Thursday, 12 April 2018

Crocodiles and Alligators; never smile, see you later...

Few reptiles exert the same (often horrified) fascination for people as do crocodilians - and I'll explain shortly why I'm using that apparently fussy and precious term instead of just 'crocodile' when talking about the overall group.  Like sharks, it seems that they are in such control in the water, where we are inevitably just visitors. We can't learn to 'read' a crocodile as we might a predatory mammal. As an excellent South African wildlife guide, who'd spent his life in the African bush, put it to me: "you can negotiate with a lion, you can't negotiate with a crocodile". (Not that I'm interested in trying either of course!)
Big male Estuarine Crocodile Crocodylus porosus crossing the Daintree River, north Queensland.
We were in a boat (large enough for us to feel comfortable!), and he definitely turned towards us
for a look, before continuing on his way.
This magnificent predator is the largest of all living reptiles.
The crocodilians comprise an Order (Crocodilia) within the Class that represents reptiles.(The other vertebrate classes represent mammals, birds, amphibians and fish, but as ever there's a complication. The fact is that crocodilians are more closely related to birds than they are to lizards - both crocodilians and birds are direct descendants of dinosaurs, while lizards are not. Given this, there has for a while now been a somewhat robust discussion about the apparent unravelling of logic here; either crocodiles should not be described as reptiles, or birds should. The latter position is favoured by quite a few respected voices, but for now I'll leave you to angst over it - sorry about that! - and return to our more immediate subject.)

There are three Families of crocodilians - the crocodiles (hence my use of 'crocodilian' for the overall group), the alligators and caimans, and the single-species Family represented by the Indian Gharial.
Gharial Gavialis gangeticus, Uttar Pradesh, India.
(Photo courtesy Wikipedia.)
This narrow-jawed fish-eating specialist is, tragically, Critically Endangered.
For the rest, there are 14 species of crocodile (four each in Africa and the Caribbean/Central/South America region, and six from Australia through south-east Asia to India), and 8 caimans and alligators (the six caimans are all South American, and there is an alligator in both the south-east USA and in China, where it is seemingly headed for extinction).
Nile Crocodile Crocodylus niloticus, Queen Elizabeth National Park, Uganda.
This species is second in size only to the Estuarine Crocodile.
The scales typical of all crocodilians are evident here; this tough waterproof outer layer forms as the skin cells
cornify, ie they die and fill with keratin. On them are rows of hard separate scales.
The powerful laterally flattened tail sweeps from side to side to drive the animal through the water.
Yacaré Caimans Caiman yacare, Pantanal, south-western Brazil.
The concentration of this species in the vast ephemeral wetlands of the Pantanal, estimated at some ten million caimans,
is the greatest in the world.
There is plenty of - often superficial - discussion of the differences between crocodiles and alligators out there, so I won't spend a lot of time on it here. Inter alia, in the wild it is only relevant in the Caribbean region, but the discussion can still raise some interesting points. One observation often made is that alligators/caimans have a broader 'U-shaped' nose, compared to the sharper 'V-shaped' nose of the crocodiles. I would add a large 'but' to that assertion, though it is generally true. You can see the narrower snout in the Nile Crocodile above, and in this Freshwater Crocodile C. johnsoni.
Freshwater Crocodile, Boodjamulla (Lawn Hill) NP, north-west Queensland.
This relatively small species mostly eats fish, and its narrow jaws (like that of the Gharial) are probably
adapted for quick movement rather than power.
This Black Caiman Melanosuchus niger and the following Yacaré Caiman display the 'typical' short broad alligator snout.
Black Caiman, Rio Madre de Dios (Tambo Blanquillo Lodge), southern Peruvian Amazonia.
Yacaré Caiman, Pantanal.
It has been claimed that this shape is an adaptation to crunching up hard turtle shells, but I've seen little evidence either that this jaw shape allows a more powerful bite than a longer more tapering snout, or that alligators and caimans eat significantly more turtles than do crocodiles. Moreover it also seems to be, in part, a function of age. As the big crocodiles age, their jaws get blunter and more massive.
Big Estuarine Crocodile (in captivity, Territory Wildlife Park, south of Darwin).
And here's a young caiman.
Spectacled Caiman Caiman crocodilus, Yasuní NP, Ecuador.
Probably a more reliable indicator is in our view of the teeth when the jaws are closed. An alligator/caiman's upper jaw is broader than its lower one, so the lower teeth tend not to be visible from side on.
Yacaré Caiman, Pantanal. Toothy enough - but only the top teeth.
On the other hand a  crocodile's jaws are roughly the same width, so top and bottom teeth intermesh, and some at least of the bottom teeth are also on view; in particular, there is a very large fourth (from the front) bottom tooth, which apparently could not comfortably fit inside the mouth, so rests in a skin indentation outside the top jaw, just behind the nostrils. This early morning sun-bathing beauty demonstrates this feature admirably.
Estuarine Crocodile, Kakadu NP, Northern Territory.
The origin of the idea of the famous crocodile 'smile'.
Recent Japanese research may have thrown up another consistent distinction too. Crocodiles have consistently longer 'upper arm bones' (humerus) and 'thigh bones' (femurs) than alligators, though this is not a very useful field character... However it does raise another issue that distinguishes crocodilians from 'other reptiles'. Their legs are structured in a way that enables them to stand high off the ground with their legs directly under them, and ankles that enable the feet to swivel, giving a walk unlike that of any other reptile, but not dissimilar to that of mammals. 
Crocodile legs, modelled by Estuarine Crocodile, Territory Wildlife Park, NT (above),
and Freshwater Crocodile, Katherine Gorge, Nitmiluk NP, NT (below).
Note that only the hind feet are webbed.
This species in particular is known to gallop across the ground.
One was measured at 17kph, the world land speed record for a crocodile!
Another generalisation sometimes encountered is that crocodiles are larger than alligators. While it is true that the largest alligators are somewhat smaller than the largest crocodiles (some living Estuarine Crocodile are over six metres long and weigh more than a tonne), they are also much larger than most crocodile species. While decades of over-hunting in northern Australia brought numbers very low, they are rapidly recovering under protection which was imposed in 1971; however they are slow-growing and it will be some time before we can test the stories of huge crocs of the past. Perhaps the best-known of these is celebrated in a supposedly life-sized statue in Normanton, north Queensland, near the Gulf of Carpentaria.
The truly magnificent animal which this statue claims to faithfully represent was 8.6 metres long and weighed
over two tonnes. It was shot on the banks of the nearby Norman River in 1957 by Krystina Pawlowski,
a Polish post-war immigrant. Her story (and that of her husband Ron), and their later conversion to campaigners
for crocodile conservation - and subsequent harassment by the Queensland government - is well worth reading.
Here's one version of it; I understand that all the essential elements of it are true.
Black Caiman, Manu River, southern Peruvian Amazon. I estimated this huge caiman to be
at least four metres long. Like the big crocodiles they are apex predators, though fish are their main prey.
In recent years I've noted that Amazon lodges are increasingly banning swimming in their lakes;
discreet enquiries by our guides have revealed some serious attacks on tourists which were not widely reported.
Wherever really big crocodilians are present, they are the top predators in their environment. (Crocodilians in fact are the only group of exclusively carnivorous reptiles.) With smaller species and individuals, there is more of a balance. In the Pantanal, Jaguars prey heavily on Jacaré Caimans; in Amazonia, Black Caimans and Giant Otters are permanently at war, each preying on the young of the other. In Australia Estuarine and Freshwater Crocodiles can coexist in inland waters - despite their name the Estuarines are quite at home in fresh water - but 'freshies' generally avoid 'salties', doubltess aware of the real hazards posed by their big relations.
Yacaré Caiman with big catfish; piranhas are probably the biggest prey item for them however.
Some aspects of crocodilian breeding seem universal, with a male controlling a territory containing several females; eggs are bird-like, with hard calcium carbonate shells (unlike reptiles). However strategies vary around this theme. For instance in Australia Freshwater Crocodiles dig out a hollow in a sandbank near the water and lay about a dozen eggs. She doesn't guard the eggs, and many are taken by goannas, but when the hatchlings start calling (before hatching) she comes and digs them out and carries them to the water. Estuarine Crocodiles on the other hand build a mound of mud and vegetation, in which she lays around 50 eggs, which she defends against predators - very few goannas manage to rob their nests. Estuarine mothers too are attentive, guarding their young for months.

(Empty) Estuarine Crocodile nest, Kakadu NP, Northern Territory.
Ancient Gondwanan crocodiles known as Mekosuchines arose some 50 million years ago, and survived in inland Australian in and around swampy areas, but most died out as Australia dried, by the end of the Pliocene 1.6 million years ago. However one genus, Kwinkana, a big long-legged terrestrial hunter, survived well into human times; as ever it is impossible to say whether humans or climate change finished it off. However modern crocodiles (Crocodylus) apparently only arrived in Australia recently, as the continent drifted north into Asia. Estuarine Crocodile are great seafarers, using ocean currents, and would have had no problem island-hopping as Australia approached Indonesia. (Inter alia it's a very short hop from eastern New Guinea to Cape York.) I am not aware of any genetic studies, but I assume that Freshwater Crocodiles evolved from these immigrant salties.

Perhaps I've not encouraged many people to love crocodiles - and I don't imagine the crocodiles want that anyway! - but I hope you can appreciate these venerable consummate hunters a little more now. Let's end on some crocodilian portraits from across the southern hemisphere.
Estuarine Crocodile cruising past a Striated Heron, East Alligator River, Kakadu NP.
Estuarine Crocodile, Kinabatagan River, Sabah, Malaysian Borneo.

Nile Crocodile and Cape Buffalo, Queen Elizabeth NP, Uganda.

Baby Spectacled Caiman, Yasuní NP, Ecuador.

Spectacled Caiman at night, Yasuní NP, Ecuador.

Yacaré Caiman and Capybara, Pantanal, Brazil.

Yacaré Caiman crossing the road, Pantanal.
Yacaré Caiman with headwear, Pantanal.
Estuarine Crocodile in the landscape, Cooinda, Kakadu NP.
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Flabmeister said...

Following our conversation in other lines about the links between birds, crocs and other reptiles I seemed to conclude that, if following the DNA-based fatwas, one no longer bothers about orders, classes and families and such like but only about clades. These seem to totally cut across the traditional boundaries.

The crocodilian in Sri Lanka has the possibly appropriate name of Mugger Crocodile.

Ian Fraser said...

I haven't followed the developing directions of cladistics in recent times, but I think I understand where they're coming from. The new information flowing from new tools is overwhelming more traditional systems of classification, and some are trying to stay true to their data, which probably means bypassing the established structures. I'm not sure that it needs to go that far, but it probably requires some massive reconstruction. (eg either crocs aren't reptiles - which is probably silly - or birds are.) I suspect that for most purposes pragmatism will ensure that the current systems will prevail, albeit with a bit of a wink and a nudge. I find it all fascinating, but of course never as much as the organisms themselves!
And yes, I do like the name Mugger!