The coconut crab that got lucky

A coconut crab climbing a tree on Christmas Island.
A coconut crab climbing a tree on Christmas Island.PHOTO: PETER NG
A coconut crab on Christmas Island.
A coconut crab on Christmas Island.PHOTO: PETER NG
Crab researcher, Dr Yoshihisa Fujita, holding a coconut crab during an expedition to Christmas Island in 2010.
Crab researcher, Dr Yoshihisa Fujita, holding a coconut crab during an expedition to Christmas Island in 2010.PHOTO: PETER NG
A coconut crab, also known as a robber crab, tearing open a coconut on Christmas Island.
A coconut crab, also known as a robber crab, tearing open a coconut on Christmas Island.PHOTO: PETER NG

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.

  • FACTS ON THE COCONUT CRAB

  • - The largest land-based invertebrate, and the largest hermit crab on the planet (though it has no need for a shell to retreat into). Following a brief larval stage in the ocean, these crabs spend the rest of their lives on land.

    - Can weigh up to 4kg, and have a leg span of more than 1m.

    - Can live more than 60 years.

    - Found on islands across the Indian and Central Pacific oceans.

    - Likes to eat coconut and is known to tear the husk off with its claws and hammer the fruit till an opening is made. Sometimes, it climbs trees while carrying a coconut and throws it to the ground to break it.

    - It is tasty: Because of the crab's diet, those who have eaten the crustacean say it has hints of coconut. Considered a delicacy in some circles, its numbers are believed to have declined significantly.

    - Its pincers generate up to an estimated 740 pound-force (3291.684 Newton) — a force about 90 times its own body weight. The biggest crabs exert a claw force that could nearly rival a lion’s bite. 

    - Some people believe coconut crabs ate American aviation pioneer Amelia Earhart’s remains after she crash-landed on Nikumaroro island in the Western Pacific Ocean.

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.


Q&A with Assistant Professor Mark Laidre of Dartmouth College

How surprising is it for a known scavenger to be seen actively hunting, like in the case of the coconut crab?

Pretty surprising! Active hunting requires skill, and for an invertebrate to take down such a large bird is rather impressive.

Is it common for animals to change their behaviour? Is it something that takes a long time to happen?

We often don't have deep knowledge about animals in different locations, and their behaviour may vary substantially between geographical locations. So it is important to study them thoroughly in many places.

What could cause such a change in behaviour?

Opportunity and hunger. If the prey animal was within reach and the potential predator was hungry, then this is the right combination.

What implications can such changes in behaviour have on the larger ecosystem?

The behavior might change the composition of the community and also result in more nutrients being deposited in the soil (after the crab drags the bird carcass into its burrow).