Red, White and Blue Crabs: These Tree-Climbing, Bird-Killing Crabs Come in Multiple Colors and No One Knows Why

Scientists are trying to solve the mystery of these big crabs' colorful differences. Photo Credit: John Tann Coconut crabs (Birgus latro) are gigantic land-dwelling crabs found on islands throughout the Indo-Pacific. They can live for decades, and can grow to be more than 3 feet wide (legs outstretched) and weigh in at more than 6 pounds. So that name isn't because they're the size of a coconut—it's because they can actually tear open coconuts to eat their tender meat. "If a coconut falls out of a tree, they'll clamp onto it on the top and then drag it back to their husking ground," explained Victoria Morgan, a PhD Candidate in the Department of Evolution and Ecology at University of California, Davis. You can always tell where a crab hides out by the piles of coconut husks lying around. And it just so happens that out these massive, tree-climbing crabs come in multiple colors. They start out white as juveniles, when they act like other hermit crabs and don a protective shell. Then, as they mature and grow, they turn either red or blue. Really, really red, and really, really blue. "It's weird that the colors are so distinctive," Morgan explained. Stark color differences within a species, or color polymorphisms as scientists call them, are found in other crab species, but they're generally in young animals.