Given that most animals are essentially big bags of water, it would seem to make sense that they have a well-sealed skin to retain precious fluid. But evaporative water loss through the skin has its uses keeping animals cool and no biological membrane can ever be completely water tight. Alex Champagne from the University of Southern Indiana, USA, explains that evaporation through the skin accounts for 65% of the water lost by birds as vapour. However, little was known about the interactions between water and the waterproofing fats (lipids) in the skin's outer layer – the stratum corneum. Champagne was also curious to know how the proportions of the different waterproofing lipids may vary through the skin's depth and how they change with the seasons, so he and his colleagues, Heather Allen and Joseph Williams, embarked on an ambitious series of experiments to get to the bottom of the house sparrow's waterproofing mechanism.
Capturing 11 sparrows during the summer of 2012 and 8 in the following winter, Champagne first measured the skin's evaporative water rates and found that the summer birds’ water loss rate (48 mg H2O cm−2 day−1) was almost twice as high as that of the winter birds (26 mg H2O cm−2 day−1). Next, the team measured the absorption of infrared light in micrometre-thick layers of the skin, to calculate the amount of water carried in the layer and how tightly the water interacted with the waterproofing lipids, before carefully measuring the different types of lipids present at different depths through the skin's structure.
Amazingly, the team found the birds were able to vary the degree of skin waterproofing through the seasons, increasing the lipid content of the skin in winter by retaining more lipids near the surface and increasing the amount of cerebrosides and ceramide II deeper in the skin. They also found that the skin was better hydrated at depth than near the surface and that the increased hydration was associated with a greater number of kinks in the long fat molecules that hold water in the lipid layers. They also suggest that water-attracting structures in the deeply buried ceramide and cerebroside molecules could hold on tightly to water molecules in their vicinity, to increase the viscosity of the water and slow evaporation.
So, the deepest layers of bird skins are the most waterproof and best hydrated, and the team adds, ‘Birds modify the locations of certain lipid classes seasonally, likely to acclimatize to changes in temperature and humidity and thus maintain heat and water balance, even in cold, dry conditions’.