The mighty pinch of the coconut crab

December 1, 2016 • 9:30 am

The coconut crab, or “robber crab” (Birgus latro), is the largest terrestrial arthropod in the world, with individuals weighing up to four kg (about 9 pounds).  They have a wide range:


But because of their size and the fact that they’re tasty, they’ve been largely driven extinct by humans on populated islands. Here’s how big they are:


They come in different colors, often red or blue. Here’s a beautiful blue one:


Two facts about these species. First, they’re fairly close relatives of the hermit crab, and in fact are descended from hermit crabs that occupied shells throughout their lives. But this species is sufficiently large as to have few predators besides humans, and so only the very young and small coconut crabs live in shells (sometimes bits of coconut shells). They’re also completely terrestrial, and will die if put in salt water.

You can see the hermit-crab ancestry in the reduced and curled abdomen behind the carapace:

Hermit crab without shell:


Coconut crab:


I’ve written two earlier posts on coconut crabs (here and here), including photos by reader Dennis Hansen from the Indian Ocean of Aldabra. There they invade the field station, making off with leftover food from the table, and they can also open garbage cans, as they have great climbing skills:


They’re HUGE! Second Fun Coconut Crab fact: they do indeed open coconuts, and can climb trees to get them, though they’re omnivores and coconuts are not a main part of their diet. Wikipedia gives the details (my emphasis):

The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccanus) and seeds (Annona reticulata), and the pith of fallen trees. However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals. They have been observed to prey upon crabs like Gecarcoidea natalis and Discoplax hirtipes, as well as scavenge on the carcasses of other coconut crabs. During a tagging experiment, one coconut crab was observed killing and eating a Polynesian Rat (Rattus exulans). Coconut crabs may be responsible for the disappearance of Amelia Earhart’s remains, consuming them after her death and hoarding her skeletal remnants in their burrows.

The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the coconut meat inside. They often descend from the trees by falling, and can survive a fall of at least 4.5 metres (15 ft) unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.

Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the nuts. In the 1980s, Holger Rumpff was able to confirm Streets’s report, observing and studying how they open coconuts in the wild. The animal has developed a special technique to do so: if the coconut is still covered with husk, it will use its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the coconut crab will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.

Here’s one at work; remember that it can take them several days to do this:

Clearly, these animals must be strong. Indeed, they can lift up to 28 kg (62 pounds)—at least seven times their own weight.

This is all leading up to a new paper published in PLOS ONE by Shin-Ichiro Oka et al. (reference and free download below), in which the authors measured the pinching force of 29 coconut crabs. As you might expect, they pinch hard, exerting more force per gram of body weight than nearly any other animal, including terrestrial carnivores.  The authors used these devices in this way (caption from the paper):

Fig 1. Measurement of the pinching force and claw morphology in the coconut crabs. (a) The force was measured with the SK-MBF-01F device (SkyScience Co. Tokyo, Japan) and related sensors and (b) demonstration of the method by which pinching force was measured. (c) Claw measurements of the coconut crab used in this study. The placement of the sensor used for pressure measurement is highlighted in green. The measurements used for claw length (CL), claw height (CH), and claw width (CW) are also indicated. L1: in-lever length from the fulcrum to the apodemes insertion; LBAE: out-lever length from the fulcrum to the tubercle (the contact point with the device sensor).

And the results? Here’s what the paper says:

The pinching force exerted by coconut crabs was extremely strong. Maximum pinching force ranged from 29.4 to 1765.2 N. The scaling factor in the allometric equation for pinching force and BW was 0.82. This value was significantly greater than the predicted isometric scaling of pinching force (proportional to muscle cross-sectional area) against BW (a = 0.67). According to a previous study, the reported maximum BW of the coconut crab is 4 kg. Applying our allometric scaling equation, the pinching force of the coconut crab of 4 kg BW was estimated to be 3300 N. This force greatly exceeds that in all other crustacean species that have been reported, as well as the bite force for the majority of modern terrestrial predators, other than alligators.

Here, from the paper, is a log/log graph showing the amount of force exerted by different groups as a function of their body weight. Of course larger animals in a group are capable of biting or pinching more strongly, but the coconut crab, judging by the amount of force exerted per kg of body weight, is the king, with only a few other crabs being relatively stronger:

Fig 3. Regression analysis of the maximum force per unit body weight vs. body mass across several animal groups, including coconut crabs. The shaded gray area represents the range of the maximum force exerted by various animal activities (running, jumping, pushing, pulling, swimming, flight, nipping, and biting) [11]. Colored lines were calculated based on the relationship between the closing forces of crustacean chelae, vertebrate jaws and body masses determined previously [1, 2].
Why did they evolve such strength and size? The authors note that these large claws not only help them procure food, including rats as well as coconuts—drupes (not nuts) native to many of their habitats—and also to drive off predators and competitors.

Because the crabs are relatively small compared to lions and alligators, I don’t think they can pinch your finger off, and I haven’t seen reports of severe wounds in humans produced by these crabs. (But they can crush plastic pens, as you can see here.) Nevertheless, if you’re lucky to come across one, you’d best handle it carefully, as the guy is doing in the photo above.


Reference:  Si, O., T. Tomita, K. Miyamoto. 2016. A Mighty Claw: Pinching Force of the Coconut Crab, the Largest Terrestrial Crustacean. PLOS ONE 11(11): e0166108. doi: 10.1371/journal.pone.0166108

33 thoughts on “The mighty pinch of the coconut crab

  1. Mammals tend to have larger brains and can make tools to compensate for lack of strength/weight.

    It would not surprise me if super intelligent aliens exist, they would be soft cuddly mice, easily crushed in the jaws of cat, but with a brain capable of anticipating any such episodes and with technologies that could subdue potential threats.

    1. Yes, but the suggestion is that they drag a de-husked coconut up a tree and drop it to break it. That might hint at smartness.

        1. Or a spot on the beach. Any crab that approaches me on a beach earns my spot as I run screaming like a baby. I have severe crabaphobia.

    2. “Mammals tend to have larger brains and can make tools to compensate for lack of strength/weight.”

      But that’s hardly a property of mammals in general. The most diverse mammal Orders are rodents and bats, neither noted for their braininess. And how many mammals can make and use tools? Humans, chimpanzees, capuchin monkeys and just maybe one or two others…not many out of roughly 5500 living species. And as for needing to compensate for lack of strength/weight, it’s worth noting that the largest and heaviest living animals on both land and in the sea are, you guessed it…mammals.

    3. Mammals tend to have their soft parts on the outside, which I’m guessing puts an upper limit on grip strength. Arthropods have the advantage here with their hard parts on the outside.

  2. Coconut Crabs are quite wonderful and fascinating creatures, I’ve never seen one in the flesh so to speak but have been reading about them and looking a pictures and videos of them for years. I read somewhere the poor Ms. Amelia Earhart might have been eaten by Coconut Crabs after she died or became to weak to fend them off. Gruesome story, hopefully BS.

  3. Why did they evolve such strength…

    Or how? Is it merely the size, or does it have to do with the way the claws are articulated or in the way the muscles are attached to afford greater leverage? Or do their actin/myosin fibers (and/or associated apparatus) carry some modifications in protein sequence that afford greater binding properties. (If I had a better grasp on / mamory of how that all works I could have written the last sentence better.)

  4. Shouldn’t coconut crabs be a sworn enemy of this (definitely not a) blog since they are known to be kitten-eaters?

  5. Only a matter of time before they start appearing in places like Florida, escapees from the exotic pet trade.
    That will be interesting!

  6. The question I always have on these types of studies, and I’ll admit I’ve never taken the time to look into it further, is how they decide where to put the force sensor, and how well this correlates to how the limb/jaw is used by the animal in life. Like in this case, it looks like they put the sensor as close to the fulcrum of the claw as they could to measure the pinching force. But watching that video of the crab trying to open the coconut, it could only apply force from the very tip of the claw. That’s a whole lot longer lever arm, and consequently a whole lot less force. I know I’ve watched documentaries on ‘science’ channels (increasingly less and less science oriented), where they measured the bite force of animals with a pressure plate out at the tips of their jaws, rather than back towards the root of the jaw, skewing the measurement lower.

    So, is the force they measured for these crabs really representative of a force they apply to objects naturally?

  7. “the largest terrestrial arthropod” — leaving open the possibility of even larger extraterrestrial arthropods.

    But I’m confused: if these crabs are completely terrestrial, and die in salt water, why does the map show their range encompassing vast expanses of open water, and largely avoiding land?

    Regarding omnivory, I saw a bear handler on Colbert last week who said something like “90% of their diet is omnivorous”, which left me wondering what the other 10% could possibly be.

    1. Per what Wikipedia says about that range map:
      “Coconut crabs live on most coasts in the blue area; red points are primary and yellow points secondary places of settlement”

      Additionally, while the adults live on land, the females release the larvae into the ocean, where they live in the pelagic zone for about a month. When they get a little bigger, they live on the shoreline like other hermit crabs for a while (Wikipedia didn’t say how long) before becoming fully terrestrial.

  8. They are pretty scary looking. I can imagine one coming into my tent at night in search of some fresh protean.
    The first reference for pinch strength that came to my mind was the snapping turtle. I’ve watched them making mince meat out of unminced meat. Looking at the chart, I see reptiles are rather low on the scale.

  9. When I was in Saipan about 15 years ago one of the locals asked me about coconut crabs in Australia (yes, I said, they can be found along much of the northern coast). I asked him why he was asking me about the crabs and he said they’re getting rare on Saipan. (Incidentally, the native fruit bat was also a favorite and typically stewed in coconut milk – but even back then it had already been declared a protected species and catching or eating the bats was forbidden by law.)

    1. I know people eat anything remotely edible if you look hard enough, but a *bat*? Wouldn’t that be like eating a mouse with extra membranes? (I know the Inuit don’t normally eat lemmings and marmots and that sort of thing because they aren’t worth the effort …)

  10. So how do these crabs figure out coconuts are edible, and how do they manage to stick with opening them for days?

    [But arthropods pack serious functionality in small packages. The videos of bumblebees pulling small carts to get at food from underneath tight glass enclosures always makes me laugh.

    Sure, its just 10 % or so that figure out how to do it. But some do!]

  11. There are relatively small turtles in a stream nearby that will just as gleefully relieve you of any fingers or toes that get too close. I’m not sure if they’re as threatening as an angry crab out to pillage my coconuts.

  12. I’ve seen these on nature shows and remember Morimoto cooking one on Iron Chef years ago (bad Morimoto!)

    I didn’t realize that they were closely related to hermit crabs, but looking at the comparison here, I can see the resemblance. I think some hermit crab species are also semi-arboreal.

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