Mark Laidre: Discovering the Coconut Crab

Caption: Mark Laidre giving his talk on his research in coconut crab behavior. (Credit: Sam Reed)

Caption: Mark Laidre giving his talk on his research in coconut crab behavior. (Credit: Sam Reed)

Coconut crabs, the largest terrestrial crab, have been studied very little because they are perceived as food and predated into scarcity. In response to the resulting lapse in knowledge, Mark Laidre, a Neukom Fellow at Dartmouth, traveled to the Chagos Islands to study the behavior and ecological significance of these crabs.

Coconut crabs get their name from their ability to open coconuts. Even small coconut crabs can husk a coconut, and, through a laborious process that takes up to twelve hours, reduce it to a nut. The crab can then pound one of the nut’s three eyeholes in with a leg, stick a claw in, and split the coconut.

Juvenile coconut crabs live their lives by means very similar to hermit crabs. As they grow, both of these crabs move between shells, which protect their abdomens. However, eventually coconut crabs grow so large that they cannot easily find a shell capable of housing them. Instead, adult coconut crabs abandon shells and recalcify their abdomens for protection – that is, they regrow layers of calcium-based tissues.

Laidre found that, as a result of their competitive internal relations, coconut crabs are very antisocial. In one experiment, he tethered adult coconut crabs to a quadrant and counted the number of new adults who entered for the purpose of interacting with the tethered crabs. Very few crabs, if any, joined their tethered brethren.

The burrows inhabited by the coconut crabs were often home to only one crab – almost exclusively a large, adult male.  Laidre found that the smaller males who attempted to take shelter in these burrows – a useful measure of staying moist – would often be cannibalized.

Laidre believes that this kind of competition within the male gender is what brought about the coconut crab’s ability to open coconuts. He states that the coconut crab’s ancestors originally would have been unable to open coconuts, as this is a skill fairly unique to modern day coconut crabs. However, over time, sexual selection pressured the crabs to utilize their claws for greater consumption. The result of this was longer claws, which can produce a force great enough to crack open coconuts.

To measure this force, Laidre’s colleagues created a pronged apparatus, which could be squeezed by the crabs and generate a measurement for the force of their grip. He found that the coconut crab could produce 1000 newtons to 1500 newtons of force with its claw – far more than any other member of the animal kingdom.

Lastly, Laidre analyzed theft behavior in coconut crabs. In the past, coconut crabs have been known to steal a number of objects from people visiting their habitats. Laidre hypothesized that the crabs were stealing them because they were able to pick up on trace scents, such as from old food particles, on the objects. To test this, he baited the crabs with objects that had been worn by the ocean, which washed away unique scents. The result was that the crabs didn’t touch the bait, showing that the crabs are only interested in food particles.

In the future, Laidre plans to return to the Chagos Islands to continue his research. Notably, he seeks to determine how the difference in housing between hermit crabs and coconut crabs affects their behavior and social interactions. Additionally, Laidre hopes to bring coconut crabs back to Dartmouth, so that their behavior can be studied by undergraduates.

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