SINGAPORE - It is a scene straight out of a horror movie: A giant, land-dwelling crustacean climbs a tree in the dead of night, seizes a sleeping bird and breaks its wing. Then, the coconut crab climbs back down to the bird, which has fallen to the ground, snaps its other wing like a twig and eats it alive.

The video of the kill, which has gone viral, had some speculating that the crab species had morphed from scavenger to hunter. It was the first time such predatory behaviour had been observed in the species, fuelling speculation that the transformation could have significant impact on the ecosystems where they live.

But other crab experts say that this behaviour is rare for a coconut crab, and that the individual in the video was likely an opportunistic arthropod which got lucky, and which will probably learn from the experience.

Coconut crabs, also known as robber crabs and by their scientific name Birgus latro - roam forests and climb trees in search of nuts and fruits, such as their favourite - coconuts. But being scavengers, they also feed on anything edible they come across, such as carrion.

They have a keen sense of smell, and can detect odours kilometres away.

Said Professor Peter Ng, programme director of Singapore's Marine Science Research and Development Programme and a renowned crab expert: "What is possible is that this particular crab smelled something on the tree and climbed up to explore, probably expecting a dead bird or something that was rotting.

"Instead, he climbs onto a bird... the crush of his claw is so powerful it breaks the bird's wing. And from then on, he responds like any scavenger does."

Prof Ng, who is also head of the Lee Kong Chian Natural History Museum, said he doubts this is something the crabs do on a regular basis, going by his observations through his research on crabs since the 1990s, including trips to Christmas Island to study coconut crabs.

None of the crab researchers he has spoken to has ever described the coconut crab as a predator either.

"Here you have a crab that got lucky," he said.

The odd behaviour was recorded by Assistant Professor Mark Laidre of Dartmouth College in the United States, while he was studying the giant crabs in the remote Chagos Islands in the Indian Ocean.

He wrote in scientific journal Frontiers In Ecology And The Environment that he had observed the coconut crab attack and kill an adult red-footed booby - a common sea bird - in the middle of the night in March last year.

The booby had been sleeping on a low-lying branch, less than 1m up a tree, when the crab grabbed its wing with its claw, broke the bone and caused the booby to fall to the ground.

The crab then clambered down and grabbed and broke the bird's other wing.

Within 20 minutes, five more coconut crabs had arrived for the feast.

"As the booby lay paralysed, the crabs fought, eventually tearing the bird apart over several hours, carrying it away, and consuming it," Prof Laidre wrote.

The coconut crab is the largest land-based invertebrate, weighing up to 4kg and with a leg span of more than 1m.

These animals have been turning heads since the time of Charles Darwin, the father of evolutionary biology, who described their size as "monstrous", and who observed them tearing off the husk of coconuts, and then cracking the fruit open by hammering it with their heavy claws.

According to a study published by researchers from the Okinawa Churashima Foundation's zoological laboratory last year, the force of the crab's pinch is almost equal to the bite force of an adult lion. This makes it four to five times stronger than a human bite.

In fact, the lab's chief Shin-ichiro Oka was pinched twice, and described the pain he felt as "eternal hell".

The crabs, however, are not known to be aggressive, and predatory attacks, like the one on the red-footed booby, are rare.

Prof Laidre noted in his paper that such attacks could potentially influence boobies' and other bird species' choice of islands, especially where they nest.

He conducted surveys on three small islands in the mouth of the Diego Garcia lagoon, and found that birds were less likely to live on islands where coconut crabs lived, and vice versa.

Prof Ng agrees that the coconut crab can influence bird populations, but not in a substantial way.

He believes that seabirds will continue to favour such isolated islands as they are free from agile predators such as lizards and weasels, which, because of their speed, can wipe out populations.

Coconut crabs move slowly and noisily, so it is near impossible for them to sneak up on a bird, he explained.

"In this case, the bird was a little slower."

While it takes a long time for an entire species to learn a new behaviour, an individual can do so in a much shorter amount of time.

So it is likely that this particular crab might try climbing a tree to get a bird again.

"If it succeeds after one or two times, will it do it repeatedly as a habit? Yes, it could," Prof Ng said.

But he noted that it is unlikely that this crab would or could teach others to do the same, as crabs are unlikely to learn by imitation the way higher mammals such as primates do.

Prof Ng and other researchers in Singapore are currently studying the biology of these crabs, including how they are distributed, and their patterns and colours, which they hope will help in conservation.

These days, this species of crab is believed to be rare, and is protected in most regions, though there is insufficient data to confirm their numbers.

Prof Ng has, however, eaten it once, decades ago. He said it tasted like any other crab but with a slight coconut flavour.

"I will never do it again. Once you work with these crabs, you will know they have character, and to eat them is such a waste," he said.

The Lee Kong Chian Natural History Museum will be holding an exhibition, Christmas Island Red, from Dec 18, on creatures found on the island, including the coconut crab.