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Tetrapod Zoology

Tetrapod Zoology

Amphibians, reptiles, birds and mammals - living and extinct

The once far and wide Siamese crocodile

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Siamese crocodile, photo by Rlevse, in public domain.

ResearchBlogging.org

Welcome to part II in the Crocodiles of the World series. In the previous part we began our tour of the crocodile species themselves by looking at the Mugger or Marsh crocodile Crocodylus palustris, a robust, (mostly) tropical Asian crocodile that’s probably closely related to the species within the Indopacific assemblage (though read on). Here we look at what might be the Mugger’s closest relative, the poorly known, highly endangered Siamese or Thailand crocodile C. siamensis.

Siamese crocodile, photographed in captivity by Dave Hone.

The Siamese crocodile is a relatively broad-snouted crocodile with raised bony crests behind each eye (superficially recalling those of the Cuban crocodile C. rhombifer, although smaller), a distinctive longitudinal ridge between its eyes, and unique granular anterior throat scales. It tends to be greyish-yellow, often with dark bands and blotches on its sides and tail. Its dorsal scute compliment is not that different from that of the Mugger, but the cervical shield in the Siamese croc is composed of smaller scutes, there is a more distinct gap between dorsal and cervical shields, and the transverse row closest to the head (the post-occipital row) is more reduced (Ross & Mayer 1983). Siamese crocodiles seem not to exceed a total length of 3.5 m and their snout shape suggests a generalised diet of diverse invertebrate and vertebrate prey.

Yai, a hybrid Siamese x Saltwater croc (or 'Siamestuary crocodile', according to some). He's gnarly.

It’s well known that Siamese crocs will hybridise with Saltwater crocs C. porosus in captivity (and in the wild as well), the resulting offspring sometimes being enormous animals. An especially famous hybrid individual known as Yai (or Jao Yai), hatched in 1972 and kept at the Samut Prakan Crocodile Farm and Zoo in Thailand, is between 5.5 and 6 m long (different sources give different lengths). Yai is apparently the largest crocodile kept in captivity (his identification as a siamensis x porosus hybrid has been informally doubted by some but it's likely that this is what he is. So far as I know, his genetics have not been investigated).

Historically, the Siamese crocodile occupied much of Southeast Asia (Thailand, Laos, Cambodia, Vietnam and Malaysia) as well as Java, Borneo and some of the adjacent small islands. Human hunting and habitat destruction has removed it from much of this range and in recent decades its persistence in some of the countries concerned has been doubted: in Laos, for example, Mateus (2002) reported the first sighting in 30 years and only a handful of individuals seem to persist in the wild in Thailand. Habitat modification for rice growing is one of the main reasons for its decline in that country. Captive breeding efforts has been reasonably successful and large numbers live in zoos, crocodile parks and other collections.

Fossil material from Java, originally described as C. ossifragus, apparently represents Pleistocene occurrences of this species.

The mysterious ‘Sulawesi lake crocodile’: does the Siamese crocodile occur on Sulawesi?

The Siamese crocodile is supposed to be a freshwater specialist that shows a strong preference for rivers, swamps and lakes in heavily forested areas. In fact, an alternative (and little-used) common name for the species is the ‘Siamese freshwater crocodile’. If this is so, what’s it doing on such far-flung islands as Borneo and Java? Its presence on Sumatra has been rumoured (e.g., Trutnau & Sommerlad 2006) but remains unconfirmed. And what about Sulawesi? The idea that Siamese crocodiles live on Sulawesi has been mentioned a few times in the literature (e.g., Trutnau & Sommerlad 2006) but also seems never to have been confirmed. In fact, reports of lake-dwelling crocodiles on Sulawesi even appear in the cryptozoological literature (Eberhart 2002), the implication being that they might represent an undiscovered species. [Map below by Achim Raschka.]

Recent distribution of Siamese crocodile (note lack of presence on Sulawesi). Map by Achim Raschka, from wikipedia, licensed under Creative Commons Attribution-Share Alike 3.0 Unported license.

Let’s suppose for a moment that the species does occur on Borneo, Java and Sulawesi. Would this distribution show that the species will actually make sea crossings when the need arises? Note that a modern sea crossing from Borneo to Sulawesi – this involves crossing the fabled Wallace’s Line – would mean swimming across the 200 km of the Makassar Strait. This body is water is nowhere less than 1000 m deep; in fact, these days, Sulawesi is surrounded on all sides by deep water.

Or, is it that the island-dwelling populations reached these islands via terrestrial routes, and became isolated by tectonic events and/or sea-level changes? Sumatra, Java and Borneo were connected to mainland Asia during the Pleistocene, and this makes it totally plausible that Siamese crocs got to these islands via terrestrial routes.

If, however, Siamese crocs are present on Sulawesi, we have an interesting problem, since Sulawesi has a very distinct geological history and probably last had a terrestrial connection with Borneo in the late Miocene (Moss & Wilson 1999). Having said that, shallow carbonate shelf regions did span part of the distance between Borneo and Sulawesi during the early part of the Pliocene (Moss & Wilson 1999): shallow seas don’t help you make a sea crossing if you’re a land mammal, but they might make things easier for crocodiles seeing as they can rest on the seafloor if it’s within easy reach of the surface.

Map of SE Asia and northern Australasia showing continental shelves exposed during the Pleistocene (in grey) and biogeographic boundaries. From Moss & Wilson (1999).

If it wasn’t possible for land animals to use a terrestrial connection to get to Sulawesi after the Miocene, Sulawesi’s Asian-style mammals and other terrestrial animals (the majority of which probably originated in the Pliocene or Pleistocene) must have crossed the Makassar Strait in risky, so-called sweepstakes fashion (Groves 2001). Could a crocodile – a crocodile with a strong preference for freshwater, at that – make a sea crossing of more than 100 km? Well, I don’t know. Maybe, maybe not. Given that even 'dedicated' freshwater members of the alligator lineage simply must have crossed marine barriers in the geological past (Common caiman Caiman crocodilus are present on Trinidad), (just about) anything’s possible.

However, supposing, again, that the Siamese croc does occur on Sulawesi, the idea that it might have made use of a terrestrial route that existed in the Miocene isn’t entirely out of the question, since there are indications from the fossil record and from divergence timings that the stem-lineages of at least some living crocodile species extend back that far (Brochu 2000a, Oaks 2011). So, hypothetically, maybe a stem-member of the Siamese crocodile lineage could have used a terrestrial route to get to Sulawesi.

However, I think that all of this speculation is moot, since – even if there are lake-dwelling crocodiles on Sulawesi – I’m not aware of any good reason to think that they’re Siamese crocodiles. Saltwater crocodiles C. porosus definitely occur on Sulawesi, Saltwater crocodiles very happily walk to, or swim to, freshwater lakes well inland, and hence Saltwater crocodiles are the species we should think of when we wonder about the identity of Sulawesi’s ‘mystery crocodiles’. Having said that, I’d love to be wrong.

Submerged Siamese crocodile, photo by Mo Hassan. This individual has especially prominent squamosal crests (those tall ridges behind the eyes), and in this respect superficially recalls the distantly related Cuban crocodile (C. rhombifer).

One more thing on distribution. Given that Siamese crocs are present on Java and Borneo, why aren't they present on Sumatra? They should be, so those rumoured, unconfirmed accounts plausibly have a basis in reality. Does the species persist on Sumatra, or has it become extinct? If it is extinct there, why? Readers with good recollection of the currently absent content of Tet Zoo ver 2 will recall the similarly selective distribution of Indopacific mekosuchines - technically, they should have been more widespread than fossils and archaeological specimens presently indicate.

Oh, for a place in a cladogram

Either way, reproductive isolation for the Siamese crocodiles on Java and Borneo is suggested by their pattern of gular scalation, which is apparently quite different from that present on the Siamese crocs of the Asian mainland (Trutnau & Sommerlad 2006). It would be interesting to know how these Indonesian animals compare genetically to the ones from the mainland, and what the data might say about the timing of their separation from mainland populations of C. siamensis. While there are a few genetic studies that incorporate data from the C. siamensis, I can’t see that anyone has done a study that compares populations in this way. Say so if you know otherwise.

Highly simplified cladogram showing one phylogenetic hypothesis proposed for crocodiles. This topology is based on a tree in McAliley et al. (2006) but other topologies have been published. Some workers, for example, find the niloticus + New World assemblage clade to be the sister-group to a (porosus + siamensis) + 'reduced' Indopacific assemblage clade. Photos (top to bottom) by Mo Hassan, Davric, Wilfried Berns, Naish, Dave Hone. Image of New Guinea crocodile (icon for 'reduced' Indopacific assemblage) licensed under Creative Commons Attribution-Share Alike 2.0 Germany license.

The phylogenetic position of the Siamese crocodile is slightly controversial, but only because there are competing phylogenies for Crocodylus. We saw in the previous article that Crocodylus crocodiles have often been thought to consist of distinct Indopacific and New World assemblages, with the Nile crocodile C. niloticus being a close relative of the New World assemblage. Morphological evidence suggests that the Siamese crocodile might be the sister-taxon to the Indopacific assemblage, or to a Mugger + Indopacific assemblage clade (Brochu 2000a, b, McAliley et al. 2006, Brochu & Storrs 2012).

However, molecular studies have confused this picture and the species in the Indopacific assemblage don’t group together. Instead, New Guinea C. novaeguineae and Philippine crocs C. mindorensis are close kin, the Australian Freshwater crocodile C. johnstoni may or may not be close to these two, and the Siamese crocodile may be the sister-taxon to the Mugger, or the sister-taxon to the Saltwater crocodile (McAliley et al. 2006, Meganathan et al. 2010, Man et al. 2011, Oaks 2011). If the Mugger, Siamese croc and Saltwater croc form a clade, this presumably originated in Asia before some lineages headed towards Australasia and the Indopacific. I've depicted this - in simplified form - in the adjacent cladogram. Because the Mugger, Siamese croc and Saltwater croc are removed from the New Guinea, Philippine and Freshwater croc in most molecular analyses, I've renamed the 'Indopacific assemblage' the 'reduced' Indopacific assemblage [the photo serving as the icon for that clade (a New Guinea crocodile) was taken by Wilfried Berns]. The content of this assemblage will be modified further in later articles in this series!

And that's all for now. Next: Saltwater crocodiles and the invasion of Australasia.

For previous Tet Zoo articles on crocodiles, see...

Refs - -

Brochu, C. A. 2000a. Congruence between physiology, phylogenetics and the fossil record on crocodylian historical biogeography. In Grigg, G. C., Seebacher, F. & Franklin, C. E. (eds) Crocodilian Biology and Evolution. Surry Beatty & Sons (Chipping Norton, Aus.), pp. 9-28.

- . 2000b. Phylogenetic relationships and divergence timing of Crocodylus based on morphology and the fossil record. Copeia 2000, 657-673.

- . & Storrs, G. W. 2012. A giant crocodile from the Plio-Pleistocene of Kenya, the phylogenetic relationships of Neogene African crocodylines, and the antiquity of Crocodylus in Africa. Journal of Vertebrate Paleontology 32, 587-602.

Eberhart, G. M. 2002. Mysterious Creatures: A Guide to Cryptozoology, Volume Two N-Z. ABC Clio, Santa Barbara.

Groves, C. P. 2001. Mammals in Sulawesi: where did they come from and when, and what happened to them when they got there? In Metcalfe, I., Smith, J. M. B., Morwood, M. & Davidson, I. (eds) Faunal and Floral Migration and Evolution in SE Asia-Australia. A. A. Balkema Publishers (Lisse, The Netherlands), pp. 333-342.

Man, Z., Yishu, W., Peng, Y. & Wu, X. 2011. Crocodilian phylogeny inferred from twelve mitochondrial protein-coding genes, with new complete mitochondrial genomic sequences for Crocodylus acutus and Crocodylus novaeguineae. Molecular Phylogenetic and Evolution 60, 62-67.

Mateus, O. 2002. The first direct observation of Crocodylus siamensis in Lao PDR in the last thirty years. Amphibia-Reptilia 22, 253-256.

McAliley LR, Willis RE, Ray DA, White PS, Brochu CA, & Densmore LD 3rd (2006). Are crocodiles really monophyletic?--Evidence for subdivisions from sequence and morphological data. Molecular phylogenetics and evolution, 39 (1), 16-32 PMID: 16495085

Meganathan, P. R., Dubey, B., Batzer, M. A., Ray, D. A. & Haque, I. 2010. Molecular phylogenetic analyses of genus Crocodylus (Eusuchia, Crocodylia, Crocodylidae) and the taxonomic position of Crocodylus porosus. Molecular Phylogenetics and Evolution 57, 393-402.

Moss, S. J. & Wilson, E. J. 1999. Biogeographic implications of the Tertiary palaeogeographic evolution of Sulawesi and Borneo. In Hall, R. & Holloway, J. D. (eds) Biogeography and Geological Evolution of SE Asia. Backhuys Publishers, Leiden, 133-155.

Oaks, J. R. 2011. A time-calibrated species tree of Crocodylia reveals a recent radiation of the true crocodiles. Evolution 65, 3285-3297.

Ross, F. D. & Mayer, G. C. 1983. On the dorsal armor of the Crocodilia. In Rhodin, A. G. J. & Miyata, K. (eds) Advances in Herpetology and Evolutionary Biology. Museum of Comparative Zoology (Cambridge, Mass.), pp. 306-331.

Trutnau, L. & Sommerlad, R. 2006. Crocodilians: Their Natural History and Captive Husbandry. Edition Chimaira, Frankfurt.

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

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