Biometeorology is the science that deals with the effects of the atmosphere on living organisms. Research has even revealed the relative insulating capacities of the winter coats of different animals.

I have just come across a charming table of information (see below). It can be found in the McGraw-Hill Encyclopedia of Science and Technology in the entry on Biometeorology, and it lists the relative insulation quality of the winter coats of selected mammals:

Humans and other warm-blooded animals are in a constant battle to maintain thermal equilibrium. We gain heat from metabolism, but suffer a net loss through heat exchange with the environment. We have had to evolve strategies to cope with changing climate throughout the year. Animals grow a winter coat or hibernate (when reduced oxygen consumption leads to reduced metabolic activity and a body temperature maintained at survival level) while humans wear clothes.

If you want to be scientific about your shopping, the unit you need to specify is the "clo" - defined as the amount of insulation that permits a heat flux from the body through a garment of 1 kcal per square metre per hour with a temperature difference of 0.18C between the inner and outer surface of the fabric. A business suit, with cotton underwear and no coat, or a dress with slip, bra, panties and tights, rates about one clo. Open-necked shirt, shorts and sandals are 0.2 clo; suit,thick overcoat, warm woolly underwear, hat, lined gloves and sweater, between 3 and 3.5.

Laboratory experiments have determined the insulating values of different animals' coats, as indicated in the table, which show that a sheep is four times as well insulated as a weasel. Hibernating animals generally have lower values of coat insulation. While the quality of insulation correlates quite strongly with the thickness of the coat, it is clear this is not the sole determinant of winter warmth. A squirrel's coat is twice as thick as a weasel's, but is only marginally more effective in keeping it warm. Rabbit fur offers perhaps the best warmth-to-bulk ratio. Warm winter coats Species Thickness Insulation Weasel 1cm 2 Squirrel 2cm 2.5 Rabbit 3cm 5 Dog 4cm 6 Beaver 4cm 5 Sheep 7cm 8