Sexual dimorphism in bite performance drives morphological variation in chameleons

Abstract

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.

Figure 1. Photographs of the five dwarf chameleon forms within the B. melanocephalum-B. thamnobates species complex from southern KwaZulu-Natal Province, South Africa.

Figure reprinted from .

Figure 2. Distributions of four of the five phenotypic forms of the B. melanocephalum-B. thamnobates species complex.

Numbers indicate field sites sampled in this study: 1, Durban; 2, Hilton; 3, Karkloof; 4, Howick; 5, Dargle; 6, Nottingham Road; 7, Kamberg Nature Reserve.

Figure 3. Nine head measurements recorded for each chameleon. Images on the left are based on a µCT-scan, courtesy of R. Boistel, Université de Poitiers.

CT, coronoid process of mandible to snout tip; QT, posterior surface of quadrate to snout tip; HW, head width.

Figure 4. Regression plots illustrating the correlation between SVL and bite force within the B. melanocephalum-B. thamnobates species complex.

Figure 5. Error plots depicting mean absolute bite force for the five phenotypic forms.

Absolute force equates to log₁₀-transformed bite force. Solid circles represent males; empty circles, females.

Figure 6. Error plots depicting mean relative bite force for the five phenotypic forms.

Relative forces represent the residual values from regressing log₁₀Bite Force against log₁₀SVL. Solid circles represent males; empty circles, females.