Picking a tree: habitat use by the tree agama, Acanthocercus atricollis atricollis, in South Africa

INTRODUCTION Habitat selection by animals has been defined as the active choice of an area from a range of alternatives in the absence of constraints (Partridge 1978). In reality, no area is free of constraints and habitat selection depends on the physical structure of the environment, the physiology of the animal, food availability and protection from predators (Ward & Lubin, 1993). There are numerous studies on habitat selection of arboreal and semi-arboreal lizards, mostly dealing with the polychrotid genus Anolis (e.g. Rand 1964; Andrews 1971; Vitt et al. 1981). For arboreal lizards, tree structure is of obvious importance because trees provide foraging sites, nest sites, and refuges from predators and trees that have greater surface area and complexity are often more favourable (Cooper 1993). Retreat sites with suitable thermal conditions are also crucial for ectotherms as behavioural and physiological processes are strongly dependent on temperature (Huey 1982). Habitat selection may also involve differences in sexual or ontogenetic responses to the physical environment (Heatwole 1977). Differences in body size have been shown to influence interspecific habitat use (Vitt et al. 1981) and should similarly influence different size classes within the same species (Jenssen et al. 1998). Often, differences in foraging behaviour (Bartosiewicz 1987) and predation risk (Shine 1989) cause males, females and juveniles to use different habitats. Competition avoidance may be another cause of intraspecific habitat segregation when individuals are competing for similar sized prey and taxa (Jenssen et al. 1998). Little data exists on habitat use in southern African lizards (but see Pianka 1986). Such data are important for comparative purposes and for understanding life history patterns. Furthermore, tree harvesting is increasingly placing pressure on the environment and baseline data on arboreal species dependent on trees may be important for future conservation plans. The objective of this study was to determine habitat selection in the tree agama (Acanthocercus a. atricollis). Tree agamas occur throughout Africa, from Ethiopia in the north to coastal KwaZuluNatal, South Africa in the south. Acanthocercus a. atricollis are large (120–150 mm snout–vent length), diurnal, arboreal lizards that are sexually dimorphic in size and colouration. Mature males are larger than females and have a bright blue head and throat and a broad yellow-green vertebral stripe, while females remain olive-coloured with black marbling (Branch 1998). They are ambush foragers, spend most of their time foraging in trees, and show no sexual differences in diet or foraging behaviour (Reaney & Whiting 2002). We set out to determine 1) if A. a. atricollis select trees nonrandomly; 2) the physical attributes of trees they use; 3) if there are sexual and age/sizerelated differences in habitat use; and 4) what nocturnal retreat sites they use.