Thermal implications of interactions between insulation, solar reflectance, and fur structure in the summer coats of diverse species of kangaroo

Abstract

Not all of the solar radiation that impinges on a mammalian coat is absorbed and converted into thermal energy at the coat surface. Some is reflected back to the environment, while another portion is reflected further into the coat where it is absorbed and manifested as heat at differing levels. Substantial insulation in a coat limits the thermal impact at the skin of solar radiation, irrespective where in the coat it is absorbed. In coats with low insulation, the zone where solar radiation is absorbed may govern the consequent heat load on the skin (HL-SR). Thin summer furs of four species of kangaroo from differing climatic zones were used to determine how variation in insulation and in coat spectral and structural characteristics influence the HL-SR. Coat depth, structure, and solar reflectance varied between body regions, as well as between species. The modulation of solar radiation and resultant heat flows in these coats were measured at low (1 m s⁻¹) and high (6 m s⁻¹) wind speeds by mounting them on a heat flux transducer/temperature-controlled plate apparatus in a wind tunnel. A lamp with a spectrum similar to solar radiation was used as a proxy for the sun. We established that coat insulation was largely determined by coat depth at natural fur lie, despite large variations in fibre density, fibre diameter, and fur mass. Higher wind speed decreased coat insulation, but depth still determined the overall level. A multiple regression analysis that included coat depth (insulation), fibre diameter, fibre density, and solar reflectance was used to determine the best predictors of HL-SR. Only depth and reflectance had significant impacts and both factors had negative weights, so, as either insulation or reflectance increased, HL-SR declined, the larger impact coming from coat reflectance. This reverses the pattern observed in deep coats where insulation dominates over effects of reflectance. Across all coats, as insulation declined, reflectance increased. An increase in reflectance in the thinnest coats was not the sole reason for the limited rise in HL-SR. Higher reflectance should increase the depth of penetrance of solar radiation, thus increasing HL-SR. But in M. antilopinus and Macropus rufus, which had the highest of coat reflectances, penetrance was relatively shallow. This effect appears due to high fibre density (M. rufus) and major modifications in the fibre structure (M. antilopinus). The differing adaptations likely relate to the habitats of these species, desert in the case of M. rufus and monsoon tropical woodland with M. antilopinus.