Critical radius of endotherms.

Area of Science:

• Thermodynamics
• Biophysics
• Zoology

Background:

• The critical radius effect, known in engineering, has been incorrectly applied to explain insulation in newborn animals.
• This effect suggests that small animals might lose less heat when nude than with fur or feathers.

Purpose of the Study:

• To re-evaluate the applicability of the critical radius effect to animal insulation.
• To develop a more accurate model for heat transfer in fur and feathers.

Main Methods:

• Analysis of existing insulation models, including solid insulation and porous media models.
• Mathematical modeling of heat transfer by conduction and radiation in porous materials.

Main Results:

• The previous analysis of the critical radius effect in animal insulation was incomplete but yielded similar results to a basic solid insulation model.
• A proper model for fur and feathers treats them as porous media.
• Neither porous media model demonstrated a critical radius effect.

Conclusions:

• The critical radius effect is not applicable to the insulation provided by fur or feathers in animals.
• Simultaneous heat transfer by conduction and radiation in porous materials prevents the logarithmic increase in thermal resistance required for a critical radius effect.