Why cant crocodiles live in the ocean

The odd shark fin, sure, but you don't necessarily expect to see a crocodile at the seashore. And yet that's just what the sunbathers of Palancar Beach on the Mexican island of Cozumel experienced in early July of this year: a ten-foot American crocodile sauntered (and, really, that's the best description) through their midst to slip into the Caribbean.

Nerve-wracking as the sight might have been, the incident turned out far better than one in Queensland's Daintree National Park back in May, when a large Indo-Pacific crocodile snatched a woman from the shallows off Thornton Beach.

So what do we know about the seafaring tendencies of crocodiles? And what to make of other reports from this year, of multiple alligators turning up in the surf along the southeastern United States? And hey – what happens when a high-seas traveller of a crocodile meets a big shark?

SEAGOING CROCS

Unlike alligators or caimans, crocodiles have functional lingual salt glands for expelling sodium chloride and a fairly impermeable lining to their inner mouth, which makes them inherently hardier than their relatives in seawater. And any good-sized crocodilian can survive a long while without drinking freshwater by satisfying its needs via food and metabolic action.

“That Indo-Pacific crocodiles are occasional mariners is well established. The historical range of this biggest living reptile is a vast ocean-dominated kingdom.”

A number of crocodile species readily reside in brackish habitats, but two are by far most comfortable in marine settings: the American croc and the Indo-Pacific (aka "saltwater") croc.

That Indo-Pacific crocodiles are occasional mariners is well established. The historical range of this biggest living reptile is a vast ocean-dominated kingdom: from the seacoast of southern China (where it was wiped out long ago) and the Sundarbans mangal of India and Bangladesh, south to northern Australia, and from the Solomon Islands and Vanuatu in the east as far west as the Seychelles, where it was killed off by the early 1800s.

As croc expert Adam Britton's website nicely lays out, this formidable beast has turned up in many far-flung locations befitting an ocean wanderer. A crocodile killed at Ailinglaplap Atoll in the Marshall Islands in 2004 was some 1,200 miles from the nearest known population. The historical record even shows vagrant crocs off Japan: in the Ryukyu and Ogasawara archipelagos, and, farther north still, in Honshu's Toyama Bay.

Indo-Pacific crocodile distribution – including the range of "wanderer" crocs – based on a map by Adam Britton.

Lately, vagabond Indo-Pacific crocs have increasingly turned up in the Maldives, most often during the rains of the Northeast Monsoon. In 2015, there were more than a dozen crocodile sightings in less than two months – making surfers in that wave-riding paradise a little nervous. And in 2010, a snorkeller was killed by a croc over a coral reef in open coastal waters off Havelock Island, in a corner of the Andaman Islands not previously known for the reptiles.

American crocs also have an extensive, mostly coastal dominion, cruising tidewater rivers, lagoons, mangrove swamps, and offshore cays and atolls from South Florida down to South America's northern Pacific and Atlantic coasts. They've been found encrusted with barnacles, and spotted munching sea turtles at nesting beaches and pelicans offshore. In fact, one study revealed that South Florida crocodiles have a diet of 65 percent marine prey – and suggested the animals "may be capable of being ecologically isolated from freshwater ecosystems".

In other words, that Cozumel beach croc was no aberration.

An American crocodile nicknamed "Cletus" is famous for being the only one of his kind to inhabit the waters of the Dry Tortugas, a remote cluster of islands west of Florida's Key West. Since he mysteriously turned up here back in 2003, Cletus has been living in an environment completely devoid of freshwater. Image: Dry Tortugas National Park/Facebook

Like their Indo-Pacific counterparts, American crocs sometimes meander well outside their established range. South Florida crocs have materialized well north up the coasts of the Sunshine State, and even off South Carolina. Crocs from the Lower Florida Keys occasionally make deepwater crossings out to the remote Dry Tortugas. In Sonora, Mexico (where they've vanished), American crocs used to periodically stray northward along the Sea of Cortez's desert shores as far as Tiburon Island.

In recent years, a few crocs – likely drifters from Cuba, which has more American crocs than anywhere else – have reared their snaggletooth heads in the Cayman Islands, which probably once supported their own resident population. 

SURF RIDERS

Research from the Cape York Peninsula in northeastern Australia, a global stronghold for the Indo-Pacific crocodile, has offered intriguing clues on how this reptile might have come to occupy such a gigantic and briny jurisdiction. It suggests that crocodiles can compensate for a lack of seafaring expertise by catching lifts on currents flowing in the direction they want to go.

Acoustic tracking has shown that some resident adult crocs in Queensland's Kennedy River make periodic long-distance forays synced with tidal currents: they coast on the prevailing flow, rest on the riverbed when the tide reverses, and then resume their journeys when the currents turn favourable again. Such tactics save precious energy, and combined with the crocs' large size and marine-adapted physiology, they might allow them to "undertake and survive considerable ocean voyages", the researchers suggest.

Another study tested the effectiveness of translocating "problem" crocodiles by trapping three large males on the Cape York Peninsula, rigging them with satellite transmitters and helicoptering them to release sites. The trio of reptiles all returned to the locations of their capture – demonstrating both their homing ability and the limitations of translocation.

One of these beasts, nearly 16 feet long, was caught in the Wenlock River on the western coast of the peninsula, flown 78 miles overland and released in Temple Bay on the other side. After chilling on the shore a few months, the croc hopped a northbound nearshore current and rode it to the tip of the peninsula in eight days. Upon his arrival, the current through the Torres Strait (which separates the Cape York from New Guinea) was flowing west to east, so the croc lingered here for three days until it reversed. He then rounded the peninsula to surf a northerly flow and re-entered the Wenlock. His circumnavigation saw the big croc travel more than 255 miles in 19 days.

The 16-foot croc's Cape York Peninsula circumnavigation spanned 255 miles and took 19 days.

So we know these reptiles are capable of long excursions through marine waters – but where are they going? "We assume they are traveling along the coastline to a different river system," suggests one of the researchers, University of Queensland's Craig Franklin. "[Most are] mid-sized males. Possibly they have been displaced by larger males in the river system they came from and are now looking for an alternative to inhabit and breed."

Given their taste for nesting sea turtles and their habit of bee-lining for certain fish runs, it's also likely that full-grown adults sometimes use the ocean as a "highway" to commute between river mouths or turtle rookeries. Crocodiles may also use marine habitat for nesting: American crocs in South Florida mostly frequent mangrove swamps, but some females enter Florida Bay off the Everglades National Park coast to lay eggs on remote keys.

But what about honest-to-goodness transoceanic journeys? Most Indo-Pacific crocs probably restrict themselves to coastal cruising, and the few that end up far offshore may have simply been swept there by accident. But perhaps, rarely, certain crocs strike off on pelagic voyages to disperse, riding an ocean gyre to distant shores. "At sea, you have no visual point of reference, but we know that crocodiles can navigate using non-visual references – magnetic fields being one – so they may be able to detect when they're in a current out at sea," suggests Britton.

The take-home message: Indo-Pacific crocs probably don't go rafting across ocean basins very often, but – given their skill at riding currents and their long-term durability in saltwater – if they do find themselves out on the high seas, they may well endure long enough to colonise some far-off beach. And that likely explains their sprawling geography. (Franklin's lab continues to track Cape York crocs to learn where they go a'roaming – you can see pictures of tagged crocs and maps of their movements on their website.)

OCEAN GATORS?

Beachgoers in the southeastern US sometimes do a double-take at the sight of a gleaming black alligator in the rollers. It's certainly not unheard of. A number of gators, for instance, were captured this year in the breakers along two neighbouring South Carolina barrier islands.

While "surf gators" are usually fairly small – maybe young animals overpowered by outgoing tides or booted by adults from coastal territory – some aren't. A handsome 10-footer came ashore at Folly Beach, South Carolina in 2014 (though it ended up shot dead), and this year the carcass of a 13-foot-long alligator washed up on a beach in Galveston, Texas.

This 10ft alligator turned up in the surf on Folly Beach in South Carolina back in 2014. Image: Folly Beach Department of Public Safety.

An alligator seen on South Carolina's Myrtle Beach earlier this year. Image: Nancy Thompson

Floods and tidal surges – and perhaps the spirit of adventure – may pull gators from coastal rivers or inlets even farther from the mainland. In 2010, biologists surveying for North Atlantic right whales came across an alligator some 20 miles off the Georgia coast. Recent deluges in the Altamaha River basin may have carried this gator out to sea: the observers noted marsh wrack and other debris near the reptile.

Even more impressively, a six- to seven-foot gator circled an oil platform roughly 40 miles off the Louisiana mainland in the Gulf of Mexico in 2005. The oil workers who spotted the seafaring beast wondered whether it was seeking a haul-out to rest. One told a biologist investigating the sighting that he'd seen an offshore alligator only once before, maybe 16 miles out in the Gulf, but none of the others had ever observed one.

In 2010, meanwhile, scuba divers photographed an alligator kicking back 60 feet deep on the sandy seafloor near Breaker's Reef, a mile off Florida's Palm Beach coast.

Alligators and caimans are mainly freshwater crocodilians – their lingual glands don't effectively secrete salt. But American alligators regularly inhabit brackish waters in estuaries, mangrove swamps and saltmarshes. The larger ones can even endure short intervals in full-strength seawater. 

More resilient in marine settings – and maybe more adept at negotiating tidal currents and wave chop – such big gators might find rich pickings on sorties into seawater. In fact, in some areas, estuarine or marine prey may compose most of the diet of coastal alligators, especially adult males. Documented saltwater gator morsels include mullet, blue crabs, yellowtail perch, horseshoe crabs and stingrays. (A Georgia state biologist noted his agency had "pulled stingray barbs out of the cheeks of 12-foot males".) There are also records of Florida alligators preying on green and loggerhead sea turtles.

Studies from places like Florida's Shark River Estuary and Sapelo Island off Georgia suggest certain coastal alligators make a tradeoff: they endure the physical hardships of brackish habitats – plus the workout of shuttling between them and the freshwater havens where they recharge – because these spots are such productive hunting grounds.

In some areas, estuarine or marine prey like crabs, stingrays and sea turtles may compose most of the diet of coastal alligators.

CROCS & SHARKS

When mulling crocodilians in marine habitats, it doesn't take long for the thought of sharks to cross the mind. Gnarly freshwater predator with 250 million years of badass evolution under its belt meets gnarly saltwater predator with an even longer track record – who prevails?

Animal Face-Off-style sensationalism aside, shark-crocodile relations can usually be summarised pretty mundanely: big crocs eat little sharks and big sharks eat little crocs. The estuaries and lagoons of the tropics, after all, find the vulnerable young of both creatures overlapping. The shark pups of mangrove nurseries make easy meals for crocodiles, and hatchling and subadult crocs are appealing morsels for river-cruising sharks.

Still, we do know that Indo-Pacific crocs in Australia's Northern Territory are opportunistic shark-eaters. In 2010, a "saltie" (estimated at some 16 feet long) dispatched a good-sized bull shark in Kakadu National Park. In 2014, the 18-foot croc "Brutus" – whose missing front leg has sometimes been attributed to shark bite – chomped one in the Adelaide River. Full-grown bull sharks, though outsized by many compatriot crocs, are heavy-jawed bruisers of fish, and likely don't fall victim to the reptiles much (except maybe to genuine giants like Brutus). Lemon sharks, on the other hand, appear to avoid the chemical traces of American crocodiles in the water. 

Brutus with a bull shark between its jaws on the Adelaide River in the Kakadu National Park. Photo: Andrew Paice/Getty Images

Adult oceangoing crocs, meantime, are hypothetically at risk from only the biggest sharks. A 14-foot tiger shark caught off the South African coast contained the head and forequarters of a Nile crocodile in its gullet (along with such sundries as a sheep leg and some cans of peas) – though there's no telling whether the shark had killed or simply scavenged the reptile. One large individual hooked in Indonesia had consumed a 6½-foot crocodile.

Tiger sharks – which commonly patrol coastal waterways – could pose a threat to any croc, given their large size, robust serrated teeth (which can saw through a sea-turtle shell like nobody's business), and up-for-anything predatory habits. So could, of course, the more massive great white shark, which grows to similar lengths as Indo-Pacific or Nile crocodiles and can substantially outweigh them. There are records of white sharks preying on adult American crocodiles in the Columbian Caribbean.

Then there's the sizeable decapitated Nile crocodile head found on a South African beach in 2013. Locals who discovered it suspected only a big shark could have made the clean cut (if poachers were responsible for the animal's death, they would presumably have kept the head for its high value).

It's important to note that a crocodile on the high seas is at a disadvantage against a shark, being a far less efficient or energetic swimmer – not to mention that the reptile's ambush-style hunting tactics wouldn't be of much use in this setting.

Just for fun, let's acknowledge here a highly suspect account reported in the October 5, 1877 issue of The Fishing Gazette, which describes essentially an all-out rumble off southeastern Florida between American alligators and an unspecified species of shark. Large numbers of gators allegedly drawn by fish schools were flushed into the ocean by a neap-tide surge. Here, inconveniently, they ran into "hundreds of enormous sharks". A bloody surf skirmish followed, according to a shore-bound witness, who saw "sharks and alligators rise on the crest of the waves and fight like dogs". Mangled carcasses – headless gators, sharks "nearly bitten in two" – washed ashore for miles up the coast, attracting hordes of vultures.

Crocodilians eating sharks, and vice versa, happens all the time, but a considerable size difference between the two is the norm. A huge shark and crocodile gnashing it out? Probably a super-rare occurrence – except, of course, in the fevered imaginations of human beings. "Predators generally don't mess with each other because the risk of a serious injury usually isn't worth it," notes Britton. "But there are always exceptions."

Two recent pieces of footage from Australia suggest what's likely more typical interplay between full-grown crocs and sharks. Earlier this year, we passed along a video from Garig Gunak Barlu National Park, where an Indo-Pacific crocodile hauled a sea turtle offshore, a jackal-like gaggle of scavenging sharks in tow. And in August, a saltie peaceably joined comparably sized lemon sharks circling a boat on a Kimberley river.

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