Critical radius of endotherms.
The critical radius effect for insulation, well known in the engineering literature, was used by other authors to explain the lack of insulation on newborn endotherms. If that effect existed in small animals, they would lose less heat if nude than if fur or feathers were present. We show 1) that the previous analysis, although incomplete, yields the same result as a solid insulation model with the required sophistication and 2) that a proper model of fur is a porous media model. Neither of two porous media versions yield a critical radius effect. No critical radius effect occurs because simultaneous heat transfer by conduction and radiation makes it impossible to obtain the required logarithmic increase in thermal resistance with increasing insulation radius in a porous medium.
D000818:Animals; D000831:Animals, Newborn / Q000502:physiology*; D001833:Body Temperature Regulation; D005241:Feathers / Q000502:physiology; D006197:Hair / Q000502:physiology; D013499:Surface Properties