Abstract: All 18 extant species of penguin are strongly countershaded, having dark dorsal and light ventral coloration. In this paper, the thermal effects of this body color in penguins are investigated through analytical and observational analyses. First, a thermal analysis that takes into account the environmental characteristics of penguins’ habitats, fluxes, and morphology is used to analytically calculate penguin dorsal surface temperature. Next, a turbulent analytical solution for a heated boundary layer over a flat plate is applied to show that the dark color on the top of the penguins’ bodies is very effective at skin drag reduction. To verify this result, a 2D model penguin is computationally analyzed at different temperatures, confirming in principle underwater skin drag reduction through color-based surface warming with efficiency savings of up to 30%. Finally, to study how color-based increases in body surface temperatures are maintained through foraging dives, thermal cameras are used to measure the surface temperature of captive penguins before and after dives. This study shows conceptually that dark dorsal coloration in penguins could have a significant influence on in-water drag.

Key words: Animal coloration , · Drag , · Energy balance , · Irradiation , · Penguin , · Thermal imaging , · Skin friction