Among the various videos that have caught the public attention in the past week is a really disturbing one showing a gigantic coconut crab – the world’s largest land invertebrate – eat attack a sleeping seagull-ish sea bird called a booby and eat it live, fighting with four other crabs over the still-kicking carcass. The video brings out our inherent species-ism – we automatically root for the warm-blooded creature over the invertebrate – as well as reminding us that Nature can be really, really mean.
Turns out the video, funded by National Geographic, was made by a Dartmouth biologist, Mark Laidre, as part of research into the effect that predators, and fear of predators, has on species behavior and possibly evolution. This is from the intro to his article in the journal Frontiers in Ecology and the Environment:
From January to March 2016, I undertook a 2-month field expedition to study the behavior and natural history of coconut crabs on the Chagos Archipelago (British Indian Ocean Territory), the Earth’s largest coral atoll and one of the biggest protected areas in the world. In contrast to other sites where coconut crabs’ natural behavior has likely been substantially modified by human activities, no indigenous humans have been present on Chagos for almost half a century. Chagos therefore provides arguably the most pristine existing environment to study the natural behavior of these animals.
In Chagos, the first hint of predation on vertebrates by coconut crabs came when I inventoried a crab’s underground lair on 5 Feb 2016. Deep inside the crab’s burrow was the carcass of a nearly full-grown red-footed booby (Sula sula), one of the largest birds on Chagos. Then on 2 Mar 2016, in the middle of the night, I observed a coconut crab attack and kill an adult red-footed booby. 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 …., Five more coconut crabs came to the site within 20 minutes, likely cueing in on the blood with their neurologically acute olfactory sense … As the booby lay paralyzed, the crabs fought, eventually tearing the bird apart over several hours, carrying it away, and consuming it.
Yuck. The effect on behavior, and thus potentially on evolution?
The pattern I found across these islands was pronounced, suggesting a strong relationship between coconut crabs and nesting birds. On Middle and East islands, there were no ground-nesting birds, nor any eggs on the ground, but in both cases I counted dozens of coconut crabs. By contrast, as I stepped onto West Island, many nesting noddies (Anous spp) took flight (WebFigure 1, top) and continued circling overhead as I undertook my transect along the island’s length. While moving across my transect I did not find a single coconut crab but here, every few strides, I encountered an egg (Figure 2) and had to be careful where I stepped.
Above and beyond the science, Laidre’s video has produced some visceral reaction, such as this comment from Science Alert: “Now we have proof that coconut crabs are way, way worse than we could ever imagine.”
He got support from National Geographic