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

Abstract

Mammalian fur often shows agouti banding with a proximal dark band near the skin and a lighter distal band. We examined the function of both bands in relation to camouflage, thermal properties of pelts, and thermal energetics of dunnarts (Sminthopsis crassicaudata), which are known to use torpor and basking. Although the distal band of dunnart fur darkened with increasing latitude, which is important for camouflage, it did not affect the thermal properties and the length of the dark band and total hair length were not correlated. In contrast, the length of the proximal dark band of preserved pelts exposed to sunlight was positively correlated (r (2) = 0.59) with the temperature underneath the pelt (T pelt). All dunnarts offered radiant heat basked by exposing the dark band of the hair during both rest and torpor. Basking dunnarts with longer dark bands had lower resting metabolism (r (2) = 0.69), warmed faster from torpor (r (2) = 0.77), required less energy to do so (r (2) = 0.32), and reached a higher subcutaneous temperature (T sub) at the end of rewarming (r (2) = 0.75). We provide the first experimental evidence on the possible dual function of the color banding of mammalian fur. The distal colored band appears to be important for camouflage, whereas the length of the dark proximal hair band facilitates heat gain for energy conservation and allows animals to rewarm quickly and economically from torpor.

Darker where cold and wet: Australian birds follow their own version of Gloger's rule

“…Similarly, if water loss is the main cost of dark plumage, this cost could be less important in areas with high rainfall and available water. It should be kept in mind, however, that plumage colour is only one component determining the thermal balance of endotherms, which also depends on the insulation properties of the concealed part of the plumage or pelage (Walsberg , Hochscheid et al ), changes in feather or hair disposition (Wacker et al ), body posture (Lustick et al ), behaviour (flight, microhabitat choice, huddling; Marder , Huertas and Díaz , Dreiss et al ) and wind speed (Wolf and Walsberg ). The fact that nevertheless a consistent correlation was found suggests that, despite this complexity, darker plumage might be under direct positive selection in colder environments and/or negative selection in warmer environments.…”

Deconstructing the long‐standing a priori assumption that serial homology generally involves ancestral similarity followed by anatomical divergence

“…These animals were preserved with hair imprints, in which guard hair can be distin- (Ji et al, 2002) and boreosphenidan mammal Synodelphys szalayi (both 125 mya) (Luo, 2003) also consisted of two types of hair, the guard hairs and dense underfur. It is likely that the appearance and diverse function of hair in modern animals is similar to those of ancestral forms, as the main function of the bottom wool layer is thermoregulation (Wacker, McAllan, Körtner, & Geiser, 2016). Long and generally coarse guard hair protects the undercoat and skin from external negative factors and provides pigmentation and camouflage (Wacker et al, 2016).…”

“…It is likely that the appearance and diverse function of hair in modern animals is similar to those of ancestral forms, as the main function of the bottom wool layer is thermoregulation (Wacker, McAllan, Körtner, & Geiser, 2016). Long and generally coarse guard hair protects the undercoat and skin from external negative factors and provides pigmentation and camouflage (Wacker et al, 2016).…”

Phoenix from the Ashes: Fire, Torpor, and the Evolution of Mammalian Endothermy

“…With time an even higher metabolism would have been selected in ancestral endotherms via increased shivering and non-shivering thermogenesis (Oelkrug et al, 2015 ) together with an external cover of fur or feathers resulting in a prolongation of periods with a high and constant T b . Importantly, and in contrast to what was believed in the past (Cowles, 1958 ), fur cover in living animals does not necessarily make basking energetically ineffective (Geiser et al, 2004 ), because unlike dead fur coats used to cloak ectothermic lizards (Cowles, 1958 ), mammals can adjust the angle of hair to maximize heat gain (Wacker et al, 2016 ). Basking mammals during normothermia can maintain resting MR near basal MR values over a wide range of T a well below the thermo-neutral zone and can reduce rewarming cost by up to 75% in comparison to endothermic rewarming from torpor (Geiser and Drury, 2003 ).…”