This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
A momentous thing is about to happen. Take a deep breath and prepare yourself. Today is the day when... I finish my series of articles on the crocodiles of the world. As you’ll recall if you read the previous six parts of this series (all of which are linked to below), I’ve managed so far to get through the Asian crocodiles, the Indo-Australian ones, and – most recently – the two distinct African lineages. That ‘only’ leaves us with the several New World crocodiles, and it’s this group of species – forming the so-called New World assemblage within Crocodylus – that we look at here.
We start with the best known member of this group: the American crocodile C. acutus. This is a mostly Central American species, well known for being tolerant of brackish and marine conditions. It’s a large to gigantic crocodile, the specific name referring to its proportionally long and slender snout. Actually, the name Crocodylus americanus Laurenti, 1768 might be a senior synonym of C. acutus Cuvier, 1807, but the former isn’t in use since the description it accompanies does not unambiguously pertain to this species. Having said that it’s “gigantic”, modern individuals tend to be between 2.5 and 4 m in total length, with lengths of 6 or 7 m being very much a thing of the past.
American crocodiles occur from coastal western Mexico all the way south to extreme northern Peru in the west and eastern Venezuela in the east. Its presence in southern Florida is well known, and in the Caribbean it occurs on Cuba, Jamaica, Hispaniola and Martinique (though, apparently, not elsewhere in the Lesser Antilles). Specimens have been reported from the Islas Marías in the Pacific (100 km off the coast of Mexico), the Caymans (370 km west of Cuba in the Caribbean), and archaeological remains show that it previously occurred on the Bahamas. Clearly, it’s able to make sea crossings and to live at sea for months at a time. Indeed, a specimen captured in 2008 off the coast of Mexico had stalked and non-stalked barnacles attached to its teeth and caudal scutes (Cupul-Magaña et al. 2011).
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Adult American crocodiles have a distinctive hump along the midline of the snout, anterior to the eyes, and their irregular and slightly asymmetrical arrangement of dorsal armour is also distinctive. The dorsal osteoderm compliment of the American crocodile is strongly reduced compared with that of most other living crocodylians; it is in fact the most reduced osteoderm compliment of any living crocodylian. In some individuals, there are only four or even just two scutes arranged transversely across the middle part of the body (six or eight are more typical), and two transverse scute rows are typically absent between the dorsal shield and neck shield, creating an unarmoured band across the base of the neck (Ross & Mayer 1983). Crocodiles normally have 16 or 17 transverse osteoderm rows in the dorsal shield, but some American crocodiles only have 14 (Ross & Mayer 1983).
This reduced armour compliment is very likely related to the American crocodile’s adaptation to life in brackish and marine environment; it probably isn’t coincidental that the other crocodile with a reduced armour compliment is the Saltwater crocodile.
Crocodylians and seed dispersal, again
American crocodiles have catholic diets, with fish dominating their diets but birds and mammals being consumed too. Juveniles eat insects, frogs, small turtles as well as fish and small mammals. Intriguingly, Casas-Andreu & Quiroz (2003) found a large number of plant seeds inside the American crocodile droppings they analysed from Mexico. They assumed that these seeds originated from the stomach and gut contents of animals that the crocodiles had eaten, and noted that crocodiles might play a hitherto overlooked, but important, role in seed dispersal.
While this all sounds likely and sensible, long-time readers with good memories might recall that occasional fruit-eating and even leaf-eating is now known for several crocodilian species: I covered this subject back in 2008; more recently, another study has reported seed dispersal and possible fruit consumption in American alligators Alligator mississippiensis (Platt et al. 2013). Could it be, then, that at least some of those seeds in the Mexican droppings represent primary acts of plant consumption on the part of the crocodiles, not secondary acts resulting from the ingestion of herbivores?
Placing C. acutus in the crocodile tree – with complications from non-monophyly
Views on how the American crocodile might be related to other crocodiles have varied quite a bit. It’s often been considered close to the Cuban crocodile C. rhombifer, Mexican crocodile C. moreletii and Orinoco crocodile C. intermedius within the ‘New World assemblage’, but some studies have – surprisingly – recovered C. siamensis and C. porosus as part of the same clade as these American species (Densmore & Owen 1989).
The hypothesis that there might be a ‘New World assemblage’ consisting only of the American crocodile, Cuban crocodile, Morelet’s crocodile and Orinoco crocodile (Brochu 2000, Oaks 2011) seems logical on biogeographical grounds (but, ha, that doesn’t mean it’s true). Several morphological characters seem to support the monophyly of the New World assemblage, most notably the presence of a median boss on the snout formed from slightly elevated nasal bones (Brochu 2000) and a reduced post-occipital scute row where the scutes are small and low in number (this latter feature has evolved convergently in some other crocodiles, most notably the Siamese and Saltwater crocodiles).
Man et al. (2011) found a strong relationship between the American crocodile and Nile crocodile in their analysis of mitochondrial genes. However, they didn’t include any other New World species, so it mustn’t be assumed that they necessarily demonstrated an exclusive relationship between these two species. Their findings do, however, add support to the view that the New World assemblage is closer to the Nile crocodile than to members of the other living crocodile lineages. Oaks (2011) supported monophyly of a New World assemblage, with the Nile crocodile and Sacred crocodile being successive outgroups to this clade (see the previous articles in this series – links below – for more on those species).
The distribution of the American crocodile is interesting in that it occurs on both the Caribbean and Pacific sides of Central America. It’s plausible that members of the respective populations might have moved across land to get to the other side of the landbridge. But it’s also plausible that the populations have been separate for a very long period of time. Given that the Panamanian Isthmus formed more than two million years ago, the potential for reproductive isolation here very much suggests that cryptic lineages await recognition. Fossils show that C. acutus was present in Costa Rica at least about two million years ago (Mead et al. 2006), supporting the view of a lengthy and perhaps complex history in the region.
There is in fact already some evidence that C. acutus as conventionally recognised is not monophyletic (though, don't get me wrong: populations don't have to be monophyletic in order to be considered a 'species'. A 'species' is, after all, pretty much whatever the hell we want it to be). Milián-García et al. (2011) found Cuban C. acutus populations to form a clade with the Cuban crocodile C. rhombifer, rather than with C. acutus populations from Central America. This is at odds with what we’d predict based on morphology, since the Cuban crocodile is extremely distinctive in appearance while the Central American and Cuban C. acutus populations are highly similar. One possible solution might be to sink the Cuban crocodile into C. acutus (perhaps down-grading it to ‘subspecies’ level); another might be to place the Cuban C. acutus population within C. rhombifer. A third possibility is that the Cuban C. acutus population actually represents an additional species that needs naming: it looks like C. acutus from Central America, but the similarity is symplesiomorphic and the two have in fact been separate for 2 or more million years (Milián-García et al. 2011). More work is needed to resolve this issue.
On Cuba, American crocodiles live in sympatry with Cuban crocodiles. Both are known to hybridise in captivity, and genetic evidence from wild animals seems to confirm that morphological ‘mosaics’ involving the characters of both species are indeed naturally occurring hybrids (Milián-García et al. 2011). Cuban crocodiles declined substantially during the 19th and 20th century, largely due to hunting pressures, and today they’re entirely restricted to the Zapata Swamp in western Cuba.
Unfortunately, illegal hunting of the species continues, as does modification of the Zapata Swamp region. Hybridisation with the American crocodile may represent a new and unanticipated problem for the persistence of the Cuban crocodile and it has been recommended that future management plans aim to reduce or prevent interspecific hybridisation occurring (Milián-García et al. 2011). As is clear from the accompanying photos, the Cuban crocodile is pretty awesome in appearance. It has especially prominent squamosal horns, an especially rugose-looking integument, and a neat, mottled pattern that involves black blotches on a greenish or yellowish ground colour. Some sources say that it has proportionally longer legs than other crocodiles: does anyone know if this has been confirmed?
The Morelet’s or Mexican crocodile
One of the world’s most poorly known crocodiles is Morelet’s crocodile C. moreletii, named in 1851 for specimens collected by French naturalist Pierre Morelet in Lake Petén Itzá (then known as Lac Florès), Guatemala. The names Central American crocodile and Mexican crocodile are sometimes used for this species. I personally prefer these to ‘Morelet’s crocodile’ since they at least remind you that the species is an American one, but I’m also a stickler for history and don’t wish to denigrate Morelet’s role in its discovery. The species has a confused early history, since at least one of the first specimens was shipped to Cuba and mixed with a consignment of Cuban molluscs. This meant that the crocodile was then assumed to be a Cuban crocodile, and distinct species status for C. moreletii wasn’t really accepted until 1924.
Morelet’s crocodile is usually dark brown or blackish with a snout of ‘generalised’ form (that is, neither especially robust nor especially elongate). In keeping with this anatomy, it seems to be a dietary generalist, with prey items including snails, turtles, mammals and fish. Platt et al. (2007) reported cases in which Morelet’s crocodiles scavenged on dead cattle and also described an incident where a Morelet’s crocodile found and partially consumed a tapir killed by a Jaguar Panthera onca.
While often stated to be strictly restricted to freshwater lakes, rivers and pools in forests and savannahs, Morelet’s crocodile also inhabits brackish lagoons in parts of its range. It recalls the American crocodile in having a reduced and somewhat asymmetrical dorsal osteoderm compliment: just three or four osteoderms might be present in each transverse row in the dorsal shield, and detached lateral scutes are always present on the flanks. At least some of the four to six scutes present in the cervical row closest to the head are larger than those of the American crocodile (Ross & Mayer 1983).
Morelet’s crocodile is generally stated to be a small species that might not usually exceed 2.4 m. However, individuals of over 3 m and even 4.1 m were observed in the 1970s and 90s (Pérez et al. 1991) and there are suggestions that animals about 5 m long might previously have existed. This is clearly not a giant species, but the fact remains that it probably can reach a greater size than usually thought, and it may owe its small modern average size to decades of intensive and selective hunting (Ludwig & Ralf 2006). The skin of this species has been used as a high quality leather and exploitation for the leather trade resulted in its local extinction across part of its range. Legal protection was granted in 1981 and the species has partly recovered since then. John Platt wrote about the conservation status of the species over at Extinction Countdown back in April 2011.
Morelet’s crocodile is known to hybridise with C. acutus on the Mexican Yucatan Peninsula (Cedeño-Vázquez et al. 2008).
The Orinoco crocodile
The Orinoco crocodile C. intermedius is a critically endangered member of the New World assemblage found only in Colombia and Venezuela [adjacent image by Greg Hume]. Its long, slender and often gently upcurved rostrum is usually regarded as distinctive, but it can sometimes look extremely similar to the American crocodile, so much so that there are cases where captive specimens identified as Orinoco crocodiles were actually misidentified members of C. acutus (Trutnau & Sommerlad 2006). The two can, however, be distinguished thanks to the convex area present on the midline of the rostrum in C. acutus (no such structure in C. intermedius), the longer mandibular symphysis of C. intermedius, and the less reduced dorsal osteoderm compliment of C. intermedius.
Like the American crocodile, the Orinoco crocodile is supposed to reach gigantic size – that is, there are dubious accounts of specimens 7 or 8 m long. 5.2 m is the more accepted maximum recorded length, though even this is exceptional compared to modern animals, adult males averaging just over 4 m. Orinoco crocodiles vary in colouration from greyish to brownish or yellowish, and dark spots and blotches are often present over the dorsal surface. Black specimens are on record.
The Orinoco crocodile seems to be especially close to the American crocodile (Brochu 2000, Oaks 2011).
And it’s at this point that I have to stop – since now, finally, I’ve completed my planned species-by-species review of the living crocodiles. Yay! Basically, this series of articles should be seen as a sort of review of ideas about the phylogenetic hypotheses and species-level diversity of extant crocs. There is, needless to say, tons on the ecology, behaviour, biology and conservation of these species that I haven’t covered. We will, of course, be coming back to crocodylians (and other crocodyliforms, and other crocodylomorphs) on numerous occasions in the near and far future. Until then...
For for other parts of this series, see...
The Saltwater crocodile, and all that it implies (crocodiles part III)
Crocodiles of New Guinea, crocodiles of the Philippines (crocodiles part IV)
The Freshie: Australian crocodile, seemingly from the north (crocodiles part V)
And for other Tet Zoo articles on crocodiles, and on other crocodylians, crocodyliforms and crocodylomorphs, see...
In pursuit of Early Cretaceous crocodyliforms in southern England: ode to Goniopholididae
Tool use in crocodylians: crocodiles and alligators use sticks as lures to attract waterbirds
Refs - -
Brochu, C. A. 2000. Phylogenetic relationships and divergence timing of Crocodylus based on morphology and the fossil record. Copeia 2000, 657-673.
Brueggen, J. 2002. Crocodilians: fact vs. fiction. In International Union of the Conservation of Nature, Crocodile Specialist Group 1, pp. 204-210.
Cedeño-Vázquez, J. R., Rodriguez, D., Calmés, Ross, J. P., Densmore, D. & Thorbjarnarson, J. 2008. Hybridization between Crocodylus acutus and Crocodylus moreletii in the Yucatan Peninsula: I. evidence from mitochondrial DNA and morphology. Journal of Experimental Zoology (Molecular and Developmental Evolution) 309A, 661-673.
Cupul-Magaña, F. G, Rubio-Delgado, A., Escobedo-Galván, A. H. & Reyes-Núñez, C. 2011. First report of the marine barnacles Lepas anatifera and Chelonibia testudinaria as epibionts on American crocodile (Crocodylus acutus). Herpetology Notes 4, 213-214.
Densmore, L. D. & Owen, R. D. 1989. Molecular systematics of the order Crocodilia. American Zoologist 29, 831-841.
Ludwig, T. & Ralf, S. 2006. Crocodilians: Their Natural History and Captive Husbandry. Edition Chimaira, Frankfurt.
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.
Mead, J. I., Cubero, R., Zamora, A. L. V., Swift, S. L., Laurito, C. & Gómez, L. D. 2006. Plio-Pleistocene Crocodylus (Crocodylia) from southwestern Costa Rica. Studies on Neotropical Fauna and Environment 41, 1-7.
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