The fur of mammals in exposed environments; do crypsis and thermal needs necessarily conﬂict? The polar bear and marsupial koala compared
Terence J. Dawson • Koa N. Webster • Shane K. Maloney
The furs of mammals have varied and complex functions. Other than for thermoregulation, fur is involved in physical protection, sensory input, waterprooﬁng and colouration, the latter being important for crypsis or camouﬂage. Some of these diverse functions potentially conﬂict. We have investigated how variation in cryptic colouration and thermal features may interact in the coats of mammals and inﬂuence potential heat inﬂows from solar radiation, much of which is outside the visible spectral range. The coats of the polar bear (Ursus maritimus) and the marsupial koala (Phascolarctus cinereus) have insulative similarities but, while they feature cryptic colouration, they are of contrasting colour, i.e. whitish and dark grey. The reﬂectance of solar radiation by coats was measured across the full solar spectrum using a spectroradiometer. The modulation of incident solar radiation and resultant heat ﬂows in these coats were determined at a range of wind speeds by mounting them on a heat ﬂux transducer/ temperature-controlled plate apparatus in a wind tunnel. A lamp with a spectral distribution of radiation similar to the solar spectrum was used as a proxy for the sun. Crypsis by colour matching was apparent within the visible spectrum
for the two species, U. maritimus being matched against snow and P. cinereus against Eucalyptus forest foliage. While reﬂectances across the full solar spectrum differed markedly, that of U. maritimus being 66 % as opposed to 10 % for P. cinereus, the heat inﬂuxes from solar radiation reaching the skin were similar. For both coats at low wind speed (1 m s-1), 19 % of incident solar radiation impacted as heat at the skin surface; at higher wind speed (10 m s-1) this decreased to approximately 10 %. Ursus maritimus and P. cinereus have high and comparable levels of fur insulation and although the patterns of reﬂectance and depths of penetrance of solar radiation differ for the coats, the considerable insulation limited the radiant heat reaching the skin. These data suggest that generally, if mammal coats have high insulation then heat ﬂow from solar radiation into an animal is much restricted and the impact of coat colour is negligible. However, comparisons with published data from other species suggest that as fur insulation decreases, colour increasingly inﬂuences the heat inﬂow associated with solar radiation.