Odatria

The subgenus ''Odatria'', sometimes known as the dwarf monitor lizards, consists of small monitor lizards found in Australia and Indonesia. Species in this subgenus include the smallest monitor species in the world, the tiny 16 gram Dampier Peninsula monitor, but also includes some more medium sized species such as the 240 gram black-palmed rock monitor.

Taxonomy

''Odatria'' was coined by John Edward Gray in 1838, albeit as a genus name.

''Odatria'' is the most species diverse subgenus of monitor lizards, with 22 different species. The subgenus also includes two species complexes represented by the spiny-tailed monitor and the Timor monitor. Alternatively, Vidal ''et al.'' 2012 splits ''Odatria'' into two species groups represented by the spiny-tailed monitor and the black-headed monitor.

In the past, tree monitors such as the green tree monitor have sometimes been included within ''Odatria'' as well as ''Euprepriosaurus'', but now form their own subgenus ''Hapturosaurus''.

A 2020 phylogenomic study by Brennan ''et al.'' found that ''Odatria'' is most closely related to much larger Australian monitor lizards from the subgenus ''Varanus,'' which includes the largest living lizard, the Komodo dragon, as well as the monotypic subgenus ''Papusaurus'' of which the crocodile monitor is the only member of''.'' Together, ''Odatria'', ''Varanus'', and ''Papusaurus'' form a monophyletic clade of Indo-Australopapuan monitors.

The same study found the black-palmed rock monitor, the largest species within ''Odatria'', as sister to the rest of ''Odatria'', making it the most basal species within the subgenus. Vidal ''et al.'' 2012 also recovers this species as the most basal, although within the ''V. tristis'' group.

During the early-mid Oligocene, the genus ''Varanus'' dispersed in two directions. The southeastern dispersion into Indo-Australia likely occurred shortly after the collision of the Asian and Australian tectonic plates, which created a connection between Sahul and Sundaland, i.e., an Indonesian land bridge that would have facilitated the dispersal of monitor lizards into Australia. The existence of this land bridge also likely allowed the Indo-Australopapuan clade including many ''Odatria'' species to disperse back into Indonesia.

Phylogeny

Species Groups

Vidal ''et al.'' 2012 separated ''Odatria'' into two species groups; the ''V. acanthurus'' group included ''V. acanthurus, V. baritji, V. brevicauda, V. bushi, V. caudolineatus, V. eremius, V. gilleni, V. kingorum, V. primordius,'' and ''V. storri,'' while the ''V. tristis'' group included ''V. auffenbergi, V. glauerti, V. glebopalma, V. mitchelli, V. pilbarensis, V. scalaris, V. semiremex, V. similis, V. timorensis,'' and ''V. tristis.''

To maintain monophyly in light of Brennan ''et al.'' 2020, ''V. glebopalma'' would be excluded from the ''V. tristis'' group, and more species would be included as in the following:

''V. acanthurus'' group - (''V. acanthurus, V. baritji, V. brevicauda, V. bushi, V. caudolineatus, V. eremius, V. gilleni, V. kingorum, V. primordius, V. sparnus'' and ''V. storri'')

''V. tristis'' group - (''V. auffenbergi, V. glauerti, V. hamersleyensis, V. mitchelli, V. pilbarensis, V. scalaris, V. semiremex, V. similis, V. timorensis,'' and ''V. tristis'')

Ecology

Behaviour

The genus is diverse in terms of behaviour, and includes terrestrial, arboreal, and semiaquatic species. Many are saxicolous, i.e., they are strongly associated with rocky habitats, while some others prefer forests.

Ritualized fighting sometimes occurs between males. Unlike many larger monitors which grapple with each other while standing on their hind legs, some dwarf monitor species instead grapple each other with all 4 limbs, belly to belly, and roll around on the ground trying to force the other onto its back. Biting may also occur.

Diet

Orthopteran insects and other species of lizards make up the mainstay of the diet for most species. One species, the rusty desert monitor, consumes a particularly large amount of lizard prey with lizards such as various species of ''Ctenotus'' accounting for up to 70% of their diet by weight. Larger species such as black-palmed rock monitors are also known to prey on other dwarf monitors.

Reproduction

Like all other monitors, all species within ''Odatria'' are oviparous. As parthenogenesis is well known in monitor lizards in some other subgenera, it is possible that dwarf monitors are also capable of parthenogenesis; there is a report of a suspected case of parthenogenesis in peacock monitors.

Parasites

Physalopterid nematodes of the genus '' Abbreviata'' are major gastrointestinal parasites in many species of reptiles. Those that parasitize dwarf monitors include '' A. hastaspicula'' and '' A. levicauda''; the former may specialize in Australian monitors. Depending on the species, more than half of all individuals collected from the wild may be infected, but in most cases the parasites do minimal harm. Various other parasites are also known, but in comparatively much low numbers. In captivity, wild caught individuals may suffer more from the effects of parasites due to the stressful process of being taken from the wild and being shipped elsewhere.

Conservation and captivity

Under the EPBC Act, the export of live Australian ''Odatria'' species for commercial purposes is prohibited. Illegal trade of these species still persist, as many dwarf monitor species are in great demand in the pet trade. In Australia, monitor lizards are threatened by the invasive cane toad, as they have no natural immunity to the toad's toxins when ingested.

Captive breeding for the pet trade has been successful in many such species however and lessens the pressure on wild populations. Some such as the often captive bred ridge-tailed monitor are popular pet lizards, in some cases as a "beginner monitor" to train those who are aspiring to eventually keep larger and less forgiving monitor lizard species.

Species

References

{{Taxonbar, from1=Q21439234, from2=Q41217723

Monitor lizards
Animal subgenera
Animals described in 1838
Taxa named by John Edward Gray