Why are small homeotherms born naked? Insulation and the critical radius concept

Abstract

1. 1. For cylinders or spheres smaller than a critical size, a layer of insulation may increase, rather than decrease, the rate of heat exchange. 2. 2. It has been suggested that the critical size for animals insulated with fur or feathers is about 4–7 g body mass, and therefore may explain why many small mammals and birds are born naked. We suggest that this is unlikely. 3. 3. We found that downy feathers on hatchlings as small as 1.6 g body mass retard rates of heat loss. Also, hatchling birds smaller than 4–7 g are not always, or even often, born naked. 4. 4. If a critical size exists at all, it is likely to occur at body masses of 0.6 g or smaller.