About Ctenophorus decresii (Duméril & Bibron, 1837)
Ctenophorus decresii (Duméril & Bibron, 1837), commonly called the tawny dragon lizard, is most often characterized by variation in the colour of its throat. Early studies thought these colour differences were tied to different stages of sexual maturity, but current research indicates the variation is most likely caused by environmental factors, including maternal effects, incubation conditions, and post-hatching conditions, and genetics may also be a primary driver of variation. Male C. decresii throat colours are polymorphic and generally fall into one of four categories: entirely orange, entirely yellow, orange surrounded by yellow, or entirely grey. These colour variations also differ in intensity and separate the species into two primary northern and southern lineages. Orange-throated males tend to be more aggressive when defending their territories. It remains unclear how female lineage affects female throat colour expression, as females from both lineages generally share the same colouration. Overall, throat colour in this species is polymorphic, with the main phenotypic difference being whether the throat is yellow or orange. While both yellow and orange colouration are polymorphic, yellow throat colour is additionally strongly influenced by environment and circulating testosterone levels. Both polymorphism and environmental factors play major roles in throat colour expression, but it is not possible to determine which factor is more significant without further genomic analysis and linkage mapping. The average snout-to-vent length (SVL) of the species is 80.76 mm (3.180 in); the largest recorded individuals measure around 89 mm (3.5 in), while the smallest recorded individuals measure around 72 mm (2.8 in). Magnetic resonance imaging (MRI) has been used to study the brains of tawny dragon lizards; one study identified 224 visible brain structures, and found that the brain's left hemisphere is associated with the optic system. This research has been foundational for evolutionary neuroscience research on lizards, as mapping lizard neuroanatomy allows scientists to investigate connections between brain structure, behaviour, and cognitive function. C. decresii is endemic to Australia. It occurs in a few distinct locations across the country and is most commonly found in rocky habitats. Geographic isolation has separated different populations into three distinct identified lineages: northern, southern, and New South Wales. In New South Wales, the species is found in the far west of the state, in locations including the Barrier Ranges near Silverton. The entire Ctenophorus genus has a generally consistent body shape adapted to the Australian climate. Climate change and rising temperatures have caused significant population declines for C. decresii. While the species can effectively regulate its body temperature in high temperatures through behavioural thermoregulation, this adaptation comes at a cost: it leads to a much higher rate of water loss, which causes faster desiccation. The combination of high temperatures and decreased rainfall has severely impacted not only the survival of these lizards, but also their fecundity and growth rate, and these negative impacts are expected to worsen as conditions become more extreme. C. decresii is also threatened by habitat fragmentation and bush rock removal. Since the species already has narrow, specific habitat requirements, any habitat loss increases its risk of further population decline. Population size is currently trending downward, as the species' total habitat range has become very small in some areas. Most threats to the species are human-driven, but grazing animals also negatively impact populations. Constant grazing by domestic sheep, cattle, and feral goats reduces the area of habitat available for the lizards to hide and live. Fewer hiding places make the lizards more vulnerable to predation. When combined with human-driven land clearing, this habitat loss also leads to reduced genetic variation within the species. C. decresii can produce multiple clutches of eggs per year, and has a short lifespan. Egg laying typically occurs between September and October, with most clutches laid early in this window. Incubation temperature affects hatchling tail length and sex determination. At intermediate incubation temperatures, the proportion of male hatchlings is much higher than at very high or very low extreme incubation temperatures, where only female hatchlings are produced. This pattern of temperature-dependent sex determination means that the season when a female lays her eggs is a good predictor of the sex and body size of resulting hatchlings. Very low incubation temperatures tend to produce slower, less developed hatchlings. When incubated at a suitable temperature, hatchlings have higher body mass, which correlates with a higher probability of reproductive success, particularly in terms of clutch size and the body mass of the resulting offspring. Incubation length also influences hatchling body mass. Eggs laid earlier in the laying season have more time to develop, which gives them a greater opportunity to gain more body mass, while eggs laid later do not get this same opportunity. Larger body size provides fitness advantages for both males and females: larger males are better able to defend territories, and larger females have higher fecundity. Males that hold more territory also tend to have larger body masses.
