The fur of mammals in exposed environments; do crypsis and thermal needs necessarily conflict? The polar bear and marsupial koala compared

Abstract

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.

Bergmann meets Scholander: geographical variation in body size and insulation in the koala is related to climate

“…This is not surprising because fur in mammals functions primarily as a layer influencing heat exchange, although fur properties can also play an important role in camouflage (Vignieri et al, 2010;Dawson et al, 2014). This is not surprising because fur in mammals functions primarily as a layer influencing heat exchange, although fur properties can also play an important role in camouflage (Vignieri et al, 2010;Dawson et al, 2014).…”

The functional requirements of mammalian hair: a compromise between crypsis and thermoregulation?

“…Although the hair properties of mammals in relation to thermal biology have been previously investigated (Cooper et al 2003;Dawson et al 2014), the possible dual function of camouflage and thermal biology of the color banding have not been examined. The aims of our study were (i) to determine if there was a correlation between the length of the dark band and temperature under preserved pelts, (ii) to examine if the color of dunnart fur was correlated with distribution, and (iii) to provide the first data on whether and how the length of colored hair bands affect the thermal properties of pelts and thermal energetics of normothermic and torpid dunnarts during basking/passive rewarming.…”

Orientation to the sun by animals and its interaction with crypsis

“…In Clusella-Trullas, van Wyk & Spotila (2009), the authors showed that the solar heating rate of melanistic lizards was higher than that of similar non-melanistic species. Dawson, Webster & Maloney (2013) addressed the putative conflict between thermal needs and crypsis in mammals. They showed that although the polar bear Ursus maritimus and koala Phascolarctus cinereus have very different fur colorations, their heat influx through solar radiation is similar and concluded that the lower the insulation power of the fur, the higher the influence of colour on solar heating.…”

“…The sun's rays most commonly strike dorsal parts of an animal, so darkening upper parts of the body could maximize heat gain from the sun and result in countershading. Note that a darker skin or pelt is not always associated with greater solar heat load (Lustick, Adam & Hinko 1980;Walsberg 1983;Dawson, Webster & Maloney 2013). Dark coloration on the back may also result from selection through protection from UV radiation (Braude et al 2001).…”

“…Next, we analyse the interplay between chosen orientation and solar heat. As no single model can account for the complex relationship between solar heat inflow and body coloration (Lustick, Adam & Hinko 1980;Walsberg 1983;Dawson, Webster & Maloney 2013), we propose two extreme views: (i) where pelt darkness has no influence on solar heat inflow balance and (ii) where pelt darkness drives solar heat inflow balance (i.e. thermal melanism).…”