Fig. 4. Sample temperature and grey intensity trace from one frog that willingly basked in the sun for 60 min. (A) Rock temperature (solid line), black body temperature (broken line; the surface temperature of a piece of black electrical tape exposed to the sun, but not in contact with substrate), and frog dorsal surface temperature (dotted line). (B) Changes in the frog–stone temperature difference (solid line) and the skin's greyscale value (broken line). (C) Correlation between the frog–stone temperature difference and the skin greyscale value (open circles). Roman numerals (i–iii) refer to (i) an early phase when the skin is dark and the frog warms up rapidly, (ii) a secondary phase when the skin has lightened up considerably and the frog and stone temperature difference diminishes, and (iii) a late phase where the frog is as light as possible and the frog equilibrates with stone temperature and eventually falls below stone temperature, presumably due to increased evaporative water loss.