And now, finally, we have video evidence that the crabs — thousands strong on one island — can scale trees and hunt full-grown birds in their nests.

“It would at first be thought quite impossible for a crab to open a strong cocoa-nut,” Charles Darwin once wrote, as that father of evolutionary biology recounted stories of a “monstrous” arthropod said to roam an island in the Indian Ocean.

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“The crab begins by tearing the husk, fibre by fibre, and always from that end under which the three eye-holes are situated,” Darwin wrote. “When this is completed, the crab commences hammering with its heavy claws on one of the eye-holes till an opening is made.”

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But Darwin would go no further than that. The genius who championed life’s endless forms gave no credence to reports that these fierce giant crabs could also climb trees.

In the decades to come, coconut crabs would be photographed not only climbing trees but hanging from them like enormous hard-shell spiders. Researchers in our own century once left them a small pig carcass to see what would happen, Smithsonian Magazine wrote.

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The crabs quickly disappeared the pig.

Now we know they are the largest invertebrate to walk the earth — more than three feet long, pincer to pincer, with claws so strong that a researcher once tried to measure the force, and described it as “eternal hell” after a coconut crab caught his hand.

But what, wondered Mark Laidre, do they eat?

“Few studies of this remarkable animal’s behavior have been undertaken since Darwin’s Beagle voyage,” the Dartmouth College biologist wrote in a paper published this month.

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That they ate coconuts had been established long ago, of course. And like other crabs, Laidre wrote, they were known to scavenge off corpses (leaving aside whether Amelia Earhart was one of them.)

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But what else did they eat? Laidre was fascinated by a story from the Chagos Archipelago in the middle of the Indian Ocean told to him by a witness in 2014.

“An adult red-footed booby had landed near the entrance to a coconut crab’s burrow,” Laidre recounted. “As the bird stood there, the crab slowly emerged from its underground lair, approaching the bird from behind. The crab then grabbed the bird by one leg and dragged it, struggling, back into its burrow.”

The witness never saw the bird again.

In all his years of research, Laidre wrote, he had never seen a crab prey on any animal besides — rarely — another crab. So two years after the reported disappearance of the booby, the biologist set off for the Chagos to find out if giant crabs really stalked birds.

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The archipelago’s largest island is ring-shaped, and three smaller uninhabited islands sit in its mouth. Laidre carefully surveyed each one.

Dozens of birds took flight the moment he stepped onto one of the smaller islands, he wrote, and “continued circling overhead as I undertook my transect along the island’s length.”

Nests and eggs covered the rocky beach of this island — and there was not a single giant crab in sight.

But on the other two small islands, Laidre wrote, he saw dozens of coconut crabs and few nests. He wrote in his paper of an evolutionary theory called “landscapes of fear” — that few animals will dare make homes in places dominated by predators.

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When he surveyed the fourth island, Laidre wondered if the crabs simply ruled it.

“I counted over 1,000 coconut crabs in single [9-mile] transects but did not observe even one ground-nesting bird,” Laidre wrote. All the nests were high up in the trees, and cracked coconuts littered the ground.

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After about a month on the island, in February of 2016, he investigated a giant crab’s underground lair.

“Deep inside the crab’s burrow was the carcass of a nearly full-grown red-footed booby,” he wrote.

This was Laidre’s first sign that the stories might be true, that giant crabs really were hunting birds.

He had his proof a month later.

“In the middle of the night,” Laidre wrote, “I observed a coconut crab attack and kill an adult red-footed booby.”

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“The booby had been sleeping on a low-lying branch, less than a meter up the tree,” he wrote. “The crab slowly climbed up.”

He watched the crab take the bird’s wing in its great claws. He watched it break the bones beneath the feathers.

The bird fell to the ground, Laidre wrote, and the crab descended in pursuit.

About 90 seconds of what happened next is now documented in the researcher’s video, if you care to watch.

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The booby pecks twice at the crab, and might as well have hit hardwood. Futile.

It makes a sort of squawking, croaking sound, over and over, at least a dozen times. Then its head rolls back and the bird simply breathes, unprotesting, as the crab’s claws sink into its down.

“Five more coconut crabs came to the site within 20 minutes, likely cueing in on the blood,” Laidre wrote. They tore it to pieces and took it away, and now yet one more thing is known about the giant coconut crabs of the Indian and Pacific oceans.

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That the crabs cannot swim well, Laidre wrote, may be why one of the four islands still belonged to the birds.

But the biologist wondered what would happen if just one crab were taken there, in the interests of science.

A juvenile would probably be overwhelmed and eaten by birds, he wrote. But “an adult crab may wreak havoc.”

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“Further research could experimentally test these ideas, although important ethical considerations would obviously arise,” Laidre wrote. “The birds would need to be protected.”