The Antibiotics of Color
Bacterial infections are problematic for any creature, even the high-flying ones. A new paper in Biology Letters shows that the colorful pigments of parrots may play a role in bacterial resistance:
We exposed a variety of colourful parrot feathers to feather-degrading Bacillus licheniformis and found that feathers with red psittacofulvins degraded at about the same rate as those with melanin and more slowly than white feathers, which lack pigments. Blue feathers, in which colour is based on the microstructural arrangement of keratin, air and melanin granules, and green feathers, which combine structural blue with yellow psittacofulvins, degraded at a rate similar to that of red and black feathers.
The paper is pretty straightforward – it’s all right there in the abstract – but the claims they make about importance aren’t really backed up by any data.
Here, they’re plotting the concentration of certain bacterial bi-products over time when an initial inoculation of Bacillus licheniformis is allowed to grow on different colored feathers (the different shapes correspond to different pigments present). On the one hand, it seems pretty clear that white feathers (the top line – upside-down triangles) are very permissive to bacterial growth, but black, red and blue feathers (the bottom 4 lines) are restrictive.
On the other hand, the claim that these pigments evolved for the purpose of blocking bacterial growth is unsubstantiated. It seems totally plausible, don’t get me wrong, but all birds have feather-degrading bacteria, and many have evolved bright pigments that don’t necessarily have this anti-microbial activity. It might be interesting to follow this up with an epidemiological study to see if these parrots in the wild are less prone to bacterial infection in order to make a more direct causal link.
Burtt EH Jr, Schroeder MR, Smith LA, Sroka JE, & McGraw KJ (2010). Colourful parrot feathers resist bacterial degradation. Biology letters PMID: 20926430