Central Bearded Dragon

The central bearded dragon (''Pogona vitticeps''), also known as the inland bearded dragon, is a species of agamid lizard found in a wide range of arid to semiarid regions of eastern and central Australia.

Taxonomy

''Pogona vitticeps'' was first described by German zoologist Ernst Ahl in 1926, who placed it in the genus ''Amphibolurus''.

Description

Mature bearded dragons can reach a total length of around , or two feet, with the tail accounting for more than half of that. Some sexual dimorphism is present, as males can be distinguished from females by a wider cloacal opening, a wider tail-base, a larger and more angular head, a more developed beard ( guttural throat-pouch), and the obvious possession of hemipenes. Males also have more pronounced femoral pores than females (these can be seen as waxy bumps on the underside of the back legs). Bearded dragons can vary widely in scale colouration, ranging from a blend of light brown, reddish-brown, red, yellow, white and orange; additionally, numerous colour "morphs" exist today, due to selective breeding by hobbyists and professionals. Somewhat like chameleons, bearded dragons are capable of inducing moderate changes in coloration based on mood. Additionally, they can slightly raise the pointed scales that run along either side of their throat, neck, head and torso by inhaling air into their lungs to maximum capacity, thus appearing larger to predators. The sharp-looking growths and protrusions are, in actuality, quite soft and delicate to the touch, but may be off-putting to predators such as birds of prey, foxes, feral dogs or dingos. When cornered or threatened, a bearded dragon will flatten its body against the ground, expand its rib cage outwards, open its mouth and expand its "beard" (guttural pouch; similar to a frilled lizard’s defense tactics, albeit on a smaller scale). The guttural pouch is what essentially earns the species its nickname of "beardie", and can darken in color when threatened or during courtship or territorial displays. Both of these characteristics appear similar to a human's beard. Males typically have a darker "beard" than females, and during mating season and courtship it will typically darken to near-black. The bearded dragon, like most agamid lizards, has strong legs which enable it to lift its body completely off the ground while it moves. This is done to reduce heat absorption from the hot ground, and facilitates airflow under the body.

A study conducted in 2014 established the existence of endogenous circadian rhythm in pigmentation changes in ''P. vitticeps;'' if exposed to sun, the dorsal skin of the lizard becomes darker, and if exposed to darkness, it becomes lighter. Under constant darkness (i.e. in the subjective night), the lizard's dorsal skin becomes the lightest.

Many species of ''Pogona'' have a parietal eye (or "third eye"), a photoreceptor found on the center of the forehead. This unique feature is responsible for thermoregulation and hormone regulation, and possibly helps the lizards make decisions based on the seasons, weather, etc. A March 2020 study of the central bearded dragon found that light-dependent magnetoreception occurs when light with a wavelength under 580 nanometers enters the parietal eye.

Ecology and behaviour

''P. vitticeps'' is native to semiarid woodland, arid woodland, and rocky desert areas of Central Australia, primarily the inland regions of New South Wales, Northern Territory, Queensland and South Australia. They are skilled climbers and often spend just as much time perching and basking on tree limbs, fence posts, and within bushes as they do on the ground. They often spend the mornings and early evenings sunning themselves on exposed branches or rocks, and retreat to shady areas or burrows during the hottest parts of the afternoon.

''P. vitticeps'' are opportunistic omnivores. They live in areas where food may be hard to find, thus they are not particularly finicky eaters. Their stomachs are large enough to accommodate significant quantities of vegetation, fruits, insects, worms, and the occasional small rodent or lizard. Favored insect prey includes crickets, grasshoppers, beetles, caterpillars, moths, butterflies, or any insect that can comfortably be swallowed. They will also eat mealworms, kingworms, parsley and kale and vegetables like pepper and sweet potato. An analysis of their summer diet as wild adults revealed that 26% of the food ingested (by volume) was '' Drepanotermes'' termite alates, which are only seasonally available. Plant matter made up 54% of the diet by volume.

Bearded dragons do not vocalize, except to hiss softly when threatened. Instead, they communicate through colour displays, posture, and physical gestures, such as arm waving and head bobbing. Bearded dragons are not social animals, but will sometimes gather in groups, especially in popular feeding or basking areas. At these times, a distinct hierarchy will emerge: the highest-ranking animals will take the best – usually the highest or sunniest – basking spots, and all other individuals arrange themselves lower down. If a low-ranking animal tries to challenge one of the dominant dragons, the dominant animal will demonstrate its superiority by bobbing its head and inflating its beard, at which point the challenger may signal submission by waving one of its arms in a slow or fast circle. If the low-ranking dragon does not submit, it will return the head bob, and a standoff or fight may ensue.

The head bob gestures are:

*Slow bowing motion – often used by adult females to signal submission to a male
*Fast bob – used by males to signal dominance (often accompanied by an inflated and/or blackened beard)
*Violent bob – used by males just before mating; much more vigorous, and usually sets the animal's whole body in motion
*Both males and females will occasionally do fast and violent head bobs, which shows they are stressed out and need to be isolated.

The male will only wave to show submission to a dominant male, whereas the female will wave, followed by a slow head bob, to show she is ready to mate. Gravid females will often refuse the advances of a male by chasing him and lying on his back.

When under direct attack, the central bearded dragon opens its mouth to display its yellow membranes and extend its beard. It darkens the colour of its skin and flattens its body, and will hiss and make small jumps towards the attacker. Bearded dragons are not known to attack humans.

Adult male bearded dragons can bite more forcefully than adult females and this difference is associated with greater head dimensions.

Bearded dragons have been shown to be able to learn from watching the behaviour of conspecifics. An experiment demonstrated that after one individual was trained to open a door to reach a food item, most other bearded dragons watching this action were able to perform it as well.

Reproduction

The age of sexual maturity has not been measured, although it is estimated to be about one or two years. Body size and growth rates are more important than age when determining sexual maturity in bearded dragons. Males will become very aggressive towards each other and will assert their dominance by inflating their beards and through fast head bobbing. Breeding typically occurs in the early spring. Females will lay a clutch of 11–30 oblong-shaped eggs in a shallow nest dug in the sand. After being laid, the eggs are buried and are left unattended. The eggs will hatch approximately 60 to 80 days later, depending on the incubation temperature. In captivity, they can be incubated in a styrofoam fish box, but without a male lizard, the female's eggs will not be fertile. However, a female bearded dragon can retain sperm, and thus produce fertile eggs even after being separated from a male.

Courtship involves the male "head bobbing" to display dominance. If the female displays submissive behaviour, the male will use his mouth to grab the back of the female's head and the male will also wrap his front legs around the female's upper torso to keep her from moving. Copulation and insemination are quick. The gestation period averages about a month and a half.

Thermally-induced sex reversal

A 2015 study showed that high-temperature incubation of eggs transforms genetically male individuals into functional females. Normally their sex is determined genetically. Males have ZZ sex chromosomes, females ZW. However, when their eggs are incubated at temperatures above 32 °C (90 °F) some genetic males are born female. These females are fertile, sometimes producing more eggs than the ZW females. As juveniles, the sex reversed ZZ females resemble ZZ males with respect to relative tail length and boldness. However, as adults sex-reversed ZZ females lack the larger head and greater bite force of ZZ males and more closely resemble ZW females.

Captive breeding

Several of the ''Pogona'' genus are bred in captivity as pets; the two most popular are this species and the Rankin's dragon (''Pogona henrylawsoni''). The majority of captive-bred bearded dragons today are thought to have originated from stock illegally exported from Australia during the 1970s. Australia has since prohibited the importation and exportation of its native species, thus guaranteeing any bearded dragons found today outside of Australia have been captive-bred, hand-raised and socialized with human interaction from a very young age. The anti-exportation policies in Australia also ensures that bearded dragons (and indeed all Australian species found abroad) are not forcibly taken or poached from the wild and shipped around the globe via the black market—as is the case with Madagascar's rare and sensitive chameleons, geckos and frogs, for example.

However, captive bearded dragons worldwide are threatened by Agamid ''adenovirus'', an HIV-like virus that compromises the immune system of the animal, and leads to death from other diseases. Still, the majority of the infections are subclinical. Subclinically-infected animals show no symptoms, but are active carriers of the disease and will thus infect other bearded dragons. As with nearly all reptiles and amphibians, captive bearded dragons are commonly susceptible to metabolic bone disease (MBD) if not provided with adequate calcium in the diet, a condition similar to human osteoporosis. Numerous supplements are available for sale to dust insect prey or plant matter with before each feeding. Additionally, they may be at-risk of constipation or digestive impactions from inadvertently ingesting decorative sand, gravel or even insects that are inappropriately sized for them to consume.

When the female is ready to lay eggs, she will generally stop eating, and spend most of her time trying to dig.

A morph with underdeveloped dorsal scales is popular as a terrarium pet under the name of "leatherback bearded dragon".

References

Further reading

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{{Taxonbar, from=Q792301

Pogona
Agamid lizards of Australia
Reptiles described in 1926
Taxa named by Ernst Ahl
Reptiles as pets