Little is known about the world’s largest terrestrial invertebrate, the coconut crab, which grows to what Charles Darwin described as “a monstrous size.”
As their numbers dwindle, coconut crabs need to be better understood, says Mark Laidre, National Geographic Explorer and an assistant professor of biological sciences at Dartmouth College. With a research grant from the National Geographic Society, Laidre has been studying the crab’s behavior and natural history in the Chagos Archipelago in the Indian Ocean, the Earth’s largest coral atoll, which boasts an undisturbed population of coconut crabs.
During Laidre’s two-month field expedition, he studied how the crabs open their preferred source of food: coconuts. Using a custom-built tool to measure their grip, Laidre found coconut crabs can produce up to 1,500 newtons of force, far more than any other member of the animal kingdom.
In addition to coconuts, Laidre observed the coconut crabs preying on other species including hermit crabs, rats, and most shockingly, live birds. In a paper published online today in Frontiers in Ecology and the Environment, Laidre describes how he watched and captured footage of a coconut crab attacking and killing a red-footed booby in the middle of the night, behavior that has rarely been documented before.
The booby had been sleeping on a low-lying branch, less than a meter up the tree. The crab slowly climbed up and grabbed the booby’s wing with its claw, breaking the bone and causing the booby to fall to the ground, where it was unable to fly. The crab then approached the bird, grabbing and breaking its other wing. The booby struggled and pecked at the crab, but the crab retained its grip with both claws, kicking at the bird with its other legs. Five more coconut crabs came to the site within 20 minutes, likely having smelled the blood with their acute olfactory sense. The attacker dragged the booby several meters away, and then released its grip. As the booby lay paralyzed, the crabs fought, tearing the bird apart over several hours, carrying it away, and consuming it.These attacks may have an important ecological impact by stimulating fear in prey, potentially influencing boobies’ and other bird species’ choice of islands, especially where they nest. This research raises an intriguing hypothesis for future testing: coconut crabs could act as “ruler of the atoll” for terrestrial communities by inducing fear, particularly in vulnerable, ground-nesting species.
Read more about the amazing discovery in this National Geographic story. Want to become a National Geographic Explorer? Learn how you can apply for a grant from the National Geographic Society here. You can support National Geographic’s efforts to enable more cutting-edge scientists, conservationists, and educators like these to get out into the field here.