However, recent research suggests that the large size of Komodo dragons may be better understood as representative of a relict population of very large varanid lizards that once lived across Indonesia and Australia, most of which, along with other megafauna,[7] died out after the Pleistocene. Fossils very similar to V. komodoensis have been found in Australia dating to greater than 3.8 million years ago, and its body size remained stable on Flores, one of the handful of Indonesian islands where it is currently found, over the last 900,000 years, "a time marked by major faunal turnovers, extinction of the island's megafauna, and the arrival of early hominids by 880 ka."[7]
As a result of their size, these lizards dominate the ecosystems in which they live.[8] Komodo dragons hunt and ambush prey including invertebrates, birds, and mammals. Their group behaviour in hunting is exceptional in the reptile world. The diet of big Komodo dragons mainly consists of deer, though they also eat considerable amounts of carrion.[4]
Mating begins between May and August, and the eggs are laid in September. About twenty eggs are deposited in abandoned megapode nests or in a self-dug nesting hole.[4] The eggs are incubated for seven to eight months, hatching in April, when insects are most plentiful. Young Komodo dragons are vulnerable and therefore dwell in trees, safe from predators and cannibalistic adults. They take about eight to nine years to mature, and are estimated to live for up to 30 years.[4]
Komodo dragons were first recorded by Western scientists in 1910.[9] Their large size and fearsome reputation make them popular zoo exhibits. In the wild their range has contracted due to human activities and they are listed as vulnerable by the IUCN.[2] They are protected under Indonesian law, and a national park, Komodo National Park, was founded to aid protection efforts.
Etymology
The Komodo dragon is also known as the Komodo monitor or the Komodo Island monitor in scientific literature,[1] although this is not very common. To the natives of Komodo Island, it is referred to as ora, buaya darat (land crocodile) or biawak raksasa (giant monitor).[10][11]Evolutionary history
The evolutionary development of the Komodo dragon started with the Varanus genus, which originated in Asia about 40 million years ago and migrated to Australia. Around 15 million years ago, a collision between Australia and Southeast Asia allowed the varanids to move into what is now the Indonesian archipelago, extending their range as far east as the island of Timor. The Komodo dragon was believed to have differentiated from its Australian ancestors 4 million years ago. However, recent fossil evidence from Queensland suggests that the Komodo dragon evolved in Australia before spreading to Indonesia.[7][12] Dramatic lowering of sea level during the last glacial period uncovered extensive stretches of continental shelf that the Komodo dragon colonized, becoming isolated in their present island range as sea levels rose afterwards.[7][11]Description
Senses
The Komodo dragon does not have an acute sense of hearing, despite its visible earholes, and is only able to hear sounds between 400 and 2000 hertz.[11][16] It is able to see as far away as 300 metres (980 ft), but because its retinas only contain cones, it is thought to have poor night vision. The Komodo dragon is able to see in color, but has poor visual discrimination of stationary objects.[17]
The Komodo dragon was formerly thought to be deaf when a study reported no agitation in wild Komodo dragons in response to whispers, raised voices, or shouts. This was disputed when London Zoological Garden employee Joan Proctor trained a captive specimen to come out to feed at the sound of her voice, even when she could not be seen.[18]
Ecology
For shelter, the Komodo dragon digs holes that can measure from 1–3 metres (3–10 ft) wide with its powerful forelimbs and claws.[19] Because of its large size and habit of sleeping in these burrows, it is able to conserve body heat throughout the night and minimize its basking period the morning after.[20] The Komodo dragon hunts in the afternoon, but stays in the shade during the hottest part of the day.[21] These special resting places, usually located on ridges with a cool sea breeze, are marked with droppings and are cleared of vegetation. They serve as a strategic location from which to ambush deer.[22]
Diet
Komodo dragons eat by tearing large chunks of flesh and swallowing them whole while holding the carcass down with their forelegs. For smaller prey up to the size of a goat, their loosely articulated jaws, flexible skull, and expandable stomach allow it to swallow its prey whole. The vegetable contents of the stomach and intestines are typically avoided.[22] Copious amounts of red saliva that the Komodo dragons produce help to lubricate the food, but swallowing is still a long process (15–20 minutes to swallow a goat). A Komodo dragon may attempt to speed up the process by ramming the carcass against a tree to force it down its throat, sometimes ramming so forcefully that the tree is knocked down.[22] To prevent itself from suffocating while swallowing, it breathes using a small tube under the tongue that connects to the lungs.[14] After eating up to 80 percent of its body weight in one meal,[8] it drags itself to a sunny location to speed digestion, as the food could rot and poison the dragon if left undigested for too long. Because of their slow metabolism, large dragons can survive on as little as 12 meals a year.[14] After digestion, the Komodo dragon regurgitates a mass of horns, hair, and teeth known as the gastric pellet, which is covered in malodorous mucus. After regurgitating the gastric pellet, it rubs its face in the dirt or on bushes to get rid of the mucus, suggesting that it, like humans, does not relish the scent of its own excretions.[14]
The Komodo dragon drinks by sucking water into its mouth via buccal pumping (a process also used for respiration), lifting its head, and letting the water run down its throat.[24]
Saliva
In late 2005, researchers at the University of Melbourne speculated that the perentie (Varanus giganteus), other species of monitor, and agamids may be somewhat venomous. The team believes that the immediate effects of bites from these lizards were caused by mild envenomation. Bites on human digits by a lace monitor (V. varius), a Komodo dragon, and a spotted tree monitor (V. scalaris) all produced similar effects: rapid swelling, localized disruption of blood clotting, and shooting pain up to the elbow, with some symptoms lasting for several hours.[31]
In 2009, the same researchers published further evidence demonstrating that Komodo dragons possess a venomous bite. MRI scans of a preserved skull showed the presence of two venom glands in the lower jaw. They extracted one of these glands from the head of a terminally ill specimen in the Singapore Zoological Gardens, and found that it secreted a venom containing several different toxic proteins. The known functions of these proteins include inhibition of blood clotting, lowering of blood pressure, muscle paralysis, and the induction of hypothermia, leading to shock and loss of consciousness in envenomated prey.[32][33] As a result of the discovery, the previous theory that bacteria were responsible for the deaths of Komodo victims was disputed.[34]
Kurt Schwenk, an evolutionary biologist at the University of Connecticut, finds the discovery of these glands intriguing, but considers most of the evidence for venom in the study to be "meaningless, irrelevant, incorrect or falsely misleading". Even if the lizards have venomlike proteins in their mouths, Schwenk argues, they may be using them for a different function, and he doubts that venom is necessary to explain the effect of a Komodo dragon bite, arguing that shock and blood loss are the primary factors.[35][36]
Reproduction
Mating occurs between May and August, with the eggs laid in September.[11] During this period, males fight over females and territory by grappling with one another upon their hind legs with the loser eventually being pinned to the ground. These males may vomit or defecate when preparing for the fight.[18] The winner of the fight will then flick his long tongue at the female to gain information about her receptivity.[8] Females are antagonistic and resist with their claws and teeth during the early phases of courtship. Therefore, the male must fully restrain the female during coitus to avoid being hurt. Other courtship displays include males rubbing their chins on the female, hard scratches to the back, and licking.[37] Copulation occurs when the male inserts one of his hemipenes into the female's cloaca.[17] Komodo dragons may be monogamous and form "pair bonds", a rare behavior for lizards.[18]The female lays her eggs in burrows cut into the side of a hill or in the abandoned nesting mounds of the Orange-footed Scrubfowl (a moundbuilder or megapode), with a preference for the abandoned mounds.[38] Clutches contain an average of 20 eggs which have an incubation period of 7–8 months.[18] Hatching is an exhausting effort for the neonates, who break out of their eggshells with an egg tooth that falls off soon after. After cutting out the hatchlings may lie in their eggshells for hours before starting to dig out of the nest. They are born quite defenseless, and many are eaten by predators.[25]
Young Komodo dragons spend much of their first few years in trees, where they are relatively safe from predators, including cannibalistic adults, who make juvenile dragons 10% of their diet.[18] According to David Attenborough, the habit of cannibalism may be advantageous in sustaining the large size of adults, as medium-sized prey on the islands is rare.[23] When the young must approach a kill, they roll around in fecal matter and rest in the intestines of eviscerated animals to deter these hungry adults.[18] Komodo dragons take about three to five years to mature, and may live for up to 50 years.[19]
Parthenogenesis
Main article: Parthenogenesis
Komodo dragons have the ZW chromosomal sex-determination system, as opposed to the mammalian XY system. Male progeny prove that Flora's unfertilized eggs were haploid (n) and doubled their chromosomes later to become diploid (2n) (by being fertilized by a polar body, or by chromosome duplication without cell division), rather than by her laying diploid eggs by one of the meiosis reduction-divisions in her ovaries failing. When a female Komodo dragon (with ZW sex chromosomes) reproduces in this manner, she provides her progeny with only one chromosome from each of her pairs of chromosomes, including only one of her two sex chromosomes. This single set of chromosomes is duplicated in the egg, which develops parthenogenetically. Eggs receiving a Z chromosome become ZZ (male); those receiving a W chromosome become WW and fail to develop.[44][45]
It has been hypothesized that this reproductive adaptation allows a single female to enter an isolated ecological niche (such as an island) and by parthenogenesis produce male offspring, thereby establishing a sexually reproducing population (via reproduction with her offspring that can result in both male and female young).[44] Despite the advantages of such an adaptation, zoos are cautioned that parthenogenesis may be detrimental to genetic diversity.[46]
History
Discovery by the Western world
Studies
Conservation
Komodo dragons avoid encounters with humans. Juveniles are very shy and will flee quickly into a hideout if a human comes closer than about 100 metres (330 ft). Older animals will also retreat from humans from a shorter distance away. If cornered, they will react aggressively by gaping their mouth, hissing, and swinging their tail. If they are disturbed further, they may start an attack and bite. Although there are anecdotes of unprovoked Komodo dragons attacking or preying on humans, most of these reports are either not reputable or caused by defensive bites. Only a very few cases are truly the result of unprovoked attacks by abnormal individuals which lost their fear towards humans.[25]
Volcanic activity, earthquakes, loss of habitat, fire,[14][51] loss of prey due to poaching, tourism, and illegal poaching of the dragons themselves have all contributed to the vulnerable status of the Komodo dragon. Under Appendix I of CITES (the Convention on International Trade in Endangered Species), commercial trade of skins or specimens is illegal.[53][54]
On Padar, a former population of the Komodo Dragon became extinct, of which the last individuals were seen in 1975.[55] It is widely assumed that the Komodo dragon died out on Padar after a strong decline of the populations of large ungulate prey, for which poaching was most likely responsible.[56]
In captivity
The first Komodo dragon was exhibited in 1934 at the Smithsonian National Zoological Park, but it lived for only two years. More attempts to exhibit Komodo dragons were made, but the lifespan of these creatures was very short, averaging five years in the National Zoological Park. Studies done by Walter Auffenberg, which were documented in his book The Behavioral Ecology of the Komodo Monitor, eventually allowed for more successful managing and reproducing of the dragons in captivity.[50]
A variety of behaviors have been observed from captive specimens. Most individuals are relatively tame within a short period of time,[58][59] and are capable of recognizing individual humans and discriminating between more familiar keepers.[60] Komodo dragons have also been observed to engage in play with a variety of objects, including shovels, cans, plastic rings, and shoes. This behavior does not seem to be "food-motivated predatory behavior".[8][11][61]
Even seemingly docile dragons may become aggressive unpredictably, especially when the animal's territory is invaded by someone unfamiliar. In June 2001, a Komodo dragon seriously injured a man when he entered its enclosure at the Los Angeles Zoo after being invited in by its keeper. He was bitten on his bare foot, as the keeper had told him to take off his white shoes, which could have potentially excited the Komodo dragon.[62][63] Although he escaped, he needed to have several tendons in his foot reattached surgically.
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