Above: two mosaics that featured in the study: the female (left) shows less red than the cock. Both photos: Ricardo Lopes


SCIENTISTS HAVE long known that genetics plays a role in the differences in male and female plumage colours, but still don’t understand how two individuals of the same species, with almost identical genomes, can look so different.

New research published in Science magazine suggests that in canaries and some related birds, the difference may involve just a single gene, which governs an enzyme that destroys red and yellow pigment molecules. 

An international research team examined the genetics of the mosaic canary, a hybrid between the red siskin (Spinus cucullatus) and monochromatic strains of domesticated canaries. When the mosaic genomes were compared with those of the monochromatic canary, the researchers found there was one region containing three genes that differed. Only one of those genes, BCO2, was expressed differently in the male and female mosaics: females have more of it in certain areas.

BCO2 governs an enzyme which breaks down B-carotene, a reddish-orange pigment. On parts of the body where males have redder plumage, female hybrid canaries expressed higher levels of BCO2 than the males. In female mosaics, the reduced level of pigmentation is a result of the enzyme in the feather follicles degrading the carotenoid pigments,  rather than any sex-specific variation in physiological functions such as pigment uptake or transport. 

One theory, the study states, is that the expression of BCO2 may be affected by oestrogen, one of the primary sex hormones in female birds. This would explain the different levels present in females and males, but the researchers note that further investigation is needed to prove this.

In wild birds, they found that BCO2 did not always play a role in colour differences between sexes. Nonetheless, the BCO2 mechanism may be one reason that carotenoid pigments are found in so many dichromatic bird species, compared with other pigments.

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