April 08, 2013

Croc Invasion Facts

  • Estuarine Crocodiles (Crocodylus porosus) are the largest living reptiles on the planet today. Adult males grow an average of five metres long and weigh nearly half a ton.

  • The largest of all male Estuarine Crocodiles reign supreme and as the wet season approaches the game is on to attract females and drive off rival males. They chase, head-slap and growl to threaten other contenders.

  • Head slapping involves the crocodile snapping its jaws shut explosively on the surface of the water to create a loud snapping sound. If need be, males will go into full-blown combat that can fatally harm the other crocodile.

  • Estuarine Crocodiles employ three highly specialised techniques to capture their prey – the ambush attack, superior night senses and the death roll.

  • In the ambush attack, they wait submerged, with only their eyes, nostrils and ears above water to sense approaching prey, then launch explosively onto the unsuspecting victim.

  • Estuarine Crocodiles are in their element in the dark, with superior night vision as well as pressure receptors for detecting the slightest vibrations.

  • Once the crocodiles have a hold on their prey, if it is large, they will execute the death roll, holding on with their jaws and using an explosive roll with the tail and body.

  • Other crocodiles can live in salt water but the Estuarine Crocodile exploits this niche best. It has salt glands on its tongue that excrete unwanted salt. So not only can they live in freshwater, they have even been found in water twice as salty as seawater.

  • We are only just finding out about Estuarine Crocodiles’ ability to make amazing ocean journeys. Ships far out at sea have sighted them.

  • They may not travel fast when ocean swimming (no more than two kilometres per hour) but tracking studies find them travelling up to 100 kilometres from creeks and rivers.

  • Crocodiles depend on outside sources of heat like the sun, or warm river banks, to keep their body temperatures at their peak. They have bones called osteoderms beneath the skin of their backs with many blood vessels to help make the most of the sun’s warmth. Once they have fed, they seek out hot places to bask so they can digest more quickly.

  • Extraordinary events occur on Crab Island in the Gulf of Carpentaria in the far north of Australia. Three large species come together in ways rarely seen before – two large predators converge on a feeding event.

  • Sea turtles like Flatback Turtles (Natator depressus) use ‘magnetic maps’ to relocate the breeding grounds on which they hatched. Recent studies show Crocodiles also use magnetic sense like other true navigators, and they also head towards the turtle breeding grounds.

  • Crab Island is in an area of huge tidal range – varying three metres. These tides create strong currents and along with the many tidal rivers entering the sea here, water turbulence is high.

  • Unlike other sea turtles, Flatback Turtles nest during the day and can withstand extremely high temperatures of up to 52 degrees celcius.

  • Flatback Turtles lay 54 eggs per clutch – a relatively small number compared to other sea turtles, and their eggs are much larger than other sea turtles in relation to their body size.

  • The turtle hatchlings only emerge at night – they take a few days to dig their way through the sand to the surface waiting for the sand to cool as a cue to emerge.

  • Once at the surface the darkness of the night offers little protection for the hatchlings from the array of predators awaiting them. On Crab Island they face an onslaught from Night Herons, Ghost Crabs, Stone Curlews, Pelicans, Storks and the ultimate predator of all, Estuarine Crocodiles.

  • Even if a Flatback hatchling does manage to reach the sea, the chances it will survive to adulthood are around one in six hundred.

  • Crab Island is one of the only rookeries in the world where so many crocodiles prey on turtle hatchlings. The crocodiles not only prey on the small hatchlings, if their jaws are large enough, they will also hunt and kill adult turtles that come to the island to lay eggs. Another apex predator, the Lemon Shark will also take both hatchlings and adult turtles in the shallows of the island.

  • Not only does it appear that the crocodiles are travelling long ocean distances to get to the island, they time their visit to coincide with the peak in turtle hatchings.

  • Usually Estuarine Crocodiles are highly territorial, not allowing any other crocodiles to come within their reach – unless for mating. But on Crab Island it seems such rules don’t exist. Here they are found in groups, hunting side-by-side. In terms of crocodile behaviour, it seems we have much more to learn.

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