Why are some animals asymmetrical?

22 March 2016

Question

People are not perfectly symmetrical (especially internally) and I suppose animals are like that too. But how does evolutionary biology explain how fiddler crabs have one claw very much bigger than the other. If one big claw is "good" wouldn't two big claws be "better"? Are there other species that are so blatantly externally asymmetrical?

Regards
Gary

Answer

We put Gary's questions to animal expert Chris Basu... Chris Basu - It's a good question. Symmetry in an evolutionary sense is really, really old so most of the animal organisms around us are symmetrical, we can see that in us. We're bilaterally symmetrical, so I have two hand and two legs but we see some animals deviate from this. The animals we tend to think of as asymmetrical - the Fiddler Crab, for example - that's one really famous example. It is actually a symmetrical creature. A lot of these so-called asymmetrical animals actually start off being symmetrical at some stage. That might be when they're either a baby or it might be before they were babies when they're an embryo or a larval stage if they're that kind of creature. The Fiddler Crabs when they're larvae actually have two identically sized claws and it's as they grow, it's usually one of them just becomes much, much, much larger.

Chris - Are they righthanded and lefthanded?

Chris Basu - I was just thinking that in my head. I believe it's the left claw but there's a 50/50 chance I could be wrong.

Chris - Or right on the other hand. That's about 100% chance that most of the people listening are not going to know. But why do they have that asymmetry - do we know?

Chris Basu - It's a difficult question; why did this evolve. All we can do is we can look at the function so we can see what they actually do and, as far as we can see, there's two things that the Fiddler Crabs do with their large claw. One is they wave it around and they try to attract a mate, so females generally like to select male Fiddler Crabs with large claws. But we've also, potentially, identified another function which is the large claw actually helps them to dissipate heat so if they don't have this large claw they're more prone to heatstroke.

Chris - As Gary says; why not just have them symmetrical because you could easily predict your heat loss if you had two the same?

Chris Basu - Yes, that's true. It's difficult to say at what point that's happened but it might be something to do with the fact that growing a large claw is actually quite costly; it's expensive. You have to devote lots and lots of resources so having one might be more economical than having two.

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