Peacock feathers are iridescent and colorful, but peahen (female peacock) feathers are not. Such visual differences between the sexes occur in many species, and several studies have addressed its evolution, yet the genetic basis of the differences is largely unknown. 

A new research study may have cracked the code. Scientists found that in a hybrid breed of canaries, differences in the expression of a single gene responsible for the degradation of pigments can account for why the feathers of the male birds are brighter than those of the females.

In the common domestic breed of canary (Serinus canaria), females and males have identical colors. But there exists a hybrid breed called mosaic, which have patches of colored feathers, and male and female mosaic birds do have color differences. Males' patches are brighter due to more carotenoid pigment in the feathers. 

The scientists, led by Miguel Carneiro of Portugal's Research Centre in Biodiversity and Genetic Resources found that the differences they observed between male and female birds could be explained by the expression of one single gene called BCO2. It encodes an enzyme involved in breaking down carotenoids. Females have a higher expression of this gene in the skin, which leads to more degradation of their carotenoids and the faded colors of their feathers. The team also found evidence that this mechanism may be widespread in nature, corroborating reasoning set forth by Charles Darwin himself.