AJP - Regulatory, Integrative and Comparative Physiology, Vol 250, Issue 4 699-R707, Copyright © 1986 by American Physiological Society
Critical radius of endotherms
W. P. Porter, D. F. Parkhurst and P. A. McClure
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.
