Tetrapod Zoology

Tetrapod Zoology

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Lean, green and rarely seen: enthralling prasinoid tree monitors


Captive Emerald tree monitor (Varanus prasinus). Image by TimVickers, in public domain.

I said in the previous Tet Zoo article on monitor lizards that I really wanted to cover the prasinoids; that is, the arboreal tree monitors of New Guinea, Cape York Peninsula and various of the islands surrounding these areas. So, let’s get to it. Tree monitors or prasinoids, also termed the Varanus prasinus species group, are slender-bodied, gracile monitors with pointed snouts and especially long, prehensile tails. They tend to be green: some are lime green and some are deserving of another common name, ‘Emerald tree monitor’. Some are patterned in ringlets or chevrons; others, however, are virtually black while others are yellowish. They have brown-orange eyes and pink tongues. Total length can be 110 cm or so but 60-80 cm is more common. [Image below by Greg Hume.] Incidentally, the quote used in the title for this article comes from an article about these amazing lizards by Sprackland (1994).

Beautiful Blue-spotted tree monitor: note the long, slender digits and very sharp, curved claws. Image by Greg Hume, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

It is, essentially, universally assumed that tree monitors are specialised for arboreal life, and that they spend much (though not necessarily all) of their time foraging high up in trees. Note that, across many animal groups, assumptions like this are sometimes based on anecdotal impressions and are not really backed up by detailed ecological studies or by the discovery of anatomical features linked with any specific lifestyle or behavioural preference. Furthermore, ‘obvious’ anatomical features assumed to be adaptations might not always mean what we assume they do: a prehensile tail looks like a good arboreal adaptation, for example, yet prehensile tails are also present in animals that aren’t specialised tree-climbers (examples: the turtle Platysternon, the alligator lizard Gerrhonotus liocephalus).

These issues inspired Greene (1986) to specifically examine the evidence for arboreality in prasinoids. He concluded that their green colouration, prehensile tails and unusual foot scales – all unusual features within the monitor lizard radiation – really are specific adaptations to arboreal life (Greene 1986).

A climbing prasinoid (I'm not sure which species this image depicts: it's redrawn from a photo in Kirschner et al. (1996)). Note use of the prehensile tail. Image by Darren Naish.

Prasinoid hands and feet are especially interesting. Black, raised subdigital scales are arranged in transverse rows and look very much as if they might provide these lizards with enhanced grip while climbing. Their palms and foot soles have been described as feeling usually soft or even sticky by some authors. The fact that the subdigital scales are black is interesting. If you have a good memory (or if you know this anyway), you’ll recall that melanin-rich wingtips in birds might represent an adaptation to minimise feather erosion and enhance resistance (see this Tet Zoo article on patterns of pigmentation in petrels). Could it be, then, that the melanin-enriched subdigital scales of prasinoids evolved under similar evolutionary pressure? (Greene 1986). It’s an intriguing idea.

Studies of prasinoid stomach contents reveal arthropod-dominated diets that include katydids, grasshoppers, beetles, centipedes and spiders. Captive animals will eat fruit but there’s no indication as yet that this is typical for wild ones – it’s certainly plausible that they might eat fruits, nectar and so on on occasion: some monitors are habitual frugivores and the ingestion of plant products by lizards has proved more widespread than long assumed. Large and formidable animal prey are eaten on occasion. One prasinoid had eaten a Eurycantha stick insect 12.4 cm long, an insect famous for its large, thorn-like leg spikes. However, the insect was missing several of its legs (Greene 1986), perhaps because the lizard had removed them before swallowing it. A specimen of the arboreal mouse Melomys has also been recovered as a prasinoid prey item (Greene 1986). The general picture that emerges is that they forage among arboreal vegetation, mostly searching for small katydids and similar insects but occasionally grabbing large insects and small mammals.

Illustration of Emerald tree monitor (V. prasinus): identifiable by dark, chevron-like dorsal markings (and, if this were in colour, overall green colour). Image by Darren Naish.

Observations of individuals seen in close proximity have led to suggestions of a gregarious nature for these varanids (Irwin 2004). Females have been reported to guard the tree hollows in which they lay their eggs, sometimes both before and after laying has occurred. Females have also been known to lay their eggs within arboreal termite nests. Prasinoids climb and scurry up trees and along branches when escaping predation or harassment but “If the threat continues, they launch themselves out with all four legs splayed – any contact with any foliage will result in the lizard clinging to it” (Irwin 2004, p. 405).

For at least a few decades during the late 20th century, just a single prasinoid species was identified: Varanus prasinus, the Green tree monitor, suggested by Robert Mertens in 1959 to consist of four subspecies. Sprackland (1991) suggested that one of these 'subspecies' should be regarded as distinct from V. prasinus, and thus resurrected V. beccarii of New Guinea and the Aru Islands. This is a very dark prasinoid – it's sometimes called the Black tree monitor – and it’s also one of the largest member of the group. [Image of Black tree monitor below by Greg Hume.]

Black tree monitor (V. beccarii), photo by Greg Hume, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

Sprackland (1991) further argued that V. kordensis – also regarded by Mertens as one of those V. prasinus subspecies (it was originally named in 1874 as a distinct species) – should be regarded as synonymous with V. prasinus, the supposed differences falling within tolerable limits of intraspecific variation. Since then, however, the case for V. kordensis as a distinct species has been made again: V. kordensis differs from V. prasinus in being darker green, in being decorated with ocelli instead of stripes, and in possessing distinct (lower) scale counts across the head and body (Jacobs 2004). Other workers also consider kordensis as distinct, but only as a subspecies of V. prasinus. Remember that these decisions are ultimately subjective. There’s agreement that kordensis represents a lineage close to prasinus, but is it close enough to prasinus to be included within it, or should it really be regarded as outside of it? Similar issues affect the other prasinoids. Regardless, kordensis is unique to western New Guinea, occurring also on the nearby islands of Biak and Yapen (Jacobs 2004).

Prasinoids in Australia: Canopy goannas

Anecdotes suggesting the presence of prasinoids on the Australian mainland were reported in 1975. These were confirmed in 1978 when Gregory Czechura collected a dark prasinoid from northern Queensland; two more Australian specimens were collected later on (Czechura 1980). Czechura identified these monitors as part of V. prasinus but Sprackland (1991) argued that their presence of conical (rather than flat) throat scales, anteriorly positioned nostrils and yellow chevrons and tail rings indicated the need for separate species status, and thus the name V. teriae was born... the name honours Sprackland’s wife. I suggested the possible common names Cape York tree monitor or Teri’s tree monitor for this species (Naish 1998).

Canopy goanna, image by Darren Naish. The fingers should be longer and claws should be larger and more strongly curved.

However, Wells & Wellington (1985) had already suggested the name Odatria keithhornei for the same taxon (V. teriae and O. keithhornei are both based on the exact same specimens). These names are objective synonyms if we accept the Wells and Wellington names as available: as many of you will know, controversy exists as to whether these names really are available. Debate continues, but many Wells and Wellington names are now in technical use, and V. keithhornei – vernacularly the Canopy goanna – was used in a major recent volume on varanids (Irwin 2004).

Having mentioned Wells and Wellington, I should note Hoser published his own ‘revision’ of monitors during 2013. This included new generic and tribe names for the prasinoids, along with much else besides. It may not surprise you to hear that I’m not going to use or mention any of his taxonomic suggestions. See Kaiser et al. (2013).

Wild Canopy goanna; image (c) Stephen Zozaya, used with permission.

The new ones

Sprackland also raised V. bogerti (originally named by Mertens in 1950) to species level. This is an especially dark prasinoid, entirely lacking stripes, rings or chevrons. And Sprackland (1991) also named the entirely new V. telenesetes, a green animal endemic to Rossel Island, way out to the west in the Louisiade Archipelago, west of mainland New Guinea. This far flung occurrence explains the species name, which means ‘far island dweller’ (Sprackland 1991).

Golden-speckled tree monitor (V. boehmei); image by Matthias Zepper, licensed under Creative Commons Attribution 3.0 Unported license.

More recently, Böhme & Jacobs (2001) named the beautiful Blue-spotted tree monitor or Blue tree monitor V. macraei. This is a highly distinctive prasinoid marked with brilliant blue ocelli, first discovered as a German import for the pet trade. So far as we know, this species is endemic to Batanta, a small island off the western coast of New Guinea’s Vogelkop Peninsula. Batanta is located to the south of the larger island of Waigeo, and Waigeo has also proved to be the home of another recently named prasinoid: the Golden-spotted or Golden-speckled tree monitor V. boehmei (Jacobs 2003). This species looks similar to the Australian Canopy goanna and some authors have argued that it’s not especially close to the Blue-spotted tree monitor, instead being closer to V. beccarii and V. kordensis (Böhme & Jacobs 2004). However, Ziegler et al. (2007) found V. macraei to be closest to a V. boehmei + V. prasinus clade.

Specimen identified as Reisinger's tree monitor (V. reisingeri) (but is it? I'm not 100% sure): there are lots of photos of captive individuals online, but this is about the only one that's clearly CC. Image by Vincent Malloy, licensed under Creative Commons Attribution-Share Alike 3.0 Unported.

Most recently, Reisinger’s tree monitor V. reisingeri was named (Eidenmüller & Wicker 2005): a greenish-yellow animal apparently endemic to Misool Island, just to the west of Vogelkop Peninsula but further south than Batanta. Very little is known about this monitor and its position relative to other prasinoids has yet to be discussed in print (so far as I know). Again, it was first discovered in captivity – as the property of an Indonesian wildlife dealer. Specimens are now available quite widely – I've seen them here in the UK. The trade in tree monitors sure looks like a serious problem that requires action... please read on.

If all the taxa discussed in this article (and elsewhere) are regarded as distinct species, the V. prasinus group includes as many as nine species (V. prasinus, V. beccarii, V. kordensis, V. keithhornei, V. bogerti, V. telenesetes, V. macraei, V. boehmei and V. reisingeri). Where do these monitors fit within the varanid radiation? Mertens included then within Odatria, the almost wholly Australian ‘dwarf monitor’ group, but it seems that he was incorrect: work on lung and hemipenial structure, and on genetics, shows that they are closer to the ‘Pacific’ Euprepiosaurus group that includes the mangrove monitor complex. A close relationship between prasinoids and the mangrove monitor complex has been repeatedly supported in all of the recent studies on varanid phylogeny (Ast 2001, Collar et al. 2011, Conrad et al. 2012, Vidal et al. 2012, Pyron et al. 2013).

Hundreds of Madagascan reptiles and amphibians, found dead in a shipment to South Africa (but destined for the USA) during January 2014. At least some of the species here are CITES-listed and shouldn't have been exported at all.

One final issue requires mention here. All of the species here are highly sought after for the pet trade and, if you’re at all familiar with the monitors kept and bred in captivity, you’ll likely already have seen one or some of the taxa discussed here. Indeed, some were initially discovered only after they’d been imported to Europe for use in the pet trade. Habitat destruction due to logging, mining and so on all represent dangers to these forest-dwelling lizards (note that other monitors are being horrendously over-exploited for the skin trade), but the impact of commercial collecting shouldn’t be underestimated. There are some indications that uncontrolled collecting – even of CITES-listed species – represents a very real danger as goes the health of these populations (Koch et al. 2013), and if you think that people interested in the commercial collection and trade of tropical reptiles are operating within sensible, responsible boundaries, think again. Do check out the depressing articles on this issue here, here and here, and spread the word.

A tree monitor montage, images by Darren Naish.

For previous articles on varanids and other platynotan lizards, see...

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The views expressed are those of the author and are not necessarily those of Scientific American.

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