Uca panacea, a fiddler crab, whose specific name refers to a goddess of healing.
When I gave a public talk earlier this year about fiddler crabs, I talked at length about the role of the large claw in signalling. Signals from males to females, signals from males to other males, deceptive signals, and more.
A new paper suggests we might have been overlooking another use of this large claw: control its temperature.
Crabs are ectotherms: they rely on using the environment around them to change their body temperature. Fiddler crabs sometimes come up to the entrance of their burrow first thing in the morning, and hold up their big claw, which hints at the claw acting as a sun gatherer.
Darnell and Munguia tested whether the claw affects the crabs ability to thermoregulate by heating them up with incandescent light bulbs and measuring their body temperature. They had four groups:
- Intact males with one large claw and one small claw
- Males with the small claw removed, which controls for effects of claw removal
- Males with the large claw removed
- Females, which have two small claws
If the large claw is acting as an aid to controlling the crabs’ temperature, you’d expect to see groups 1 and 2 faring much better against the heat than 3 and 4.
And that’s what you find. Everyone heats up, but the males with their large claw are doing so more slowly.
You do have to dig for this finding, though. Considering that this is the main point of the paper, it’s a bit odd to find this key graph buried as Figure 5B.
Fiddler crabs can also change their colour to some degree. The crabs tended to get lighter on their backs when placed under the bulbs. This makes sense, as this would reflect more light. This was also affected by the treatment, with the males who had lost their claws becoming the lightest.
And that’s a problem. There’s no control here for carapace colour, which means that the claw removal and colour are intermingled. The authors say:
This difference in shade, however, does not explain the observed differences in heating rate, as males with the major claw removed had the greatest heating rate, and not a lower heating rate as predicted for a light-colored crab.
I would have suggested painting the carapaces to prevent colour changes from altering the crab’s reflectance. Then, there would be no doubt.
It may be that the claw is just a passive radiator, though with all that claw waving fiddlers normally do, you might expect behaviour to be involved. The authors note that the crabs didn’t wave in this experiment, possibly because they were in the room with the crabs. They think that if the crabs could wave, the effect would be even bigger than recorded here.
I also would have been interested to see if the behaviours changed when the crabs got cold. Thermoregulation cuts both ways, after all. But this paper is very much a starting point for new directions in research on crabs that have already taught us much.
Darnell MZ, Munguia P. 2011. Thermoregulation as an alternate function of the sexually dimorphic fiddler crab claw. The American Naturalist: in press. DOI: 10.1086/661239
Photo from here.