Coconut Crabs, Birgus latro, are the largest terrestrial Crustacean species, and indeed largest terrestrial Arthropod of any description. They are found across much of the Indian and Pacific Oceans, living on most landmasses in tropical areas. They are descended from Hermit Crabs, and younger Coconut Crabs still use borrowed Snail shells for protection, but the adult Crabs have developed tough calcareous exoskeletons. They get their name from their ability to crack open and consume Coconuts, but will also consume animals, whether as carrion or by hunting, and are sometimes blamed for the disappearance of the early aviator Amelia Earhart.
In a paper published in the journal Frontiers in Ecology and the Environment on 1 November 2017, Mark Laidre of the Department of Biological Sciences at Dartmouth College reports observing Coconut Crabs consuming large Seabirds in the Chagos Archipelago (British Indian Ocean Territory).
Laidre had heard anecdotal evidence of incautious Seabirds landing too close to the burrows of Coconut Crabs and being dragged in, never to re-emerge. This led him to excavating the burrow of a Crab on Diego Garcia Island, in February 2016, where he discovered the remains of a Red-footed Booby, Sula sula, a large, predatory Seabird reaching about 60 cm in height and a metre in wingspan.
A few weeks after this Laidre was able to directly observe an attack on a Red-footed Booby by a Coconut Crab, again on Diego Garcia Island. The Bird was not attacked on the ground, but rather on a low branch of a tree, which the Crab climbed in order to sneak up on its prey. The Booby was seized from behind, breaking one of its wings instantly, then released and allowed to drop to the ground, where it was attacked by the Crab a second time, having its other wing broken before the Crab attempted to drag it away. However this attempt was of limited success, as the Crab was quickly joined by a number of other Crabs, apparently drawn by the scent of blood, who, after a brief confrontation, dismembered the Bird, dragging large pieces of it off to several burrows.
A Coconut Crab (Birgus latro) kills an adult Red-footed Booby (Sula sula) on Chagos Archipelago (British Indian Ocean Territory). Laidre (2017).
Predation, even at low levels, can have distinct impacts upon the behaviour of prey species, and that of Coconut Crabs seems to be no exception. Uninhabited islands are usually important breeding grounds for Seabirds, with large populations of Birds densely packed on any suitable ground. However on islands where Coconut Crabs were present, Laidre only observed nests on the very highest tree branches, suggesting that the Birds had learnt to avoid nest-building in any location that could be reached by the Crabs. However on one of the Chagos Islands, West Island, there are no Coconut Crabs, and where a population of ground-nesting Noddies, Anous spp., have covered much of the island with nests.
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