[1] The Komodo dragon (Varanus komodoensis), alternatively and arguably more accurately known as the Komodo monitor, is the world’s largest lizard. Reaching lengths of more than 3 meters and a mass of 90kg, these reptiles primarily inhabit the tropical savannahs, and their boundary woodlands, of several islands in the Lesser Sunda region of southeast Indonesia. Although familiar to the inhabitants of these islands, the giant reptile did not become known to Europeans until 1910, being first described scientifically by P.A. Ouwens in 1912.
[2] The Komodo dragon has the distinction of possessing the most restricted range of any carnivore and is an endangered species of monitor lizard, confined to the small eastern Indonesian island of Komodo, the smaller neighboring islands of Rinca, Gili Dasami and Gili Motang, and parts of the large island of Flores. In the late 1970s, V. komodoensis became extinct on the tiny island of Padar, located between Komodo and Rinca, probably owing to the reduction of deer, its primary prey, from poaching. The species has also been reported from the island of Sumbawa, but its present status there remains unclear. There is also some suggestion that Komodo dragons are referred to in legendary events located in the Bimanese region of eastern Sumbawa.
[3] In coastal regions of Flores, the Komodo dragon is sympatric with another monitor, Varanus salvator, usually called the water monitor. The two species are quite distinct. Mature Komodo dragons can reach a length of over 3 m; in contrast, the Flores sub-species of V. salvator is relatively small, rarely growing to more than 1.5 m. With a weight that can approach 90 kg in mature wild specimens, the dragon is also much heavier than the water monitor and displays a distinctive form, especially the shape of the head and the overall build. When walking or running, Komodo dragons stand higher off the ground than do water monitors. Their colors also differ. While the more arboreal immature specimens of V. komodoensis display yellowish markings on a darker background and in this respect may resemble immature and even adult water monitors, mature Komodo dragons are largely a uniformly light brown, brownish-grey, or grey.
[4] While the Komodo dragon will opportunistically consume carrion, the animal is nevertheless a capable hunter that regularly preys on large mammals, both wild and domesticated. In fact, the dragon’s status as a predator has recently been highlighted by a study reporting the presence of a hitherto undiscovered venom gland with ducts leading to the teeth. Adult lizards kill the largest ungulate prey found on the islands – water buffalo, pigs and Timor deer – that often equal or exceed its own body mass. Individual dragons often kill their prey directly but also feed on carcasses of prey killed by other lizards or other agents. A large carcass enables multiple dragons to feed on one carcass at the same time.
[5] How Komodo dragons kill their prey is a controversial topic. Komodo dragons are thought to kill prey items with a lethal bite, which injects toxic bacteria into the blood stream of these animals. Until recently, the ‘‘bacteria as venom’’ dogma prevailed, based largely on a report by Auffenberg in the early 1980s. After spending a year observing Komodos in the wild, Auffenberg reported that when large Komodos attacked larger prey such as deer or water buffalo, those animals that were only injured would be overcome by infection and vulnerable to future predation. It was therefore postulated that ‘‘induction of wound sepsis and bacteremia through the bite of the Komodo dragon may be a mechanism for prey debilitation and mortality.’’ Other researchers later concurred with this model, citing the results in their study of aerobic oral flora; however, this study was before the discovery of venom glands in Komodo dragons.
[6] Regarding the possible origins and ecological bases of sepsis-inducing bacteria in the mouths of komodo dragons, some researchers had proposed that the bacteria were beneficial to the lizards, in essence a slow-acting venom that facilitated prey capture by the attacking lizard or other lizards, termed the ‘bacteria as venom’ model. Meanwhile, other researchers had questioned this interpretation, and proposed that sepsis-inducing bacteria were more plausibly acquired passively from prey and other environmental sources, with no role in prey acquisition, termed the ‘passive acquisition’ model. That model fits the observation that captive lizards and presumably newborns lack sepsis-inducing bacteria.
[7] In recent studies, researchers at the Venomics Research Laboratory have shown that Komodo dragon saliva also contains a potent and complex venom, which causes anticoagulation and induction of systemic shock in prey animals. After days, or even weeks, depending on the size of the prey and the number of bites, the prey succumbs to the lethal effects of systemic infection or shock. However, despite the fact that Komodo dragons fight and inflict bites on each other, these lizards are not known to be affected by septic infections. This may indicate that these animals have developed some type of immune mechanism(s) that protect against potential sepsis due to bites from other Komodo dragons.
[8] In addition to the mechanism of the Komodo dragon's venom, its reproductive behavior in captivity has also lead to research to investigate its genetic plasticity. Parthenogenesis, the production of offspring without fertilization by a male, is rare in vertebrate species, which usually reproduce after fusion of male and female gametes. However, research has indicated that female Komodo dragons may switch between asexual and sexual reproduction, depending on the availability of a mate.
[9] There are only two sexually mature female Komodo dragons in Europe, both of which were bred in captivity and are crucial to the success of the regional breeding programme. One of these at the Chester Zoo in the UK has never been kept with a male but has nevertheless produced a clutch of 25 eggs, of which 11 seemed to be viable. Three of these eggs collapsed early during incubation and provided embryonic material for genetic fingerprinting. Moreover, another captive-bred female, at the London Zoo, UK, now deceased, produced four viable eggs (from a clutch of 22) 2.5 years after her last contact with a male, which could be explained either by long-term sperm storage or by parthenogenesis. The eggs hatched 7.5 months later, and the young appeared to be healthy.
[10] Parthenogenesis has been reported in about 70 vertebrate species (roughly 0.1%). It occurs in captive snakes and has been implicated in one other species of Argus monitor lizard (Varanus panoptes). Researchers' observations of two separate occurrences of parthenogenesis at two different institutions indicate that the reproductive strategy might not be so unusual when Komodo dragons are isolated, even though reproductive plasticity in species thought to reproduce sexually is unexpected owing to the strict requirement to maintain diploidy.
[11] Parthenogenesis presents a previously unrecognized problem for the genetic management of threatened populations. Although captive breeding can be an essential part of a species’ conservation, the results indicated that studbook records of reptile species might no longer be accurate. A pressing concern with parthenogenesis is instantaneous homozygosity of the entire genome, as this inbreeding carries an associated risk of reduced fitness and an increased probability of extinction.
[12] It is common practice to keep extra females without males in captivity to maintain a sex-ratio bias towards the reproductively limiting sex and, because these are solitary animals, to reduce the risk of aggressive interactions. However, they are then subjected to strong selective pressures — as experienced by island colonists of a sexually reproducing damselfly (Ischnura hastata), for example, that became exclusively parthenogenetic. Parthenogenesis can also bias the sex ratio: in Varanus species, females have dissimilar chromosomes (Z and W), whereas the combination ZZ produces males, so the parthenogenic mechanism can produce only homozygous (ZZ or WW) individuals and therefore no females.
[13] Parthenogenesis in wild Komodo dragons is thought to be adaptive, given that viable offspring are always male and that sexual reproduction can resume, albeit between related individuals, in a colony founded by a single unfertilized female. Fewer than 4,000 Komodo dragons remain in the wild, of which perhaps fewer than 1,000 are mature females.
[14] Researchers' discovery of the potential for asexual reproduction in this species, and possibly in other reptiles presumed until now to be exclusively sexual, calls for further investigation into the genetic load experienced by the parthenogens, the frequency with which asexual offspring occur in captive and in wild populations, and the fitness consequences associated with facultative parthenogenesis.