacional de Ciencias Naturales in Madrid, has shown that for wild boars (Sus scrofa), red hair is a significant indicator of oxidative stress, which has negative implications for an animal’s overall fitness.
It is known that pheomelanin, a pigment that produces reddish colors in mammalian fur, is synthesized in a molecular process that consumes glutathione (GSH). In contrast, eumelanin, which produces black and dark brown pigments, does not consume GSH, and in fact eumelanogenesis does not activate until cells contain relatively low levels of GSH.
GSH is a tripeptide molecule that serves an important antioxidant function in animal cells. It acts as a reducing agent in intracellular chemical reactions, limiting the cellular damage that can accumulate from free radicals released during oxidation-reduction reactions. Numerous studies have shown that oxidative damage tends to increase as organisms age, and many organisms concordantly display reduced GSH levels as senescence progresses. Thus, Galván and colleagues hypothesized that that GSH levels and senescence would be positively associated with eumelanogenesis and negatively associated with pheomelanogenesis, since pheomelanogenesis relies on abundant GSH.
The team obtained samples from one hundred wild boars in Doñana National Park. The age of each animal was quantified using tooth growth patterns and the ratio of pulp cavity width to overall tooth width. These boars represented five different color morphs, which ranged along a spectrum of eumelanic and pheomelanic color scores. The pigmentation of each animal’s pelage was compared to its levels of intramuscular GSH and oxidative stress.
The results were quite interesting: the more pheomelanin a boar had in its fur, the lower its levels of GSH, and the higher its levels of oxidative stress. This supports the author’s hypothesis that pheomelanogenesis can be of negative consequence for fitness, possibly because it consumes GSH that could otherwise be functioning to protect cells from oxidative damage.
The study also challenged the idea that graying hair indicates oxidative stress, as is often assumed given its common association with aging. The gray boars actually showed the lowest levels of oxidative stress, indicating that their aged appearance was not indicative of oxidative damage as a result of senescence.
Red is a color that carries heavy cultural significance. Its distinction as the hue of blood means that it often symbolizes aggression or sin, but it is also often used in badges of valor. In some Chinese cultures, it is a traditional wedding color, and is often used to package New Year’s gifts. It appears that red’s significance in the natural world is equally variable. “Redder is better” if you are a migrating monarch butterfly, but ruddy boars may be unlucky relative to their black and gray counterparts. We have just uncovered the tip of the iceberg with regards to the study of color and pigmentation, and it will be interesting to follow further studies on the association of hue with physiological and/or ecological advantages.
Davis, A. K., Chi J., Bradley C., and Altizer S. 2012. The redder the better: wing color predicts flight performance in monarch butterflies. PLoS ONE 7(7): e41323. doi:10.1371/journal.pone.0041323
Davis, A. K. 2009. Wing color of monarch butterflies (Danaus plexippus) in eastern North America across life stages: migrants are ‘redder’ than breeding and overwintering stages. Psyche 2009. DOI: 10.1155/2009/705780.
Galván, I., Alonso-Alvarez, C., and Negro, J. J. 2012. Relationships between hair melanization, glutathione levels, and senescence in wild boars." Physiological and Biochemical Zoology 85:4.
Photos obtained from WikiCommons