Morphology, ornaments and performance in two chameleon ecomorphs: is the casque bigger than the bite?

The evolution of ecomorphs within a species may represent either unique evolutionary events or multiple convergent events in similar environments. Functional studies of differing morphological traits of ecomorphs have been important to elucidate their role in adaptive radiations. The Cape dwarf chameleon, Bradypodion pumilum, has two ecomorphs: a large, brightly colored, ornate form found in closed habitats, and a small, dull form with reduced ornamentation found in open vegetation. The typical form is known to use casque size to communicate fighting ability, but it is unknown whether this is an honest signal and whether casque size is related to bite force. We show through a population genetic analysis that these ecomorphs are not separate genetic lineages but the result of multiple transitions between closed and open habitats. From measurements of ornamental and non-ornamental morphological characters and bite force in 105 chameleons, we find that bite force is significantly related to head size and is best predicted by head width. Bite force was reasonably predicted by casque height in ecomorphs from closed habitats, but not in those from open habitats. For size-adjusted data, open habitat males had wider heads, biting harder than closed habitat males. Our data suggest honesty in signaling for closed habitat ecomorphs, but for open habitat ecomorphs communication is different, a finding commensurate with the common framework for species radiations.