13 January 2011

Sexy swords support startled swimming

I’ll give you that a massive sword looks cool...

But is it practical? What if you came up against someone with, I dunno, a longbow and you had to run? Things could be worse, though. At least that sword can be dropped. If you’re Xiphophorus helleri, you’re not so lucky: you’re stuck with your sword.

ResearchBlogging.orgXiphophorus helleri is a good old swordtail, known to anyone who’s ever been in a pet store. Why does this fish have a sword? The answer seems obvious once you realize that only males have the sword:

Chicks dig it.

Consequently, the role of swords in swordtail courting and mate selection has been well studied. But Baumgarter and colleagues are more interested in what the sword is doing all the rest of the time when a male isn’t trying to catch the eye of fa female.

That long sword could impose a cost on the fish. It might make it harder to swim, particularly when time is of the essence. Swordfish, like many other fish, will escape from a sudden stimulus by bending into a C shape that pulls the head away from the source of the stimulus before the animal swims off. I’ve talked a little about C-starts before (here and here).

Baumgarter and company tested the C-start performance of males with a range of sword lengths, and found no significant differences in their behaviour. They also did before and after experiments where they removed the sword from the tails, and again found no differences in the before and after abilities of the fish to perform C-starts.

That the sword doesn’t impact on this particular behaviour doesn’t mean it’s “free,” though. Those long tailed individuals may have to perform those C-starts a lot more, as the authors note that it’s entirely possible those big, showy tails could translate into more predator attacks.

C-starts are explosive and typically executed in high-risk situations – but they’re rare. Steady swimming is the norm, and the researchers note that long swords seem to make steady swimming more tiring, turning the sword into an ongoing energy sink.


Baumgartner A Coleman S, Swanson B. 2011. The Cost of the Sword: Escape Performance in Male Swordtails PLoS ONE 6(1): e15837. DOI: 10.1371/journal.pone.0015837

Sword picture by DJOtaku on Flickr. Fish picture by mineobskuriteter on Flickr. Both used under a Creative Commons license.


AK said...

Studies like this always remind me of the Handicap Principle

Zen Faulkes said...

The handicap principle is lurking the background for these stories.

My recollection is that the evolution for swords seems to be because females prefer large males, but females check the size of males mainly by looking at the bottom of the fish. Swords that are artificially added to the top of the tail, instead of the bottom, aren’t attractive to females.

But laziness prevented me from finding the reference to that study. Sorry.