This is Uca mjoebergi, a colourful crab from south Pacific shores. They're signalling to someone - but to whom?. To their own species? Their own sex? To predators?
I had fun recently giving a talk about fiddler crab signalling at a local nature center. I had seen a decent amount of research on fiddler crabs, but had never had the opportunity to review it and try to pull it into a story before. And while I was doing that, a new paper appeared about fiddler crabs.
While you're preparing talk on something, new research gets published. It's a variation of the law of maximum inconvenience.One of the problems with signalling is that it implies that there is some sort of intended audience. Spending your time and energy waving a lot can be worth it if your intended audience is always nearby.
In this case, this nicely coloured species, Uca mjoebergi, the males' audience is mainly females of the same species. In most of the range of this species, it's the only fiddler crab in the environment. But in some locations, there are a couple of other fiddler crab species. Some of them look distinctly different to us, but fiddler crabs' eyes don't have the same power as us.
Could the males of this species tell the females apart? Or would they be all, like, "Hey baby... hey baby... hey baby..." to any vaguely crab-like shape that was nearby?
The answer was... it depends.
When single females were released into a colony of U. mjoebergi males, the males waved regardless of whether it was a U. mjoebergi female (smooth moves!) or U. signata female (sorry, but you're not really my type).
Femalesof both species were also presented simultaneously, but in a slightly less natural scenario. Instead of females walking of their own volition through the colony, they were tethered so they wouldn't move out of range of a male's burrow. Booksmyth and colleagues also put up barriers so that males wouldn't be able to see other distractions.
Under these conditions, the males would often wave at both females... but they would wave longer and faster to the females of the same species, and were much more likely to try to mate with the female of the right species. But the match wasn't perfect; some males still tried to mate with the wrong species. Interspecies harassment, if you will.
This suggests that the guys can tell if the girl is the right species. So why don't they, especially if waving is costly?
One possibility is that by having the males generally waving regardless of species, a male colony might become almost like a lek or mating "hotspot" that attracts more females of the right species.
Another interesting one is that the females can tell the males apart better then the males recognize females. The females might "reject" inappropriate males so quickly that the cost to the male is minimized.
Finally, the mix of "right species" to "wrong species" might matter. The "right species" was far more prominent in this area. An interesting follow-up would be to repeat these experiments in a place where the "right species" was in the minority, and males might waste a lot more time and energy courting indiscriminately.
Reference
Booksmythe I, Jennions M, & Backwell P. 2011. Male fiddler crabs prefer conspecific females during simultaneous, but not sequential, mate choice. Animal Behaviour: In press. DOI: 10.1016/j.anbehav.2011.01.009
Photo from here.
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