The animal kingdom features an astonishing variety of colors, but one striking phenomenon is the dramatic difference in appearance between males and females of the same species. This disparity raises the question of why one sex, such as the male peacock, possesses brilliant plumage while the female peahen remains subtly camouflaged. This difference in hue is known as sexual dichromatism, a widespread trait across birds, fish, and reptiles, describing a distinct variation in color patterns between the sexes.
Defining Sexual Dichromatism
Sexual dichromatism is a specific type of sexual dimorphism, the broader term for any physical difference between males and females of the same species. Dichromatism focuses exclusively on differences in coloration and patterning. This phenomenon is most frequently observed in species where the male provides little to no parental care for the offspring.
The difference can be dramatic, as seen in the Northern Cardinal, where the male is a vivid red and the female is a dull reddish-brown. The male is typically the brightly colored sex, while the female maintains a more cryptic, or camouflaged, appearance. However, some species display “reverse sexual dichromatism,” where the female is the more colorful sex, a pattern seen in shorebirds like phalaropes.
Biological Mechanisms of Color Production
Animal coloration is produced through two distinct mechanisms: pigments and physical structures, which the sexes often employ differently. Pigmentary colors are derived from chemical compounds synthesized by the animal or obtained through its diet. Melanins are the most widespread group of pigments, responsible for producing shades of black, brown, and grey.
The yellows, reds, and oranges often seen in conspicuous displays are frequently created by carotenoid pigments. Animals cannot produce carotenoids themselves and must ingest them through their diet, making these colors a direct signal of an individual’s foraging success and overall health. These pigments absorb specific wavelengths of light, reflecting the colors that are ultimately perceived by an observer.
Structural colors, conversely, are not produced by chemicals but by the physical architecture of the animal’s surface, such as the keratin in feathers or the micro-ridges on scales. These nanostructures scatter light waves, creating iridescent, metallic, or glossy effects that shift in appearance with the viewing angle. Blues, violets, and the intense iridescence of many species, including hummingbirds and the male peacock, are produced by this light-scattering mechanism. Both pigmentary and structural colors often work together, with pigments providing a base layer that filters light before it interacts with the structure.
The Evolutionary Drivers of Color Differences
Sexual dichromatism is driven primarily by sexual selection, which favors traits that enhance mating success, even if they pose a risk to survival. This selection operates in two main ways to promote color differences. Inter-sexual selection involves mate choice, where the brightly colored sex develops showy traits to attract the opposite sex.
These conspicuous displays act as honest signals of an individual’s genetic quality or fitness, especially when the color is derived from carotenoids, which are also used for immune function. A male that diverts these valuable, diet-acquired pigments to his plumage demonstrates superior foraging ability and robust health. The second aspect, intra-sexual selection, involves competition between members of the same sex for access to mates or territory.
Bright colors and intricate patterns can be used as a form of non-physical contest, intimidating rivals and advertising dominance. The evolution of dichromatism is not solely about males becoming brighter, as it can also result from the female losing her bright coloration due to other pressures. The combined effect of selection for showiness in one sex and selection for subtlety in the other leads to the distinct color separation observed in many species.
The Predation Trade-Off
The development of elaborate, bright coloration comes with an inherent cost, establishing an evolutionary trade-off between reproductive success and survival. Conspicuous colors, while effective for attracting mates, simultaneously make the individual more visible to predators. This dynamic creates a balance where the benefit of attracting a mate must be weighed against the increased risk of being eaten.
The less colorful sex, which is typically the female, often retains cryptic coloration as camouflage. This is important because females are frequently the sex responsible for incubating eggs or caring for immobile young, which forces them to spend prolonged periods in vulnerable locations, such as a nest. Natural selection strongly favors camouflage in the parent responsible for offspring care, as her survival is directly linked to the survival of the next generation. Therefore, the final expression of sexual dichromatism is a visible record of this constant tension between the need to advertise for mating and the need to hide for survival.