Varanus (Hapturosaurus)
Green tree monitor (Varanus prasinus)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Squamata
Family: Varanidae
Genus: Varanus
Subgenus: Hapturosaurus
Bucklitsch, Böhme, and Koch, 2016
Type species
Varanus prasinus
Schlegel, 1844
Species

Hapturosaurus, sometimes known as the tree monitors, is a subgenus of lizards, consisting of slender-bodied arboreal monitor lizards mostly found in the tropical rainforests of Indonesia and Papua New Guinea.

Taxonomy

The type species is the green tree monitor, originally designated by Hermann Schlegel in 1844 as Monitor prasinus. Robert Mertens placed tree monitors within the subgenus Odatria in 1942. In 1988, tree monitors were instead placed within Euprepriosaurus alongside the mangrove monitors.[1] Nevertheless, there was a distinction between mangrove and tree monitors that was clear even then, so Euprepriosaurus was commonly considered to consist of two species complexes, i.e., the V. indicus complex and the V. prasinus complex. In 2016, Yannick Bucklitsch, Wolfgang Böhme, and André Koch found the two species complexes sufficiently morphologically, ecologically, and biologically distinct, and so all species within the V. prasinus complex were moved under a newly erected subgenus, i.e., Hapturosaurus. Hapturosaurus diverged from Euprepriosaurus during the late Miocene.[2]

Etymology

The name Hapturosaurus is derived from the Greek words "haptein" (to grasp), "ouros" (tail), and "sauros" (lizard), in reference to the prehensile tails of tree monitors.[2]

Ecology

Behaviour

All species are highly arboreal. In captivity, tree monitors demonstrate the capacity to play, in the form of destructive behaviour such as systematically shredding the leaves on plants with teeth and claws.[3] Play-like behaviour is also documented in other less closely related monitor species, such as Komodo dragons.[4]

Feeding habits

Tree monitors are primarily insectivorous, but also consume other small invertebrates such as spiders, or occasionally small mammals, lizards or the nestlings and eggs of birds. In captivity they are occasionally seen eating plants although the gut contents of wild individuals were not reported to contain plant matter.[5] This may however be a result of accidentally ingesting leaves when playing with them, which is documented in captivity.[3]

Captive hatchling often refuse food for more than two weeks, although force feeding may be recommended before then and until they begin feeding by themselves.[5]

Like other monitor lizards, tree monitors are highly intelligent reptiles, but tree monitors demonstrate particularly complex problem solving abilities, fine motor coordination, and skilled forelimb movements when hunting prey. When they cannot reach prey in tight crevices and holes with their jaws, they instead extract prey by reaching for it with their forelimbs—which are some of the longest and most slender forelimbs of all monitors and end in elongated digits tipped with large claws and adhesive soles—and hooking it out with its claws, allowing them to exploit a wider range of niches.[6][7][8][9]

It is recommended that in captivity this behaviour be encouraged by placing food items in places that are only accessible by reaching for them with their forelimbs, as a form of enrichment.[6]

Range

Most species are allopatric and confined to single islands in Indonesia and Papua New Guinea, with the exception of the canopy goanna which is endemic to northeast Australia and the green tree monitor which has a much wider distribution than other tree monitors. The green tree monitor has even been purportedly sighted on the Cape York Peninsula on multiple instances, which if true would make it sympatric with the canopy goanna.[10]

Their colonization of the Indo-Pacific islands occurred during the Pleistocene glacial periods, when lower global sea levels caused the Sahul Shelf to form a continuous land-bridge out of the shallow waters around the Torres Strait Islands. This allowed the exchange and dispersal of fauna between New Guinea and Australia each time the land-bridge formed.[10]

Venom

Like all other monitor lizards, tree monitors possess venom glands in their lower jaws, giving them a noticeably venomous bite.[11] The venom is an anticoagulant, and has two known mechanisms for disrupting blood clotting: by fibrinogenolysis (the destructive cleavage of fibrinogen) and by blocking platelet aggregation. The venom also causes hypotension. Monitor lizard venom is extremely complex and diverse due to the great range of ecological niches that they occupy.[12]

Tree monitors have the most potently fibrinogenolytic venoms of all monitor lizards, matched only by the also arboreal banded monitor from the subgenus Odatria. This may be because arboreal monitor species experience strong selection pressure to quickly subjugate prey items before they break free and escape by falling out of the trees or flying away. As fibrinogenolytic activity is not painful, the venom likely has a primarily predatory role in subjugating prey, instead of acting as a predator deterrent.[12]

It is suggested that higher prey escape potential may cause increased venom potency, as the lethally toxic eastern green mamba and boomslang are also arboreal. This may also apply to the piscivorous and highly venomous cone snails, given their highly mobile fish prey.[12]

As the venom is an anticoagulant, monitor lizard bites often bleed far more than what the mechanical damage of the bite alone would have induced.[12]

Species

Species Taxon Author Common Name Picture
V. beccarii Doria, 1874 black tree monitor
V. boehmei Jacob, 2003 golden-spotted tree monitor
V. bogerti Mertens, 1950 Bogert's monitor
V. keithhornei Wells & Wellington, 1985 canopy goanna
V. kordensis A.B. Meyer, 1874 Biak tree monitor
V. macraei Böhme & Jacobs, 2001 blue tree monitor
V. prasinus Schlegel, 1839 green tree monitor
V. reisingeri Eidenmüller & Wicker, 2005 yellow tree monitor
V. telenesetes Sprackland, 1991 mysterious tree monitor

References

  1. ↑ Böhme, Wolfgang (2003). "Checklist of the living lizards of the world (family Varanidae)" (PDF). Zoologische Verhandelingen. 341: 4–43.
  2. 1 2 Bucklitsch, Yannick; Böhme, Wolfgang; Koch, André (2016-08-17). "Scale morphology and micro-structure of monitor lizards (Squamata: Varanidae: Varanus spp.) and their allies: implications for systematics, ecology, and conservation". Zootaxa. 4153 (1): 1–192. doi:10.11646/zootaxa.4153.1.1. PMID 27615821.
  3. 1 2 Kane, Daniel (2019-10-01). "Play behaviour by captive tree monitors, Varanus macraei and Varanus prasinus". Herpetological Bulletin. 149 (Autumn 2019): 28–31. doi:10.33256/hb149.2831. ISSN 1473-0928.
  4. ↑ "Reptiles: Such jokers, those Komodo dragons". Science News. 162 (5): 78. 2002. doi:10.1002/scin.5591620516. ISSN 1943-0930.
  5. 1 2 Fischer, Dennis (December 2012). "Notes on the Husbandry and Breeding of the Black Tree Monitor Varanus (Euprepiosaurus) beccarii (Doria, 1874)" (PDF). BIAWAK Journal of Varanid Biology and Husbandry. 6: 79–87.
  6. 1 2 Mendyk, Robert; Horn, H.-G (2011-09-01). "Skilled forelimb movements and extractive foraging in the Arboreal monitor lizard Varanus beccarii (Doria, 1874)". Herpetological Review. 42: 343–349.
  7. ↑ Green Tree monitor enrichment, retrieved 2021-06-26
  8. ↑ #ZooEnrichment: Green Tree Monitor at the Smithsonian's National Zoo, retrieved 2021-06-26
  9. ↑ Challenging my TREE MONITORS | WHACK-A-ROACH, SPIN THE BOTTLE + more!, retrieved 2021-06-26
  10. 1 2 Koch, Andre; Eidenmüller, Bernd (2019-08-01). "Is the New Guinea Emerald Tree Monitor Lizard (Varanus prasinus) Native to Mainland Australia?". Biawak. 13: 32–42.
  11. ↑ Fry, Bryan G.; Wroe, Stephen; Teeuwisse, Wouter; Osch, Matthias J. P. van; Moreno, Karen; Ingle, Janette; McHenry, Colin; Ferrara, Toni; Clausen, Phillip; Scheib, Holger; Winter, Kelly L. (2009-06-02). "A central role for venom in predation by Varanus komodoensis (Komodo Dragon) and the extinct giant Varanus (Megalania) priscus". Proceedings of the National Academy of Sciences of the United States of America. 106 (22): 8969–8974. Bibcode:2009PNAS..106.8969F. doi:10.1073/pnas.0810883106. PMC 2690028. PMID 19451641.
  12. 1 2 3 4 Dobson, James S.; Zdenek, Christina N.; Hay, Chris; Violette, Aude; Fourmy, Rudy; Cochran, Chip; Fry, Bryan G. (2019-05-07). "Varanid Lizard Venoms Disrupt the Clotting Ability of Human Fibrinogen through Destructive Cleavage". Toxins. 11 (5): 255. doi:10.3390/toxins11050255. PMC 6563220. PMID 31067768.
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