The coconut crab, Birgus latro, is the world’s largest terrestrial arthropod, found on islands across the Indian and Pacific Oceans. With a leg span exceeding three feet and a weight reaching up to nine pounds, this animal is massive. Its size and scavenging habits have led to documented accounts of it interacting with skeletal remains. Coconut crabs do eat bones, a behavior deeply tied to their unique physiology and the demands of their massive land-based existence.
The Coconut Crab’s Unusual Diet
The coconut crab is an omnivore and scavenger, feeding on virtually anything it can find. While its name suggests a diet focused on coconuts, its menu is varied, including fallen fruit, seeds, and the pith of dead trees. They are also aggressive scavengers and predators, consuming carrion, smaller animals like rats, and even other coconut crabs.
This non-discriminatory feeding habit attracts them to any organic matter, including the bodies of dead animals. Their acute sense of smell is highly sensitive to the chemicals released by decaying matter, guiding them to potential meals from a distance. Experiments using pig carcasses have shown that the crabs rapidly dismantle large remains, dragging pieces away into their burrows.
Scavenging involves consuming soft tissue and interacting with the skeleton. Since these crabs devour the molted exoskeletons of other crustaceans, they clearly have a drive to consume hard, mineral-rich material. The consumption of bone is an extension of this broad scavenging behavior.
The Biological Drive for Calcium
The primary reason coconut crabs actively scavenge and process bones is the biological necessity for calcium. As the largest land-dwelling arthropod, the crab requires large quantities of calcium carbonate to maintain its massive exoskeleton. Unlike their aquatic relatives, who absorb calcium directly from seawater, these terrestrial crabs must obtain all their calcium from their diet.
This need is most acute during the molting process, or ecdysis. To replace its old, restrictive shell, the crab must first draw back and store calcium from the old shell before shedding it. Once the old shell is cast off, the new, soft exoskeleton must rapidly harden, a process that demands a huge influx of calcium.
The isolated, tropical islands where these crabs live often have soil and vegetation poor in readily available calcium. Bones, whether from a seabird, a small mammal, or a larger carcass, represent one of the most concentrated sources of calcium phosphate and calcium carbonate available. By consuming these skeletal remains, the crabs gain the raw materials necessary to calcify their new shell.
The adult crab no longer carries a protective shell like its hermit crab ancestors. Instead, it develops a tough, calcified outer skin. This hardened body covering, particularly on the abdomen, requires a continuous dietary supply of calcium to remain strong and protect the crab from predators and dehydration. This physiological demand elevates bones from a mere food source to a resource that directly supports the crab’s survival and ability to grow.
Processing Hard Materials
The ability of the coconut crab to consume bone is made possible by the power of its chelipeds, or claws. These pincers are among the strongest tools in the animal kingdom, capable of generating a crushing force up to 3,300 Newtons (about 740 pounds of force). This force is sufficient to crack open a coconut shell and break apart dense bone material.
A large coconut crab can generate a crushing force up to 90 times its own body weight, a ratio among the highest of any animal. The crab uses this immense strength to fracture the hard, outer bone cortex and access the nutrient-rich marrow and spongy bone structure inside. The claw itself is durable, with an outer layer structured in a laminated pattern of calcified tissue, giving it a hardness comparable to steel.
Once the hard material is broken down, the crab’s digestive system extracts the minerals. The complex calcium compounds found in bone are processed and deposited into the crab’s tissues, supporting the development of its new, hardened shell after molting. Without the mechanical advantage of its powerful claws, the crab could not access this vital mineral resource on its calcium-poor island home.