The coats of birds and mammals typically vary through their depth in structure, insulation and optical qualities. Physical models predict that such variation can substantially affect the solar heat load acquired by an animal. This study quantifies the consequences of complex coat structure for solar heat gain in the rock squirrel (Spermophilus variegatus (Erxleben, 1777)), a species normally exposed to intense solar radiation. This species' pelage consists of two well-defined layers: a dense inner coat of fine, dark hairs, and a sparse outer coat of coarse, light hairs. The optics, structure and thermal insulation of the inner and outer coats are quantified and used to predict rates of radiative heat gain using a physical model. The radiative heat load measured at the skin compares well with model predictions. The validated model is then used to explore the consequences for solar heat gain of varying the relative proportions of the inner and outer coat layers. Results demonstrate that the ratio of inner to outer coat depths occurring in rock squirrels is very near that theoretically predicted to minimize solar heat gain. This indicates that optimization of fur structure may represent an effective means of adjusting solar heat gain independent of coat insulation and surface coloration.

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