June 18, 2012 | 49
The Saltwater crocodile Crocodylus porosus, also known as the Estuarine crocodile, Indopacific crocodile or Saltie, is one of the world’s most famous crocodile species, probably being second in line after the Nile croc C. niloticus. Part of the reason this species is so well known to the public is that it often features in films and on TV; it’s also famous because it can be large or very large, because it’s a capable macropredator of big mammals, including humans, and because it’s at home in marine habitats as well as terrestrial ones.
As is well known, there are stories of Salties exceeding 8 m, 9 m and even 10 m in total length (a specimen killed in Bangladesh in 1840 was said to be 10.05 m long). It shouldn’t be assumed that these sizes are impossible – maybe individuals did reach them in prehistoric or historic times – but the maximum lengths of authenticated individuals have been about 6.2 m (for the Fly River 1982 specimen and the Mary River animal from the 1980s). Such large animals are – in the modern world – exceptional, and a big adult male Saltie is more typically between 4 m and 5 m long.
Incidentally, the thing often said about crocodilians exhibiting indeterminate growth and growing continually throughout life is probably not true. Determinate growth has now been demonstrated for the American alligator Alligator mississippiensis (Woodward et al. 2011) and is likely present across Crocodylia. Determinate growth is also known for various turtles, snakes, lizards and tuatara.
Saltwater crocs often frequent estuaries, lagoons and mangroves, but animals in some populations spend some or all of their time at sea. Extralimital records from the Cocos Islands southwest of Sumatra, from Fiji, and even from 48 km north of North Cape in New Zealand (Steel 1989) demonstrate an ability to travel far out to sea. Given this ability to live in the ocean and travel so far, why hasn’t the species spread further? Maybe it has, since a skull from the Seychelles show that it has occasionally moved west across the Indian Ocean to within just 1500 km of the African coast (Gerlach & Canning 1993). How far east have they travelled? I’ll leave that one to the cryptozoologists… Anyway, recent satellite tagging work has shown that Saltwater crocs exploit sea-surface currents when travelling at sea – a behaviour that became tagged as ‘surfing’ in the popular media – and that this exploitation of marine currents is an important bit of dispersal behaviour in this species (Campbell et al. 2010).
Saltwater crocodiles are one of the easiest crocodile species to identify, mostly because they (normally) entirely lack large scutes between the cervical shield and the back of the head. [Adjacent image by Holger Krisp, Ulm, Germany.] In addition, an obvious gap is also present between the cervical and dorsal shields, and small, triangular scutes are present between the posterior edges of the large, transversely arranged scutes in the dorsal shield (Ross & Mayer 1983). This combination isn’t present in any other species, and it’s a ‘reduced’ compliment compared to what’s present in most other crocodiles. Elsewhere in living crocs, a reduced osteoderm compliment is also present in the American crocodile C. acutus. It’s probably not coincidental that this is also a species with a strong preference for swimming at sea.
The evolving view of crocodile phylogeny once again
We saw in previous articles that crocodiles have often been imagined to consist of distinct Indopacific and New World assemblages, with the Nile crocodile being a close relative of the New World assemblage. Within this (morphology-based) framework, the Saltie is a member of the Indopacific assemblage, and thus close to the Freshwater crocodile C. johnstoni, Philippine crocodile C. mindorensis and New Guinea crocodile C. novaeguineae (Brochu 2000a, b).
However, molecular work has indicated that things may actually be more complicated, with the Indopacific assemblage being non-monophyletic. Rather than being closest to the Freshwater croc and so on, some authors have reported a close affinity between the Saltie and the Mugger (e.g., Densmore & Owen 1989, Gatesy & Amato 2008); others have advocated a sister-group relationship between the Saltie and the Siamese crocodile (McAliley et al. 2006, Meganathan et al. 2010); and yet others find a close relationship between the Saltie and a Siamese crocodile + Mugger clade (Man et al. 2011, Oaks 2011). On balance, it does seem that the Saltwater crocodile is closest to the Mugger and/or the Siamese crocodile. Purely for convenience, I’ll call this the ‘porosus clade’.
With the three members of the ‘porosus clade’ separated from the remainder of the Indopacific assemblage, we’re left with a ‘reduced’ Indopacific assemblage as mentioned last time. Is the ‘porosus clade’ closer to the Nile croc + New World assemblage clade than is the ‘reduced’ Indopacific assemblage? (as per Oaks 2011). Or is the ‘reduced’ Indopacific assemblage closer to the Nile croc + New World assemblage clade than is the ‘porosus clade’? (as in McAliley et al. 2006). We’re not sure – more work is needed.
Anyway, what we do know has some interesting implications. Firstly, it doesn’t seem that Australia’s two native crocs – the Saltwater and Freshwater crocodile – are all that close phylogenetically.
Secondly, given that most phylogenetic analyses find the crocodiles of southern Asia and Australasia to be outside the clade that includes the Nile crocodile and the New World assemblage, an Asian-Australasian/Indopacific centre of origin for crocodiles currently looks more likely for Crocodylus (Oaks 2011) than the African origin favoured traditionally. Then again, Osteolaemus and Mecistops are African (as are other, fossil, osteolaemines), and there are fossil members of Crocodylus in Africa too, like the Miocene C. checchiai and the Plio-Pleistocene C. anthropophagus and C. thorbjarnarsoni (Brochu et al. 2010, Brochu & Storrs 2012) (note that other alleged African species of Crocodylus – like ‘C.’ gariepensis from the early Miocene of the Namibia/South Africa border and ‘C.’ pigotti from the early Miocene of Kenya – are not actually within Crocodylus). Is it that all African members of Crocodylus invaded the continent following origination in Asia or Australasia? Or might it still be possible that Crocodylus began its history in Africa and/or Asia? We’ll come back to this issue again in a later article.
If there is a ‘porosus clade’ as discussed above, the fact that Muggers and Siamese crocs are both Asian might mean that the Saltie originated in Asia before colonising Australasia. But, then, people have assumed this anyway given that the Saltie’s Australasian range ‘only’ encompasses New Guinea and the northern, coastal parts of Australia (plus the island groups between and around these regions).
Crocodylus porosus, the… species complex?
The Saltwater crocodile varies a reasonable amount in appearance and body size across its extensive range. For these reasons there have been various suggestions that C. porosus of tradition is actually a species complex that needs splitting up. [Image above of C. porosus skull by Mariomassone.]
In 1844, S. Müller and H. Schlegel suggested that a distinct blunt-snouted population could be recognised among crocodiles then known as C. biporcatus (a name now regarded as a junior synonym of C. porosus); they named this new animal C. raninus. Of the several Javanese and Bornean specimens used in the naming of C. raninus, the two Javanese ones proved to be Siamese crocodiles (Ross 1992). However, the remaining, Bornean individuals could, according to Ross (1992), be reliably distinguished from both the Siamese crocodile as well as from unquestionable C. porosus on the basis of ventral scale counts and on the presence of four postoccipital scutes (the ones arranged just behind the rear margin of the head). Ross’s (1990, 1992) support for the distinction of C. raninus – sometimes known as the Indonesian crocodile or Bornean crocodile – has been followed by some other authors, but the name can’t yet be said to be in universal use. A skull, discovered in Brunei in 1990, has been identified as that of C. raninus (Trutnau & Sommerlad 2006).
Those with a good knowledge of Australasian herpetology will be familiar with Richard W. Wells and C. Ross Wellington’s several publications on the Australasian herpetofauna. This is not the time and place to discuss their articles or the controversy and debate that has surrounded them, but I do need to note very briefly that the numerous taxonomic revisions and proposals made by these authors remain (for the most part) highly controversial. Anyway, Wells and Wellington made two suggestions about Saltwater crocodiles that should be noted here.
Firstly, they suggested that C. porosus included a previously overlooked species of especially large, proportionally short-tailed, large-headed crocodile native to the Finnis and Reynolds Rivers in Northern Territory (Wells & Wellington 1985). They named this supposed species C. pethericki (after Australian biologist Ray Petherick) and designated ‘Sweetheart’ as the holotype. ‘Sweetheart’ was a male Saltwater croc (5.1 m long), captured in July 1979 following a number of incidents where he attacked and damaged boats. Unfortunately, he drowned during capture and is today preserved as a taxidermy mount at the Museum and Art Gallery of the Northern Territory [see photo below; by Jpatokal].
According to Wells & Wellington (1985), C. pethericki differs from C. porosus in details of scalation, overall colour (blackish with white venter vs browner with yellowish venter) and eyeshine colour (whitish-blue vs reddish), as well as in proportions. However, their proposal of taxonomic distinction for this form has not been accepted by other workers and it’s generally assumed that the differences they reported are within individual variation, or are related to ontogeny or adaptation to local conditions.
Secondly, Wells & Wellington (1985) questioned the otherwise widely-held opinion that the Saltwater crocs of Australia are conspecific with those of Asia, and hinted at the idea that more than one overlooked species might exist in Australia. This didn’t result in any additional nomenclatural acts, however. The majority of crocodilian experts have not regarded Wells and Wellington’s suggestions as worthy of proper investigation. As we’ll see in a later article, they made yet other suggestions about the taxonomy and phylogeny of Australian crocodiles.
Here end our all-too-brief look at one of the world’s largest and most charismatic predators. Time to move on. What about the other members of the Indopacific assemblage: the New Guinea and Philippine crocodiles, and the Freshwater crocodile? That’s where we’re going next.
For previous articles on crocodiles, see…
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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.
- ., Njau, J., Blumenschine, R. J., & Densmore, L. D. 2010. A new horned crocodile from the Plio-Pleistocene hominid sites at Olduvai Gorge, Tanzania. PLoS ONE 5(2): e9333. doi:10.1371/journal.pone.0009333
- . & 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.
Campbell, H. A., Watts, M. E., Sullivan, S., Read, M. A., Choukroun, S., Irwin, S. R. & Franklin, C. E. 2010. Estuarine crocodiles ride surface currents to facilitate long-distance travel. Journal of Animal Ecology 79, 955-964.
Densmore, L. D. & Owen, R. D. 1989. Molecular systematics of the order Crocodilia. American Zoologist 29, 831-841.
Gatesy, J. & Amato, G. 2008. The rapid accumulation of consistent molecular support for intergeneric crocodilian relationships. Molecular Phylogenetics and Evolution 48, 1232-1237.
Gerlach, J. & Canning, L. 1993. On the crocodiles of the western Indian Ocean. Phelsuma 2, 54-58.
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.
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.
Oaks, J. R. 2011. A time-calibrated species tree of Crocodylia reveals a recent radiation of the true crocodiles. Evolution 65, 3285-3297.
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- . 1992. Designation of a lectotype for Crocodylus raninus S. Müller and Schlegel (Reptilia: Crocodylidae), the Borneo crocodile. Proceedings of the Biological Society of Washington 105, 400-402.
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Trutnau, L. & Sommerlad, R. 2006. Crocodilians: Their Natural History and Captive Husbandry. Edition Chimaira, Frankfurt.
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Woodward, H. N., Horner, J. R. & Farlow, J. O. 2011. Osteohistological evidence for determinate growth in the American alligator. Journal of Herpetology 45, 339-342.
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