Boundary layer: a tool to protect against the cold

Endotherms, like mammals and birds, mainly use two methods to minimize heat loss by convection in cold environments:

  1. .Increasing heat production

  2. Reducing heat loss by increasing the efficiency of the insulation, but how? By empowering the BOUNDARY LAYER.

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Diagram showing how the coat acts as a thermal insulator and can change its insulating properties, thus varying the thickness of the boundary layer. Personal archive

The boundary layer is a layer of fluid (water or air) adjacent to the surface of an object, and it has different properties from the fluid further away from the object. It is important because it influences the exchange of heat, which occurs first through the layer and then through convection to the adjacent airflow. Therefore, the greater the thickness of the boundary layer, the less heat is lost.

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The thickness of the boundary layer depends mainly on three factors. Personal archive


Most birds and mammals increase this layer by piloerection and ptiloerection mechanisms. The pylomotor muscles contract, erecting the hairs and feathers respectively and increasing the layer of air that acts as an insulator. We, who have lost much of our body hair, increase the boundary layer by clothing.

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Birds use the ptiloerection mechanism (ptilo: from the Greek, feather) to increase the boundary layer and improve their insulation against the cold. Ray Hennessy
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Similarly, mammals use the mechanism of piloerection (pilo: from Latin, hair). Noaa

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