Critical size of newborn homeotherms

It is shown that for cylindrical and spherical bodies there is a critical radius below which the addition of any form of insulation to the body will increase rather than decrease the cooling of the body. It is proposed, therefore, that it would be thermally detrimental to newborn homeotherms to be born with a protective covering (fur or down) if their body size were less than this critical size, and consequently that the degree of natal covering is not necessarily related to the overall development of the species when the birth size is less than this critical size. A critical weight is derived from the critical radius for basically spherical animals which compares favorably with typical birth weights of various altricial homeotherms. The effect of the overall conductive-convective heat transfer caused by a basically cylindrical animal rolling up into a ball is also discussed.