Ctenophorus decresii, also known commonly as the tawny dragon, tawny crevice-dragon, or tawny rock dragon, is a species of lizard in the family Agamidae. The species is endemic to Australia.
C. decresii is known for its variations in throat colours which change based on environmental conditions. Its primary food sources consist of both vegetation and invertebrates, and it prefers to live in rocky habitats. The average snout-to-vent length (SVL) of the species is , with larger individuals being around and smaller individuals around . The optimal time for mating in this species is two to three weeks after the females emerge from hibernation. Eggs are typically laid from September to October with most of them being laid earlier in the period.
Etymology and taxonomy C. decresii is a member of the genus Ctenophorus, which is a very diverse group of lizards found throughout Australia. The specific name, decresii, refers to L'Île de Decrès, which was the French name for Kangaroo Island in 1837.Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). The Eponym Dictionary of Reptiles. Baltimore: Johns Hopkins University Press. xiii + 296 pp. . (Ctenophorus decresii, p. 67). The usual common names for the species are tawny dragon, tawny crevice-dragon, and tawny rock dragon.
The entire genus of lizards is sexually dimorphic, in which males and females exhibit different characteristics. Neck and overall colouration distinguishes male lizards from female and juvenile lizards.
Description C. decresii are often characterised by the colour of their throats. Variations of colour in these lizards were previously attributed to different levels in sexual maturity. However, these variations are most likely due to environmental conditions such as "maternal effects, incubation conditions or post-hatching conditions". There is also a possibility that genetics are primarily responsible for the variations. The colours of male lizards can vary, but they are usually orange only, yellow only, orange surrounded by yellow, or grey only throats. The colours can also vary in degree of intensity and are due to different lineages, northern and southern. Males with orange throats tend to be more aggressive when it comes to defending their territory. It is still unclear how the lineages of females impact the expression of their colours especially since females from wither lineage tend to have the same colouration. From this, it can be concluded that throat colour in these lizards is polymorphic, specifically in determining whether the phenotype is yellow or orange. Although both colours are polymorphic, the yellow colour is also greatly influenced by environment and levels of testosterone present. Polymorphism and environmental factors play major roles in colour expression, but it is impossible to say which one is more significant without genomic analysis and linkage mapping.
The average snout-to-vent length (SVL) of the species is with larger individuals being around and smaller individuals around .
Magnetic resonance imaging (MRI) has been used on the tawny dragon lizards to examine their brains. A study found that the brain had 224 structures that could be seen. The left hemisphere in the brain is associated with the optic system. This research was fundamental in the study of evolutionary neuroscience in lizards. By having the neuroanatomy of lizards, scientists are able to see how behaviour and cognitive function are related to structures of the brain.
Polymorphism Ctenophorus decresii is known to display polymorphisms in throat colouration. Within the species C. decresii, morphs can range from grey and white to a bright red. Some of the variants include multi-coloured, grey, yellow, orange, blue, and red-throated lizards. This variation comes from the diverse geographic locations in which C. decresii can be found, such as South Australia, New South Wales, and other areas throughout Australia. One geographic location may favor a certain throat colour for C. decresii, while a different location may favor a different colour.
Although there is a lot of diversity in throat colours, the colour variants in the throats are discrete, meaning the morphologies of individual lizards could be placed into specific categories After using objective methods to identify the colour morphs, statistical tests were run. The tests analyzed variation based on granularity, segmentation, and comparison with visual background. One group of lizards had similar granularity, segmentation, and colouration. Another group also had similar granularity, segmentation, and colouration that differed from the first group. There were very few lizards with in-between phenotypes. However, within the categories, there still is slight variation in the shades of colours of the lizards. Because throat colouration is a discrete trait, it is highly heritable. This is a key reason that the discrete colour variation has been maintained over multiple generations. The offspring will have similar or the same colouration as the parents, therefore making the colouration carry out over generations.
C. decresii's diversity in discrete throat colour may be caused by a combination of sexual selection and natural selection.
The C. decresii throat colours can be classified into two main, discrete categories—dull and bright-coloured. The dull-coloured throats give lizards a fitness advantage because the lizards are harder for predators, like birds, to see. The dull throats allow the lizards to avoid predators and survive longer; therefore, this trait became more prevalent in that population of lizards.
However, bright-coloured throats also give the lizards a fitness advantage because the bright-coloured throats attract more females, and therefore those males are more likely to reproduce. Although the bright-coloured throats increase the likelihood of being eaten by predators, they also increase sexual success. Therefore, the prevalence of bright-coloured throats is maintained in this population of lizards. Based on the conflicting benefits of dull and bright-coloured throats, it is understandable that both morphs have been maintained in this species. Each trait gives a lizard a different evolutionary advantage.
Diet C. decresii consume a wide variety of foods and are omnivores. Their main diet consists of invertebrates and vegetation. Invertebrates include crickets, moths, fly maggots, and locusts.
Habitat and geographic range C. decresii is endemic to Australia. While the species is most commonly found in rocky areas throughout Australia, its exact habitat varies as it is found in a few distinct locations throughout Australia.
Scientists have identified the different populations of lizards as separate lineages due to geographic isolation. The three lineages of C. decresii are the northern, southern, and New South Wales lineages. In New South Wales, they are found in the far west of the state, in the Barrier Ranges, near Silverton, among other locations. The entire genus Ctenophorus maintains a generally constant body shape, adapted for the Australian climate.
Behaviour Reproduction C. decresii are characterized by their ability to produce multiple clutches in a year but a short lifespan.McFadden, Michael, and Peter S. Harlow. "Captive reproduction and longevity in Tawny Crevice (Ctenophorus decresii) and Central Netted Dragons (C. nuchalis)". HERPETOFAUNA-SYDNEY- 37.1 (2007): 22. Eggs are typically laid from September to October, with most of them being laid earlier in the period.
Incubation temperature affects the hatchling tail length and sex. At intermediate incubation temperatures, the proportion of males to females was much higher compared to extreme incubation temperatures (very high or very low) where females were the only sex seen. This shows that the season in which the females lay the eggs is good indicator of which sex will be seen in the hatchlings and body size, also known as temperature-dependent sex determination. Very low incubation temperatures are often associated with slower, less developed hatchlings. If incubated at a suitable temperature, hatchlings will have a higher body mass. These individuals will also have a higher probability of reproductive success especially in terms of clutch size and offspring body mass. Studies suggest that body mass is also influenced by the length of incubation. Eggs laid closer to the beginning of the season have more time to develop thus giving them more of an opportunity to gain more body mass. On the other hand, those laid later in the season do not have the same opportunity. Larger body sizes provide an advantage to males and females. Males are better able to defend their territories, and females have higher fecundity. Males with more territory also had larger body masses.
Mating As mentioned in the description section, the colouration of the male tawny dragon lizards can vary, especially by region (northern and southern). For mating, southern females are more likely to mate with southern males however, they are much less selective in their mate preferences than northern females. Both southern and northern males are more likely to be rejected by northern females. Southern males also prefer to mate with southern females because they share the same lineage. This suggests that preferences by either sex are driven by behavioural differences regionally. These lizards primarily prefer to mate with partners that are in the same region geographically. Overall, it appears that speciation is primarily driven by sexual selection with neither sex being more selective than the other.
Male-male interactions In these lizards, northern males are considered more aggressive than southern males. Orange, northern males are the most aggressive, and southern males are the least aggressive. But, regardless of lineage, males can differentiate their aggression. Aggression occurs at a higher rate when two males have not interacted with one another. When encountering a male that he had encountered before aggression was significantly lower. Repeated interactions were shown to more effective in limiting aggression between males. If there has already been a previous encounter, it is less likely that the individual will be a threat to the livelihood or territory of that male. Being able to recognize which lizards they have already encountered gives lizards the opportunity to limit costly interactions. They are able to tell which individuals they have already won or lost to. To distinguish between individuals, the lizards use physical characteristics as opposed to chemical or behavioural cues. This species uses colouration of the throats to distinguish between lizards, similar to how scientist do in experiments. When researchers controlled for throat colouration, it was more difficult for lizards to determine if they had already encountered the lizard. There were similar levels of aggression for familiar and unfamiliar males.
Another indicator of aggression is the presence of black chest patches. Males with these patches are not only more aggressive but also more likely to initiate and win fights. Size and residency status also seem to be deciding factors in the winner of territories with size being a bit more important. Regardless of size, residents are more effective at defending their territories against non-residents. In these cases, larger males do not have the advantage. However, when both lizards are residents (or non-residents), the larger lizard has more success at defending their territory.