Tuesday, February 9, 2010

Estuarine Crocodile: An Ambush Predator

Last month, Peiting of The Simplicities in Life wrote about an estuarine crocodile (Crocodylus porosus) stalking a common sandpiper (Actitis hypoleucos). Since the crocodile did not end up with the bird in its jaws, it led to some speculation that it could have been just on its way up to its favourite busking spot.

Today at the mangrove reserve, two estuarine crocodiles were present and a similar stalking event occurred. One of the subadult crocodiles glided stealthily from the west bank of Sungei Buloh Bersah and approached a common sandpiper on the opposite bank.

Here, it can be seen how having its eyes and nostrils positioned on top of the head allows the crocodile to see above the water and breathe while keeping the rest of its body hidden. Since the species is able to hold its breath comfortably for up to 5 minutes and if forced, stay submerged for more than an hour, it is likely that this exercise which happened within 5 minutes could have been solely to watch the sandpiper.

The reptile then swam past where the sandpiper stood and positioned itself where the bird was heading - in other words, was leading the bird. In an ambush set up, what the crocodile is doing is essentially predicting where its prey will be and positioning itself in the predicted path of its prey.

It seemed that the crocodile's prediction was right and the sandpiper passed right in front of the crocodile. A few moments passed. Both animals appeared to hesitate and that gave the sandpiper an opening to take off, landing on the fallen tree behind.

Despite the fact that no bird ended up in the jaws of this crocodile, this event strengthens the view that the previous record was probably not born out of coincidence and sheds some light on the hunting technique employed by one of our top mangrove predators. As for the crocodile, after the sandpiper's timely flight, it turned itself around to face the opposite end of the river bank.

Though a small bird like the sandpiper may seem like an insignificant meal for a reptile about 1.6 m long, for a "cold-blooded" animal which does not require to burn energy to maintain a constant body temperature, a small bird meal can probably go a long way.

Also, bird-eating is in agreement with studies done on the diet of the species where smaller crocodiles fed mainly on insects, crabs and shrimp then graduate to larger vertebrate prey such as fish, birds and mammals.

After all, it would be a waste for an animal to have evolved one of the strongest bites over the millions of years of its existence if it was hunting only shrimps, beetles and halfbeaks.

Further reading:
  1. Taylor, J. A. 1979. The foods and feeding habits of subadult Crocodylus porosus Schneider in Northern Australia. Australian Wildlife Research 6(3): 347-359.