Alice Leavey: 3D anatomical atlas of the heads of male and female adult Chamaeleo calyptratus

However, their potential in developmental studies has often been limited by the fact that most reptile embryos are deposited at relatively advanced stages, restricting access to the earliest phases of embryogenesis (Diaz Jr et al., 2015). Recently, the veiled chameleon, Chamaeleo calyptratus, has been adopted as a model system for the study of reptile development, evolution, and ecological niche specialization (Diaz Jr et al., 2015). For example, C. calyptratus has previously been used to study embryonic development (Andrews & Donoghue, 2004; Buchtová et al., 2013; Diaz Jr et al., 2019, 2022), disease (Hoby et al., 2010; Paré et al., 2006), sexual dimorphism (Bauerová et al., 2020; Vanhooydonck et al., 2007), social behavior (Ballen et al., 2014; Drown et al., 2022), communication (Denny et al., 2023; Ligon & McGraw, 2018; Tegge et al., 2020), phenotypic plasticity (Andrews, 2018), feeding mechanics (Herrel et al., 2014), and climbing mechanics (Krause & Fischer, 2013; Luger et al., 2021). 

Several factors have prompted this promotion to model organism. Unlike most other squamate reptiles, several chameleon species present an embryonic diapause shortly after fecundation, leading to its development at the time of oviposition being only at the early gastrula stage (Andrews & Donoghue, 2004; Measey et al., 2014). 

Thus, females do not need to be sacrificed in order to study early embryonic stages (Diaz Jr et al., 2017). They also display rapid growth within the first year of life (Karsten et al., 2008). Veiled chameleons have simple husbandry requirements, large clutch sizes, and relatively short life spans compared to other reptiles (Molnar et al., 2017). Furthermore, this species could be an excellent model for understanding how multiple factors influence skull morphology. Habitat or intraspecific aggression and communication have been linked to differences in chameleon skull morphology in general (Measey et al., 2009; Stuart-Fox et al., 2006), and in veiled chameleons in particular (Ligon & McGraw, 2018). Veiled chameleons are also sexually dimorphic in both body and casque size (Bauerová et al., 2020; Vanhooydonck et al., 2007), which has been linked to differences in skull morphology in other chameleon species (Dollion et al., 2017; da Silva et al., 2014). 

Changes in veiled chameleon skull morphology—particularly the enlargement of their distinctive casque—likely permit changes in the size and orientation of the jaw muscles, which in turn could facilitate dietary differences among age groups and between dimorphic males and females. For example, some dietary differences between sexes have been described for Bradypodion species (da Silva et al., 2016, but cf. Measey et al., 2011). Detailed descriptions of the structure, orientation, and distribution of the cranial muscles in C. calyptratus would allow future studies to be better equipped to address how structural variation is linked to biomechanical function, behavior, and ecology in chameleons.

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