Sunday, March 6, 2011

A Change of Colour: Tawny Owl

Recent studies in Finland, show that tawny owls (Strix aluco) populations are changing their plumage colour from grey to brown. It seems that the increase in owls with brown plumage may be related to climate change and the consequent decrease in snow cover.

Feather colour is hereditary, that is, is passed genetically from generation to generation, and the grey allele is said to be dominant over the brown allele, therefore, when both alleles are present, the progeny is grey-feathered, and only when it possess two brown alleles, the plumage is brown instead.

The terminology of colour change is misleading as tawny owls do not suffer any colour changes during its lifetime, they remain with the same colour, so why are scientists saying that the colours are changing?

Well, the truth is, that the brown phenotype (colour) is becoming more predominant, with a 20% increase. Thus nowadays the population of brown tawny owls makes up 50% of the entire population.

Grey and Brown Tawny Owls

Why this increase?

Population phenotypes change due to environmental pressures that select for advantageous characteristics, that is, if there is an advantage of having brown feathers, then the grey individuals will die, and the brown individuals will survive and pass on their winning genes. This is called Natural Selection and the 'survival of the fittest'.

So in the case of the tawny owl, it has been suggested that due to the decrease in snow cover, brown owls have more chances of survival as predators cannot detect them as easily as when there is thick snow cover.

The grey owls, conversely, are more easily camouflaged by the snow, and thus had higher survival rates, which counted for the predominance of this phenotype in the population.

However, these brown individuals possess disadvantageous genetic traits such as a weaker immune system and higher metabolism, requiring more foraging in search for food. Such characteristics are not found in the grey populations thus the increase of this phenotype may increase the vulnerability of the species overall.

Thus, it has been found that climate-driven selection is causing the species to evolve and raises concerns as to whether the grey 'gene pool' may disappear, causing a lower genetic diversity and a reduction in the variety of heritable material, which can cause the species to become genetically vulnerable to other environmental pressures and be at risk.

Such studies are important in determining not only the impacts of climate change on species but also the mechanisms from which species evolve and attempt to overcome adverse conditions, important information that is useful to our understanding of ecological interactions and allows scientists to devise more accurate and efficient conservation measures in order to prevent the detriment caused in our environment by human and non-human induced threats.

(click to see bigger) Survival estimates are based on averaging all candidate capture–mark–recapture models considering the effects of food supply (vole abundance), average snow depth and average temperature during a critical period in winter (see Supplementary Information). Climatic data are derived from the database of the Finnish Meteorological Institute. (a) Survival of brown and grey tawny owl colour morphs in relation to snow depth. (b) Snow depth during the critical period for tawny owl survival during 1981–2008 in Southern Finland. (c) Survival of grey and brown tawny owl colour morphs during 1981–2008. Grey tawny owls are denoted by grey circles and brown ones by red diamonds. Error bars are standard errors of the estimated survival and the lines are regression slopes based on the data.

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