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Animal coat color and radiative heat gain: A re-evaluation

Summary

Thermal resistance and heat gain from simulated solar radiation were measured over a range of wind velocities in black and white pigeon plumages. Plumage thermal resistance averaged 39% (feathers depressed) or 16% (feathers erected) of that of an equivalent depth of still air. Feather erection increased plumage depth four-fold and increased plumage thermal resistance about 56%. At low wind speeds, black plumages acquired much greater radiative heat loads than did white plumages. However, associated with the greater penetration of radiation into light than dark plumages, the radiative heating of white plumages is affected less by convective cooling than is that of black plumages. Thus, the heat loads of black and white plumages converge as wind speed is increased. This effect is most prominent in erected plumages, where at wind speeds greater than 3 ms−1 black plumages acquire lower radiative heat loads than do white plumages. These results suggest that animals with dark-colored coats may acquire lower heat loads under ecologically realistic conditions than those forms with light-colored coats. Thus, the dark coat colors of a number of desert species and the white coat color of polar forms may be thermally advantageous.

These results are used to test a new general model that accounts for effects of radiation penetration into a fur or feather coat upon an animal's heat budget. Even using simplifying assumptions, this model's predictions closely match measured values for plumages with feathers depressed (the typical state). Predictions using simplifying assumptions are less accurate for erected plumages. However, the model closely predicts empirical data for erected white plumages if one assumption is obviated by additional measurements. Data are not sufficient to judge whether this is also the case for erected black plumages.

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Abbreviations

A :

body surface area (m2)

a L :

long-wave absorptivity of coat

a s :

short-wave absorptivity of coat

d :

characteristic dimension (m)

E :

evaporative water loss (kg m−2 s−1)

h :

coat thermal conductance (W m−2 °C−1)

k :

convection constant (s1/2 m−1)

l :

coat thickness (m)

L i :

long-wave irradiance at coat surface (W m−2)

M :

metabolic heat production (W m−2)

m :

body mass (kg)

P:

plumage mass (kg)

p :

probability per unit coat depth that a penetrating ray will strike a coat element (m−1)

q(Z) :

radiation absorbed at level z (W m−2)

R abs :

radiation absorbed by animal (W m−2)

r e :

external resistance to convective and radiative heat transfer (s m−1)

r Ha :

boundary layer resistance to convective heat transfer (s m−1)

r Hb :

whole-body thermal resistance (s m−1)

r Hc :

coat (plumage) thermal resistance (s m−1)

r Ht :

tissue thermal resistance (s m−1)

r s :

apparent resistance to radiative heat transfer (s m−1)

r(Z):

thermal resistance from level z to coat surface (s m−1)

S i :

short-wave irradiance at coat surface (W m−2)

S :

radiant flux going toward skin surface (W m−2)

S + :

radiant flux going away from skin surface (W m−2)

T a :

air temperature (°C)

T b :

core body temperature (°C)

T e :

equivalent black-body temperature (°C)

T′ e :

air temperature plus temperature increment due to longwave radiation (°C)

u :

wind velocity (m s−1)

V :

heat load on animal from short-wave radiation (W m−2)

z:

depth within coat (m)

α:

short-wave absorptivity of individual hairs or feather elements

ε:

emissivity

η:

{ie211-1}

λ:

latent heat of vaporization of water (J kg−1)

ρ:

short-wave reflectivity of individual hairs or feather elements

{ie211-2}:

short-wave reflectivity of coat

{ie212-1}:

short-wave reflectivity of skin

ρc p :

volumetric specific heat of air (J m−3 °C−1)

σ:

Stefan-Boltzmann constant (W m−2 °K−4)

τ:

short-wave transmissivity of individual hairs or feather elements

{ie212-2}:

short-wave transmissivity of coat

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Walsberg, G.E., Campbell, G.S. & King, J.R. Animal coat color and radiative heat gain: A re-evaluation. J Comp Physiol B 126, 211–222 (1978). https://doi.org/10.1007/BF00688930

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Keywords

  • Wind Speed
  • Heat Load
  • Coat Color
  • Heat Gain
  • Simulated Solar Radiation