Linking microhabitat structure, morphology and locomotor performance traits in a recent radiation of dwarf chameleons

Abstract

Summary Evidence that morphological traits associated with particular environments are functionally adapted to those environments is a key component to determining the adaptive nature of radiations. Adaptation is often measured by testing how organisms perform in diverse habitats, with performance traits associated with locomotion thought to be among the most ecologically relevant. We therefore explored whether there are relationships between morphology, locomotor performance traits (sprint speed, forefoot and tail grip strength on broad and narrow dowels) and microhabitat use in five phenotypic forms of a recent radiation of dwarf chameleon – the Bradypodion melanocephalum–Bradypodion thamnobates species complex – to determine whether morphological differences previously identified between the forms are associated with functional adaptations to their respective habitats, which can be broadly categorized as open or closed‐canopy vegetation. The results showed significant differences in both absolute and relative performance values between the phenotypic forms. Absolute performance suggests there are two phenotypic groups – strong (B. thamnobates and Type B) and weak (B. melanocephalum and Types A and C). Relative performance differences highlighted the significance of forefoot grip strength among these chameleons, with the closed‐canopy forms (B. thamnobates, Types B and C) exceeding their open‐canopy counterparts (B. melanocephalum, Type A). Little to no differences were detected between forms with respect to sprint speed and tail strength. These results indicate that strong selection is acting upon forefoot grip strength and has resulted in morphological adaptations that enable each phenotypic form to conform with the demands of its habitat. This study provides evidence for the parallel evolution of forefoot grip strength among dwarf chameleons, consistent with the recognition of open and closed‐canopy ecomorphs within the genus Bradypodion.

The relationship between cranial morphology, bite performance, diet and habitat in a radiation of dwarf chameleon (Bradypodion)

“…thamnobates radiation would result in dietary differences between the morphological forms, our analyses did not detect significant differences between them. The closed-canopy chameleons, B. thamnobates and Type B, which possess larger heads across all dimensions measured, as well as harder absolute bite forces (da Silva & Tolley, 2013;da Silva et al, 2014b), did not consume larger or harder prey than the open-canopy forms. We also predicted that females would consume larger and harder prey than males as a result of them being larger and biting harder; however, no differences were detected between the sexes within the open-canopy forms, and closed-canopy females typically consumed softer prey or prey of equal hardness.…”

“…This spatial displacement and the associated differences in macro-and microhabitats, which primarily involve differences in their exposure to predators and communication ability, resulted in different selective pressures acting upon these chameleons. Sexual selection appears to be the predominant force within the closed-canopy habitats, driving the development of conspicuous secondary sexual characteristics, such as a proportionally larger head and casque, which results in a greater absolute bite force (da Silva & Tolley, 2013;da Silva et al, 2014b). These features better enable chameleons to communicate to conspecifics from a distance, reducing the need for confrontations, which can be harmful, especially malemale encounters (Stuart-Fox et al, 2006;Tolley & Burger, 2007).…”

“…Open-canopy habitats are more exposed to aerial predators compared to the closed-canopy forests . Consequently, the need to communicate to conspecifics is outweighed by the need to avoid predation, thus explaining the diminutive size (in both absolute and relative terms) of B. melanocephalum and Type A chameleons and their weaker bite forces (da Silva & Tolley, 2013;da Silva et al, 2014b).…”

“…forests, transformed shrubby landscapes) that contain broader perching substrates arranged both vertically and horizontally. These ecological differences were found to correlate with functional differences in forefoot grip strength, suggesting that the forms are adapted morphologically to their different environments (da Silva et al, 2014b).…”

Repeatability of locomotor performance and of morphology – locomotor performance relationships

“…Common patterns of repeated ( parallel or convergent) evolution of the same performance-environment relationships emphasise the role that selection plays in generating among-taxa variation in locomotor performance, and in the physiological, morphological and behavioural traits that determine performance (Taylor and McPhail, 1985;McGuigan et al, 2003;Langerhans and DeWitt, 2004;Langerhans et al, 2006;Dalziel et al, 2012;Franssen et al, 2013;Fu et al, 2013;da Silva et al, 2014;Haas et al, 2015;Nelson et al, 2015). Despite the adaptive significance of locomotion, how the variation in locomotion is generated among individuals within a population, which is what natural selection acts upon, is relatively poorly understood.…”