Sexual dimorphism in bite performance drives morphological variation in chameleons.
Phenotypic performance in different environments is central to understanding the evolutionary and ecological processes that drive adaptive divergence and, ultimately, speciation. Because habitat structure can affect an animal's foraging behaviour, anti-predator defences, and communication behaviour, it can influence both natural and sexual selection pressures. These selective pressures, in turn, act upon morphological traits to maximize an animal's performance. For performance traits involved in both social and ecological activities, such as bite force, natural and sexual selection often interact in complex ways, providing an opportunity to understand the adaptive significance of morphological variation with respect to habitat. Dwarf chameleons within the Bradypodion melanocephalum-Bradypodion thamnobates species complex have multiple phenotypic forms, each with a specific head morphology that could reflect its use of either open- or closed-canopy habitats. To determine whether these morphological differences represent adaptations to their habitats, we tested for differences in both absolute and relative bite performance. Only absolute differences were found between forms, with the closed-canopy forms biting harder than their open-canopy counterparts. In contrast, sexual dimorphism was found for both absolute and relative bite force, but the relative differences were limited to the closed-canopy forms. These results indicate that both natural and sexual selection are acting within both habitat types, but to varying degrees. Sexual selection seems to be the predominant force within the closed-canopy habitats, which are more protected from aerial predators, enabling chameleons to invest more in ornamentation for communication. In contrast, natural selection is likely to be the predominant force in the open-canopy habitats, inhibiting the development of conspicuous secondary sexual characteristics and, ultimately, enforcing their overall diminutive body size and constraining performance.
Applied Biodiversity Research Division, South African National Biodiversity Institute, Cape Town, Western Cape Province, South Africa ; Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, Western Cape Province, South Africa.
Jessica M
JM
da Silva
Département d'Ecologie et de Gestion de la Biodiversité, Centre National de la Recherche Scientifique/Muséum National d'Histoire Naturelle, Paris, Île-de-France, France ; Evolutionary Morphology of Vertebrates Research Group, Department of Biology, Ghent University, Ghent, East Flanders, Belgium.
Anthony
A
Herrel
Applied Biodiversity Research Division, South African National Biodiversity Institute, Cape Town, Western Cape Province, South Africa ; Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, Eastern Cape Province, South Africa.
G John
GJ
Measey
Applied Biodiversity Research Division, South African National Biodiversity Institute, Cape Town, Western Cape Province, South Africa ; Department of Botany and Zoology, Stellenbosch University, Stellenbosch, Western Cape Province, South Africa.
Krystal A
KA
Tolley
D000220:Adaptation, Biological / Q000502:physiology*; D000704:Analysis of Variance; D000818:Animals; D005075:Biological Evolution; D001732:Bite Force; D001837:Body Weights and Measures; D017753:Ecosystem; D005260:Female; D008116:Lizards / Q000033:anatomy & histology* / Q000502:physiology*; D008297:Male; D012641:Selection, Genetic; D012727:Sex Characteristics; D013019:South Africa