Over 200 modern lizard species possess slits, apertures and folds in the skin that lead to epidermal structures often termed mite pockets…
Corucia of the Solomon Islands, Most Amazing of Skinks
SKINKS! Again.
A captive Corucia in a commercial collection. Image: Darren Naish.
Skinks are an enormous, globally distributed group of lizards. As of December 2019, there are around 1685 recognised species, accounting for about 25% of living lizard diversity (there are about 6780 lizard species in total), and – perhaps unsurprisingly – I’ve written about them quite a lot at TetZoo… though it’s now hard to appreciate this, since the articles concerned have variously been vandalised, curtailed or paywalled by the hosters of TetZoo ver 2 and ver 3. See links below for the wayback machine versions of these articles.
There’s a lot about skinks in the TetZoo archives, please see the links below. Thank Christ for wayback machine.
Among the most remarkable and striking of skinks is the large Solomon Islands skink or Monkey-tailed skink Corucia zebrata, a prehensile-tailed, mostly green, arboreal skink, and the only member of its genus (though read on). Not only is this amazing lizard green, arboreal and equipped with a powerful prehensile tail, it’s also a giant, especially big specimens reaching 72 cm in total length. This makes it the biggest known skink. It first became known to science in 1856 when indefatigable taxonomist John E. Gray tersely described specimens brought to London by John MacGillivray after his voyage aboard the HMS Herald, the type specimens coming specifically from San Christoval (today termed San Cristobal or Makira) in the Solomon Islands (Gray 1856).
The Solomon Islands. Image by OCHA (original here), CC BY 3.0.
The lizards appear widespread throughout the archipelago (Makira is one of the most southerly islands there) and are variable, differing in eye colour, size, and in the configuration and size of their scales. Some experts think that subspecies should be named to reflect this variation, and the smaller, paler-eyed northern form was named C. z. alfredschmidti in 1997 (Köhler 1997). Maverick Australian bad boy herpetologist Raymond Hoser has claimed the existence of several entirely new species of Corucia, one of which he named for his mother. If you want to know more about Mr Hoser (and why he’s a total joke) see the TetZoo article here.
A captive Corucia in a private collection. Note the dark irides which make this individual look different from some of the other animals shown here. Image: S. Hilgers.
So far, all published work on the phylogeography and variation within Corucia finds it and its divergences to be young; as in, younger than about 4 million years old (Hagen et al. 2012). Yet it must have diverged from its closest living relatives 20 million years ago or more (we can infer this because fossils of other members of the same skink group are this old or older), meaning that the vast bulk of its lineages’ history remains completely unknown, for now.
Gray described Corucia as a new member of the ‘fish-scaled’ skink group. This seems a bit odd today, because we don’t refer to any skink by this moniker (to my knowledge). He evidently regarded it as part of the Australasian skink group that includes Egernia, Tiliqua (the blue-tongues) and kin though. Today we think (on the basis of molecular phylogenetics) that this is correct, and that Corucia is a lygosomine skink (Skinner et al. 2011, Pyron et al. 2013).
Representatives of most (but not all) of the skink lineages currently regarded as ‘families’ by Hedges and colleagues. 1: Mabuya, of Mabuyidae. 2: Acontias, of Acontidae (I think it should really be Acontiidae). 3: Ristella, of Ristellidae. 4: Scincus, of Scincidae. 5: Lygosoma, of Lygosomidae. 6: Egernia, of Egerniidae. 7: Eugongylus, of Eugongylidae. These images are from my in-prep textbook, progress of which can be observed here. Images: Darren Naish.
Traditionally, all skinks are combined in the single family Scincidae. Most herpetologists argue that we should stick with this taxonomic system since there’s no dispute that Scincidae is a clade and thus no real need to shake things up. But some argue that putting all the species of this enormous, complex group into the same single ‘family’ obscures and under-emphasises its diversity and disparity and that it would be more realistic to split it into a whole bunch of families (nine in fact: Acontidae, Atechosauridae, Egerniidae, Eugongylidae, Lygosomidae, Mabuyidae, Ristellidae, Scincidae and Sphenomorphidae) (Hedges & Conn 2012, Hedges 2014). I’ve written about this situation before: see the articles below for more. If we follow this revised family-level classification, Corucia is part of Egerniidae.
Substantially simplified cladogram depicting lygosomine skink phylogeny, mostly based on Pyron et al. (2013). Images (top to bottom): Wolfgang Wuster, H. Zell, $Mathe94$, Benjamint444 (all CC BY-SA 3.0), Mark Stevens (CC BY 2.0), W. A. Djatmiko, S. Caut et al. (both CC BY-SA 3.0).
The name Corucia is derived from ‘coruscus’ (meaning shimmering, and referring to the shiny scales), while zebrata is a reference to the stripes present in the specimens Gray was familiar with. Given that Solomon Islanders know this lizard and eat it, there was and is surely indigenous knowledge of the species and probably lore about it, though I haven’t encountered such so far. It’s generic name shouldn’t be confused with that of the Cretaceous fossil lizard Carusia, a possible relative of the living xenosaurids.
Here in the UK, it’s currently not difficult to encounter Corucia in captivity. I should add that it does well if conditions are right: as a canopy-dwelling lizard it needs tall branches with suitable retreats, and some collections (most notably the Philadelphia Zoo) have been breeding Corucia for over 40 years now. They’re not especially active during peak visitor time at zoos, mostly because they’re crepuscular. They’re also exclusively herbivorous and are in fact the only skinks said to be committed to a plant diet. Leaves, shoots, flowers and fruit are all consumed, including those of toxic species. Their dung has a distinctive aroma and it’s apparently possible to locate trees inhabited by this species by smell alone: Harmon (2002) used this technique, making his study “the first documented use of olfactory cues to locate skinks in the wild” (p. 177).
Fine side-eye from this captive Corucia at Bristol Zoo, UK. Image: Darren Naish.
Corucia is viviparous with a 6 to 8 month gestation, but the big deal about its viviparous strategy is that its babies are proportionally enormous, being about half the size of the mother. They can be over 30 cm long and weigh 175 g. Unsurprisingly, only a single baby is normally produced, though rare cases of twins and triplets are on record.
Corucia is also a social skink. In this, it’s far from unique, since egerniids of more than 20 species live together in family groups and even exhibit monogamy, kin recognition, colonial living and co-operation. Juvenile Corucia sometimes stay with their parental group for an extended period and mothers are reported to be highly protective of newborn juveniles (Wright 2007), which is what theory predicts given the substantial material investment involved in growing such a large baby. Also interesting is that not all the adults which form social groups in this species appear related (Wright 1996), and that Corucia groups are even known to allow orphaned juveniles to join their groups (read on…). Some juveniles do apparently leave their parental group to join others (Wright 2007).
Dark-eyed captive Corucia, and here’s proof that this arboreal lizard will - in captivity - drink from standing water (at least some arboreal lizards don’t do this, they rely only on water droplets on leaves). Image: S. Hilger.
Studies of wild-living Corucia on Ugi Island in the Solomon archipelago showed that individuals living less than 150 m apart were likely to be related, but also that individuals wandered for several kilometres (Hagen et al. 2013). Telemetry results obtained in an earlier study (Hagen 2011) indicate that this sort of dispersal is unusual, however, given that Corucia is mostly sedentary with home ranges being equivalent to the canopy of a single tree. Maybe this explains why groups are apparently happy to ‘adopt’ lone youngsters – they may well be related to the members of the group already. After all, we know that kin selection is at play elsewhere in social egerniids.
One of the latest papers discussing social behaviour in these skinks is also among the most shocking, since it reports the occurrence of a Corucia group living together in a deep tree hole, and one that was flooded at its bottom. Remarkably, some of the Corucia in the hole were fully submerged and located beneath the water surface at the time of discovery. To my supreme frustration, I can’t locate this publication right now, even though I recall downloading it (it was a short note in, perhaps, Salamandra or Journal of Herpetology). Let me know if you know the paper concerned. It was such a bizarre report that more information is needed. And I guarantee that it’s legit and that I didn’t dream it.
A captive Corucia at Bristol Zoo. Note the sharply curved claws and interesting nose in these lizards. Image: Darren Naish.
Finally, what does the future hold for this amazing lizard? Unsustainable destruction of forests on the Solomon Islands poses a problem, as does local hunting for the pot and collecting for the pet trade: between 1992 and 1995, 12000 animals were exported for this reason, mostly to the USA (Mann & Meek 2004). Consequently, Corucia is now being considered for inclusion on Appendix I of CITES, with captive breeding likely being crucial to its persistence.
Another captive Corucia. This image is useful and interesting because it shows the cross-sectional shape of the body: note that the side of the body is flat and that there’s an obvious change in angle between the side and dorsal surface. Image: TimVickers (original here), public domain.
A giant, fully herbivorous, slow-breeding, social skink is such a special animal that we must make effort to ensure its survival into the future. And that’s where we must end.
For previous TetZoo articles on skinks, see…
Mystery emo skinks of Tonga!, October 2010
Isopachys: worm-like skinks from Thailand and Myanmar, November 2010
Hammer-toothed skink SMASH!, November 2012
Skinks, Skinks, Skinks!, October 2014
Terror skinks, social skinks, crocodile skinks, monkey-tailed skinks… it's about skinks (skinks part II), October 2014
Sandfishes and kin: of sand-swimming, placentation, and limb and digit reduction (skinks part III), November 2014
The Madagascan Skink Amphiglossus Eats Crabs, February 2016
Refs - -
Gray, J. E. 1856. New genus of fish-scaled lizards (Scissosarae) from New Guinea. Annals and Magazine of Natural History (2) 18: 345-346.
Hagen, I. J. 2011. Home ranges in the trees: radiotelemetry of the Prehensile tailed skink, Corucia zebrata. Journal of Herpetology 45, 36-39.
Hagen, I. J., Donnellan, S. C. & Bull, C. M. 2012. Phylogeography of the prehensile-tailed skink Corucia zebrata on the Solomon Archipelago. Ecology and Evolution 2, 1220-1234.
Hagen, I. J., Herfindal, I., Donnellan, S. C. & Bull, C. M. 2013. Fine scale genetic structure in a population of the prehensile tailed skink, Corucia zebrata. Journal of Herpetology 47, 308-313.
Harmon, L. J. 2002. Some observations of the natural history of the Prehensile-tailed skink, Corucia zebrata, in the Solomon Islands. Herpetological Review 33, 177-179.
Hedges, S. B. 2014. The high-level classification of skinks (Reptilia, Squamata, Scincomorpha). Zootaxa 3765, 317-338.
Hedges, S. B . & Conn, C. E. 2012. A new skink fauna from Caribbean islands (Squamata, Mabuyidae, Mabuyinae). Zootaxa 3288, 1-244.
Köhler, G. 1997. Eine neue Unterart des Wickelschwanzskinkes Corucia zebrata von Bougainvillle, Papua-Neuguinea. Salamandra 33 (1), 61-68.
Mann, S. L. & Meek, R. 2004. Understanding the relationship between body temperatureand activity patterns in the giant Solomon Island skink, Corucia zebrata, as a contribution to the effectiveness of captive breeding programmes. Applied Herpetology 1, 287-298.
Pyron, R. A., Burbrink, F. T. & Wiens, J. J. 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Evolutionary Biology 2013, 13:93.
Skinner, A., Hugall, A. F. & Hutchinson, M. N. 2011. Lygosomine phylogeny and the origins of Australian scincid lizards. Journal of Biogeography 38, 1044-1058.
Wright, K. 1996. The Solomon Islands skink. Reptile & Amphibian Magazine 3 (2), 10-19.
Wright, K. M. 2007. Captivating giants. Reptiles Magazine 15 (12), 54-68.
The Mostly Arboreal African Gastropholis Lizards
TetZoo hasn’t had nearly enough lizards lately. If you’re a regular reader you might recall the slow-burn series on LACERTIDS, the Old World group that are the ‘typical’ lizards of western and central Europe and hence the first group to become known to European scientists.
Caption: if you’re European, lacertids are the lizards you know best. In the UK, we only have two natives, one of which is Zootoca vivipara, the Viviparous lizard. Here’s a wild individual. Image: Darren Naish.
European lacertids – thinking here of Zootoca (viviparous lizards), Lacerta (sand lizards, green lizards and kin) and Podarcis (wall lizards) in particular – are quite well known and it’s easy to find information on them; if, that is, you’ve combed through books on lizards in quest of hot lacertid info. But there are a whole bunch of African taxa within Lacertidae: what’s the deal with them, and why don’t we hear more about them? Well, comparatively little is known about them, good photos of them aren’t that abundant, and they’re rare in museum collections (Arnold 2004).
Caption: a captive Green keel-bellied lizard Gastropholis prasina, in repose. Check out that tail! Images: Darren Naish.
I recently got to see a live lacertid of a type I’d never seen before: a Green keel-bellied lizard Gastropholis prasina, a shockingly long-tailed, green, east African lacertid endemic to the coastal forests of Mozambique, Tanzania and Kenya. It reaches 40 cm in total (which seems pretty huge for a lacertid), but is more usually 25-35 cm. Most of this is formed by the slender, prehensile tail. These are diurnal lizards, and one of several things that makes them unusual among lacertids is that they’re arboreal, and do just about everything up in the trees. They sleep there, fight there, mate there and lay their eggs there (in damp hollows). Individuals of G. prasina have been found 12 metres up in trees (Spawls et al. 2018).
Caption: an 1886 illustration of the Striped keel-bellied lizard G. vittata, from J. G. Fischer’s publication. Image: J. G. Fischer, public domain.
Well…. the Green keel-bellied lizard lives this way, anyway. Animals refuse to be constrained by anatomy and not all members of this genus are arboreal. The Striped keel-bellied lizard G. vittata is apparently terrestrial despite being proportionally pretty much exactly like G. prasina and being equipped with a really long, prehensile tail (Spawls et al. 2018). What this most likely shows is that G. vittata has only recently transitioned to terrestrial life, and that its anatomy still harks back to an arboreal ancestry.
Caption: you can see why this species is called Gastropholis echinata (the species name meaning ‘spiny’). It’s from a 1919 description of the species by K. P. Schmidt. Image: K. P. Schmidt/AMNH, public domain.
Two other species are presently included in Gastropholis. G. echinata occurs throughout those countries bordering the Gulf of Guinea while G. tropidopholis is endemic to the Democratic Republic of the Congo.
Caption: another Gastropholis species: the Congolese G. tropidopholis. Again this image is from a 1919 paper by K. P. Schmidt. Image: K. P. Schmidt/AMNH, public domain.
Convergence with grass lizards…. and monitors? As implied by the name, one of the unusual things about these lizards is that their ventral scales are keeled. That’s pretty odd, since lizards (and other reptiles) ‘ordinarily’ have smooth ventral scales. Keeled ventral scales are also present in at least some species of Adolfus – another climbing African lacertid – and also in Takydromus, the east Asian grass lizards. Like Gastropholis, Takydromus has a strikingly long tail which it uses in climbing (albeit in grasses rather than high in trees).
These lizards also (together with Philochortus, another poorly known African lacertid) share especially tall, blade-like neural spines on some of their caudal vertebrae (Arnold 1997, 2004). Neither Takydromus nor Philochortus are at all close to Gastropholis in lacertid phylogeny, so any similarities have to be a product of convergence (Arnold 2004). If you want to learn more about Takydromus, I wrote about it here at ver 2, back in 2013.
Caption: a Takydromus - an Asian grass lizard - in captivity. These lacertids are similar to the African Gastropholis species in several respects, but are not that closely related to them. Image: Darren Naish.
On the subject of convergence, I can’t help but also regard these mostly arboreal, often green lacertids as convergent with the also long-tailed, also green prasinoid tree monitors of northern Australasia. Prasinoids – the Green tree monitor Varanus prasinus and its several close relatives – are larger than Gastropholis species (40 cm vs 60-110 cm, total length) and have several varanid-flavoured features lacking in lacertids (a long forked tongue, blade-like teeth, longer and more dextrous digits and – probably – enhanced cognition). Prasinoids seem more formidable as predators than is Gastropholis, since they predate on small mammals, big spiny insects and so on. This could just be a consequence of size though, and Gastropholis is reported to predate on smaller lizards so might well be quite formidable too (Spawls et al. 2018). If you’re wondering why there are both Varanus and Gastropholis taxa within the species name prasinus (or prasina), it’s because this means ‘light green’.
Caption: I’ve had reason to draw prasinoids - green tree monitors - on several occasions, and here’s a montage depicting several species. Can these varanids be considered convergent with the long-tailed, arboreal lacertid Gastropholis? Hmm, maybe. Images: Darren Naish.
I’ve written about prasinoids at TetZoo: please go here (reminder that I now link to wayback machine versions of my ver 3 articles, since SciAm has either mutilated my articles or paywalled them).
Where in the family tree? Where do the Gastropholis species fit within the lacertid radiation? None of these lizards are well known and they tend to be missed entirely from popular works that mention or discuss lacertids, this creating the impression that Lacertidae doesn’t really have a presence in the African tropics, which it totally does. Within recent decades it’s been widely agreed that these lizards are part of the equatorial lacertid clade that includes Adolfus and kin. This whole group is probably part of Eremiadini, the lacertid clade that also includes the fringe-toed lizards (Acanthodactylus) and racerunners (Eremias) (e.g., Peréz I de Lanuza & Font 2014)… though be sure to see the TetZoo article on racerunners and kin to get some perspective on the different phylogenetic proposals that have been put forwards for these animals.
Caption: a substantially simplified lacertid phylogeny, showing the approximate structure pieced together in assorted studies. Gastropholis by Darren Naish, Acanthodactylus by Richard Hing, Eremias by Yuriy75 (CC BY-SA 3.0; original here), Takydromus by Acapella (CC BY-SA 3.0; original here), Lacerta by Darren Naish, Gallotia by Petermann (CC BY-SA 3.0; original here), Psammodromus by Wolfgang Wüster. Image CC BY-SA.
Arnold (1989) examined anatomical features and found Gastropholis to form a clade with Bedriagaia, Holaspis and Adolfus, the four forming what he termed ‘the Equatorial African clade’. I have to mention in passing that Holaspis is especially interesting because it’s able to glide and has seemingly evolved this ability as an exaptation (Arnold 2002). That’s a story for another time though. Greenbaum et al. (2011), using data from several genes, found Gastropholis to be the sister-group to Adolfus sensu stricto (Adolfus of tradition is paraphyletic with respect to Holaspis, so some of its populations are now separated and in the 2011 genus Congolacerta).
A note on the future. What else do we know about these lizards? They’re suspected to be in decline, given that the forests they inhabit are being destroyed, but hints that they might be adaptable enough to persist come from the fact that they’ve moved into cash crop plantations in some places (Spawls et al. 2018). The individual shown in my photos above and below is a captive and part of the pet trade. The trade in exotic reptiles (and amphibians) is mostly evil, exploitative and the opposite of anything that can be considered sustainable and beneficial. Having said that, the fact that populations of animals endangered in the wild are now being bred in captivity might provide some kind of safeguard for the future. That’s where we’ll end for now, but we’ll return to lacertids - and other lizards - sometime in the near future.
Caption: Green keel-bellied lizard in profile. Image: Darren Naish.
For previous TetZoo articles on lacertids (linking here to wayback machine versions to avoid issues with the hosting sites, SciAm in particular), see…
The New Forest Reptile Centre, August 2012
Tale of the Takydromus, September 2013
Racerunner Lizards of the World Unite, October 2014
Refs - -
Arnold, E. N. 1997. Interrelationships and evolution of the east Asian grass lizards, Takydromus (Squamata: Lacertidae). Zoological Journal of the Linnean Society 119, 267-296.
Arnold, E. N. 1989. Towards a phylogeny and biogeography of the Lacertidae: relationships within an Old-World family of lizards derived from morphology. Bulletin of British Museum of Natural History (Zoology) 55, 209-257.
Arnold, E. N. 2002. Holaspis, a lizard that glided by accident: mosaics of cooption and adaptation in a tropical forest lacertid (Reptilia, Lacertidae). Bulletin of British Museum of Natural History (Zoology) 68, 155-163.
Arnold, E. N. 2004. Overview of morphological evolution and radiation in the Lacertidae. In Pérez-Mellado, V., Riera, N. & Perera, A. (eds) The Biology of Lacertid Lizards. Evolutionary and Ecological Perspectives. Institut Menorquí d’Estudis. Recerca 8, 11-36.
Greenbaum, E., Villanueva, C. O., Kusamba, C., Aristote, M. M. & Branch, W. R. 2011. A molecular phylogeny of equatorial African Lacertidae, with the description of a new genus and species from the Democratic Republic of the Congo. Zoological Journal of the Linnean Society 163, 913-942.
Peréz I de Lanuza, G. & Font, E. 2014. Ultraviolet vision in lacertid lizards: evidence from retinal structure, eye transmittance, SWS1 visual pigment genes and behaviour. The Journal of Experimental Biology 217, 2899-2909.
