Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

Goanna-eating goannas: an evolutionary story of intraguild predation, dwarfism, gigantism, copious walking and reckless thermoregulation


It’s well known that monitor lizards (or varanids) sometimes practise cannibalism (that is, predation within their own species), and it should be no surprise to learn that big monitor species sometimes (or even often) prey on and eat smaller ones. The phenomenon whereby predators predate on other, typically smaller, predators is termed intraguild predation, and it’s a subject that's been covered here at Tet Zoo on at least a few occasions.

This new photo – provided courtesy of Craig Stewart and John Scanlon – has been doing the rounds via email and is definitely worthy of wide attention. We haven’t been able to track down the photographer: please do say if you know or can provide a lead. Anyway, the image (definitely taken in Australia, and most likely in northern Western Australia) shows a fairly spectacular instance of intraguild predation within monitors. We can see a Perentie Varanus giganteus consuming an Argus or Yellow-spotted monitor V. panoptes.

There’s no reliable scale, but it’s obvious that the V. panoptes is proportionally huge compared to the Perentie. Eating such a proportionally large item poses potential risks for the predator (do you remember the series of articles I published on ill-fated acts of swallowing?). Furthermore – assuming that the V. panoptes was alive when the Perentie encountered it – subduing such a proportionally large animal would have been a risky endeavour. That’s about all that I can say about this remarkable photo, but I’m now left with a good opportunity to talk a little bit about Perenties and Argus monitors, two of Australia’s largest living lizards. Seeing as monstersaurians got some love the other day, it’s only fair that goannasaurians get some too.

Perentie: shy and elusive giant of the arid centre

Perentie close-up, by Richahel, released into public domain.

All monitors are awesome, but the Perentie just has to be one of the most awesome of all monitors. This is a huge lizard (the largest alive in Australia today) with a few reports describing individuals 2.5 m long in total. Anything at or over 2 m should, however, be regarded as exceptional. The Perentie has a highly distinctive, dorsally spotted livery and is comparatively long-necked. It’s an arid-land specialist of the Australian interior, though it does seem to be expanding its range southwards along the coast of Western Australia (Horn & King 2004).

While predominantly terrestrial and preferring sandy and rocky deserts as well as scrubby habitats, perenties have been known to climb, and there are peculiar (and admittedly very rare) accounts of individuals seen foraging in shallow water (e.g,. Pianka 1982). In some areas, they take shelter in burrows. These can be enormous: 1 m deep, and up to 8 m long (Pianka 1982). Perenties are well known for standing bipedally when surveying the surroundings, a behaviour sometimes known as ‘tripoding’. They’re elusive, wary, and with an air of intelligence that frequently impresses those familiar with them. ‘Intelligence’ of a sort is widespread in monitors and I hope you’ve read the stories of Komodo dragons and other monitors that enjoy human company and even engage in obvious and deliberate play behaviour.

Perentie, photographed in Western Australia by Kaiwhakahaere, released into public domain.

So, while perenties most typically walk and run on the ground, they can climb, they can swim, they can dig, and they can stand bipedally. I don’t mean to imply that this makes them ‘special’ compared to other monitors – indeed, it’s this behavioural flexibility that helps explain why monitors are so outstandingly successful.

Stomach content data shows that perenties feed on a broad diversity of arthropods, reptiles, birds and mammals, and they also eat eggs. Pianka (1994) suggested that, prior to recent centuries, perenties might have preyed on hare-wallabies and other mid-sized marsupials that are now extremely rare or extinct. Essentially, they eat whatever they can overpower, they’re consummate scavengers, and we know that they sometimes eat other monitors. But are they on record as eating monitors, or any lizards, as large as the V. panoptes prey item shown above? Horn & King (2004) refer to the swallowing of a c. 50 cm Long-nosed water dragon Lophognathus longirostris by a c. 60 cm perentie and King (1999) reported a case where a 1.5 m perentie ate another perentie that was 1.2 m long. The complete act of swallowing wasn’t witnessed in the latter case – the animal walked away with about half the length of the prey’s tail still hanging from its jaws. The photo I show here does not, therefore, represent a ‘world first’ or anything like that: we already knew that perenties did this sort of thing. Getting a good photo of it, however, is often the great challenge.

The mandatory bit about phylogeny

Within the Indo-Australian varanid assemblage, the Perentie has been found in molecular phylogenies to group together with Gould’s goanna V. gouldii and the Argus monitor V. panoptes (Ast 2001), two species we look at more below.

Reconstructed skeleton of V. priscus on display at Melbourne Museum. Photo by Cas Liber, released into public domain.

A Mertens' water monitor V. mertensi + Spencer’s monitor V. spenceri clade appears to be especially close to this ‘gouldii group’, while both the odatrians (a group of mostly small Australian monitors) and a (Crocodile monitor V. salvadorii + (Lace monitor V. varius + Komodo dragon V. komodoensis)) clade form successively more distantly related lineages (Ast 2001; see also Baverstock et al. 1993, Fuller et al. 1998). Megalania – that is, Varanus priscus – was suggested by Lee (1996) to be the sister-taxon to the Perentie since both share several skull roof characters. However, Head et al. (2009) argued that the characters concerned (they include a raised interfrontal suture and a rugose texture on the frontal bones) are widely distributed in monitors and sometimes size-related; they showed, via the presence of other detailed skull characters, that V. priscus seems instead to be a member of the (Crocodile monitor V. salvadorii + (Lace monitor V. varius + Komodo dragon V. komodoensis)) clade (Head et al. 2009). The as-yet-unnamed giant monitor known from the Pleistocene of Timor (Hocknull et al. 2009) belongs here as well.

Body size in varanids mapped on to Ast's (2001) phylogenetic hypothesis: from Gould & MacFadden (2004).

A few really interesting things have emerged from examination of Ast's (2001) phylogenetic hypothesis for monitors. Odatrians are dwarfed relative to what seems to be an ancestral total length of c. 1-2 m. And the Komodo dragon is not giant due to island gigantism – it belongs to a clade that was probably large-bodied ancestrally, and was already large when it colonised Komodo, Padar, Rintja, Flores, Gili Moto and Oewada Sami (Gould & MacFadden 2004). In fact, big-bodied monitors identified specifically as V. komodoensis are now known from the Pliocene and Pleistocene of mainland Australia (Hocknull et al. 2009), indicating that the Komodo dragon was a big animal on a continental landmass before it colonised those Indonesian islands (where, today, it’s an endangered relict). Note also that V. komodoensis and V. priscus were contemporaneous on Australia. The Lace monitor might be secondarily small since it ‘only’ reaches 2.1 m and yet is surrounded in the phylogeny by the Crocodile monitor and Komodo dragon (Fuller et al. 1998, Ast 2001, Gould & MacFadden 2004).

If the gigantic V. priscus is close to the Komodo dragon as Head et al. (2009) proposed, this giant species evolved from already large ancestors and took size to an extreme. The phrase “Runaway selection for increased body size” has already been used in the literature on body size evolution in monitors with reference to both V. priscus and the Komodo dragon (Pianka 1995); this ‘runaway selection’ still seems to have occurred, just not in an Indonesian island setting, and perhaps driven by the presence of large and vulnerable mammal prey. Incidentally, Pianka (1995) was prescient enough to note that the idea of relatively large ancestral size for monitors was equally as plausible as the idea of small size and independent acquisitions of large size.

Komodo dragon photographed at ZSL London Zoo in 2009.

On not stegodont eating, and on not taking the role of big mammalian predators

There are two other things I just have to mention while I’m here. One is that the large ancestral size of the Komodo dragon lineage helps kill stone-dead the appealing but fanciful notion that this lineage spawned giants under selective pressure to prey on island-endemic stegodont elephants (Diamond 1987). Maybe Komodo dragons did meet, kill and eat miniature stegodonts; the point is that members of the Komodo dragon lineage were, however, large before this ever happened (Hocknull et al. 2009). [Image below by Conty.]

Komodo dragon (1) and Perentie (2) compared to different reconstructions of V. priscus. Created by Conty, licensed under Creative Commons Attribution 3.0 Unported license.

The other thing concerns the idea that Australia could only produce super-giant monitors because the continent’s low-energy ecosystems were unable to support big mammalian carnivores, hence the prevalence of big lizards (Flannery 1994). Since that hypothesis was first mooted, the diversity of (formerly well-forested) Australia’s marsupial predators has increased; furthermore, it’s been increasingly recognised that some of those marsupial predators were as big and formidable as placental predators from elsewhere, if not more so (Wroe et al. 1999). We now have to contemplate a prehistoric Australia where there was, after all, enough ecological and literal, geographical space to allow the evolution of big thylacines, thylacoleonids and other mammalian predators as well as the evolution of several species of giant lizard.

The goanna that walks

In case you’ve forgotten, we’re here because of that photo shown at the top. It features a Perentie and an unfortunate V. panoptes.

Argus monitor, photograph by Jeanette Covacevich, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

V. panoptes – variously termed the Argus monitor, Yellow-spotted monitor or Floodplain goanna – was only recognised as a distinct species in 1980. Prior to this, its populations had erroneously been identified as part of V. gouldii (the Sand goanna, Gould’s goanna or Bungarra). This particular situation is complicated by the fact that, after realising that ‘V. gouldii’ of tradition actually represented two species, Storr (1980) erroneously gave the name V. panoptes to the form that included the type specimen of V. gouldii. In other words, he immediately created an objective junior synonym and left the new form without a name.

A series of exchanges in the Bulletin of Zoological Nomenclature ensued; the outcome is that V. gouldii is now applied to the extremely widespread, sandy-habitat goanna that typically has a bright yellow tail tip while V. panoptes goes to the yellow-spotted animal of northern Australia and New Guinea. This confusing situation (which I've deliberately glossed over here*) means that many references to V. gouldii in pre-1980 literature actually refer to V. panoptes. Things aren’t totally resolved yet, since the substantial variation seen within V. gouldii sensu stricto indicates that it’s a species complex that will eventually be split up. And V. panoptes is already polytypic, with distinct western (V. p. rubidus), northern and eastern (V. p. panoptes), and New Guinea (V. p. horni) subspecies. [Gould's monitor show below; photo by Peripitus.]

* I didn't even mention the name V. flavirufus.

Gould's monitor - similar to an Argus monitor, but, well, different. Photo by Peripitus, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

V. panoptes is large: males can be 1.6 m long while females rarely exceed 1 m. It occurs in diverse habitats, including mangrove edges, floodplains, wooded grassland and even urban fringes. Again, it climbs, and also forages in water on rare occasion. Again, it’s a dietary generalist, eating animal prey of all sorts. Insect larvae, turtle eggs, lizards and snakes are all recorded prey items. And it too is on record as eating members of other monitor species: V. panoptes has been observed preying on V. gouldii several times.

Me and monitors go way back - these portraits were drawn over 10 years ago. Top to bottom: Mertens' water monitor (V. mertensi), Crocodile monitor (V. salvadorii), Green tree monitor (V. prasinus), Bengal monitor (V. bengalensis). Or, I think that's right.. I've lost the labels.

Two of the best known things about this species are that it’s a consummate walker, with tracked individuals spending as much as 6.6 hours a day walking, and that it’s a fairly reckless* thermoregulator compared to other monitors (Christian & Weavers 1996). Floodplain-inhabiting V. panoptes walk long distances even during the dry season, though they do become inactive during the latest, driest part of the dry season (Christian et al. 1995). No other lizards are known to walk this much, especially during the dry season, though it has been noted that comparative data is fairly scarce (Christian 2004). Another really neat aspect of its behaviour is that it may be a communal nester: Christian (2004) described the presence of warren-like, ‘aggregated burrows’ where females might gather together to lay eggs. Whether this is a planned social strategy or the coincident use by females of the same ideal nesting sites remains unknown.

* Or, more accurately, “least careful”.

I’ve been saying for quite some time that I’d like to write about monitor lizards at length. They’re fascinating and wonderful animals, and so many amazing new things have been learnt about them in recent years. This article started life as a few short paragraphs written to accompany the photo of intraguild predation used above, but it quickly got out of hand, and here we are. Consider it a teaser; there is so much more to come.

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

Mosasaurs - historically associated with monitors or even with snakes - have been included within Platynota by some authors. This position is looking increasingly unlikely. That is, mosasaurs are probably not platynotans. Nevertheless, if you want to see some Tet Zoo articles on them, go to...

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

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