Doing the wave, crab style

Editor's Selection IconWhy do animals of a particular species living in different regions show different behaviours? One possibility is that animals have a big behavioural repertoire, and that they tweak their behaviour to suit the particular conditions they find themselves in. Alternately, animals might be somewhat more stereotyped in their behaviour, and different populations have been selected to perform different behaviours over evolutionary time.

This new paper by Zayasu and Wada looks at this question in a little crab, Ilyoplax pusilla. I’d never heard of this species before this paper (not surprising, because they are an Asian species), but now I’d like to see them. They do a behaviour called waving with their claws, shown at right. I think it looks cute.

The function of the wave is not 100% clear. The authors imply that only the males wave, and that it can be either a sexual display for females or a threat display towards other males.

Some researchers had noticed that in one region, the Yakugachi River, the crabs waved more than the other populations. The river region there is described as somewhat gravelly area, near mangroves.

To answer whether there was something about this one region that led the crabs to wave more often, and whether the crabs could adjust their displays, they simply carted a bunch of crabs from Yakugachi River to Uchinoura (a bare, muddy region) and vice versa. If there was something about the environment the crabs were responding to in real time, you would expect all crabs, both natives and imports, to do the same amount of waving.

The crabs were all housed individually at these sites in small boxes, roughly one metre apart, and video recorded at set times for almost a week. Because there are regions where these crabs live, they could also measured how many non-captive crabs were around their subjects.

The bottom line is that the crabs kept right on doing what they were doing, regardless of the new environment. The Yakugachi River crabs kept waving way more than the others. The difference in frequency didn’t seem to affect how many crabs were around at all.

This difference in waving behaviour could be the result of natural selection. If so, the question becomes what advantage does waving provide to the crab?

Zayasu and Wada also raise the possibility that, because these were adult animals, they could have learned what the typical waving rate when they were young, and are now stuck with the waving of the crab society they grew up in, much like human accents. If that were so, you might expect the “norm” to drift from year to year, but the authors imply that this is not the case. It’s hard to judge how plausible such a learning hypothesis might be without knowing more about the natural history of the species.

Reference

Zayasu, Y., & Wada, K. (2009). A translocation experiment explains regional differences in the waving display of the intertidal brachyuran crab Ilyoplax pusilla Journal of Ethology DOI: 10.1007/s10164-009-0177-5