A red-headed rock agama (Agama agama… the agama!) flashes its brilliant orange and metallic blue colors at us atop his highest basking rock. This species is one of the most common reptiles at Mpala, especially on hot sunny days. Like other agamids they are territorial. Males actively antagonizing one another through elaborate displays as well as direct aggression, which often results in stubbed tails and jaw and limb injuries. Agamas are able to change color depending on social context and stress. These color transformations are important signals between conspecifics, and lizards are able to drastically change their appearance within a single minute. A single dominant male occupies a home range that encompasses many females and subordinate males. Subordinate males that reside near dominant males are less brightly colored and adopt submissive postures, backing down readily in the presence of a bright male performing push-ups. In addition, when distanced further away from dominant males, subordinates put on a more brightly-colored suit. If a dominant male leaves (or is experimentally removed from) his territory, a subordinate male can ascend to a reproductively-active status. Subordinate males have larger home ranges than both females and dominant males, which increases their probability of becoming a dominant male.
In response to being captured, this agama’s orange head color receded almost entirely into dull gray. On the other hand, the blue chest and belly coloration remained as vibrant and saturated as before. This is because the blue color is structural as opposed to a pigment, the latter produced chemically through selective absorption of certain wavelengths of light. Agamids have a single iridophore layer, in which photonic crystals (a type of nanostructure) are both geometry-dependent in refracting light and invariant to the angle of incident light. In contrast to the scales of a morpho butterfly, the agama’s blue color is therefore non-iridescent and does not change depending on the position of the observer. Recent work on chameleons has also revealed the presence of photonic crystals and their role in color change. At least in some chameleon species, there are two layers of iridophores, and lizards are able to fine-tune the lattice of nanocrystals to achieve dynamic color change, in complement with migration of pigments within dermal chromatophores.
