Coloration reflects skin pterin concentration in a red-tailed lizard

When integumentary tissue pigments are contained in chromatophores, tissue color might not depend exclusively on the amount of pigment. Whether coloration does or does not reflect pigment concentration may be very significant for intraspecific communication, for example when pigment concentration provides fitness-related information. We studied the pigment responsible for the orange/red ventral tail coloring in a lacertid lizard species (Acanthodactylus erythrurus), and whether the color was related to skin pigment concentration. The pigment was identified as a pterin, a higher concentration of which resulted in darker, more red-saturated, redder (less orange) ventral tail skin color. The dorsal tail integument, even though it appears mostly gray to the naked eye, also contained pterins, and furthermore, the dorsal and ventral pterin concentrations were positively correlated. A possible explanation for these results is that pterins accumulate in the skin of the whole tail, even if only needed in the ventral part, but are concealed in the dorsal part. In this way, ventral orange/red coloration would accurately reflect pterin concentration, which provides the basis for a signaling function, while dorsal coloration would become less conspicuous as an anti-predatory mechanism.