11 October 2010

Should sprinting shape scorpion’s stingers?

ResearchBlogging.orgWe may think of scorpions as all bad ass, but scorpions still have to be careful. They have a painful sting, but some animals have evolved immunity to that. Even if they can drive off a predator with a sting, a scorpion close enough to sting its attacker is close enough to be damaged by its attacker.

In many cases, the best bet for a scorpion is to run away.

Temperature could play a big part whether scorpions get away from an attacker. Scorpions are ectotherms, so their performance is profoundly shaped by the external temperature. Daily temperatures can vary quite widely where scorpions live, particularly in desert regions.

Carlson and Rowe took a look at how temperature and drying affected bark scorpions (Centruroides vittatus). They didn’t have real predators in the lab, but instead tested how those two factors affected scorpion’s running and stinging abilities.

Cooled animals ran slower, which was not surprising. What was surprising was that scorpions that were partly dried out ran faster than normal controls. The authors speculate that this is is because animals that have lost water are lighter. It suggests that the physiological tolerance of these scorpions to water loss is fairly high, because you would expect at some point that water loss would be bad.

Tangentially, Carlson and Rowe rated the running speed of their animals, and concluded that males ran faster than females, who ran faster than juveniles. Unfortunately, this comparison isn’t very informative, because they didn’t measure the size of the animals. The authors mention that many of the females were gravid, and imply that mass could be factor. And it’s easy to imagine body size would also be a factor: a bigger body means longer legs, which means a longer stride length. All animals ran the same distance (half a meter), rather than some distance relative to their body size.

One hypothesis floated in the discussion is that the sprinting results might explain a difference in the shape of the stingers between males and females. The females are slower when reproducing. Running is not a good option for them compared to males. They authors suggest that the longer, thinner stinger of the males not be as good for delivering repeated stings as the thicker stinger brandished by females.

This hypothesis is somewhat deflated, because they found no significant differences between males and females in stinging behaviour! The authors do make the claim that some of the comparisons between males and females are “almost significant.” Sorry, but I’m hard-assed about this: you either meet the criterion, or you do not. It’s possible that a more detailed analysis, with a different threat stimuli, might reveal some differences.

Chilly scorpions took longer and stung less often than those at room temperature and up. Stinging was not affected by temperature as much as running was. These particular scorpions are not all that aggro: the authors had more success in getting them to run away.

The authors did not test whether scorpions’ stinging behaviour was affected by drying them out. This is an odd omission, given that the title of this paper promising an examination of both temperature and drying on antipredator behaviours in general.


Carlson B & Rowe M. 2009. Temperature and desiccation effects on the antipredator behavior of Centruroides vittatus (Scorpiones: Buthidae). Journal of Arachnology 37(3): 321-330. DOI: 10.1636/Hi09-06.1

Top photo by Wyatt Berka 2010 on Flickr; used under a Creative Commons license.


Bradley Carlson said...

Sorry to comment over a year after your initial post - I just found it now by chance. I'm the Carlson in the paper, which was research I performed as an undergrad. I appreciated your comments (even the critical ones) and interest. I will point out 2 quick things to address a couple of your concerns: 1) we didn't test dehydration effects on stinging because the dehydration experiment was an afterthought that turned out to be the most interesting element. Following it up would be interesting. 2) We were cautious about our interpretation of sex effects on sting rates. The p-value was 0.07, and of course 0.05 is an arbitrary cut-off point - is it completely meaningless to say that there is a 93% chance that there is a difference between sexes? I think too many interesting biological patterns may be missed by dogmatic understandings of what really amounts to "rules of thumb".

That being said, I wanted to let you know that I am following up this work to explicitly look at the effects of sex, mass, limb length, and tail morphology on stinging and sprinting to better tease apart what is going on here. My newest suspicion - females are actually just more aggressive. If you want, I can let you know when I've got some results. Best, Brad

Zen Faulkes said...

Thanks for your comments!

On the p values, I've had cases where early p values were (say) 0.06, but that that went, when I added a few more data points, the "wrong" way to something not even close to statistically significant.

I'm becoming more convinced that confidence intervals are a better way of conveying the degree of certainty or not.

Would be happy to hear of any follow-ups!

Bradley Carlson said...

Good point, although one could always argue that the same thing could happen with p=0.04 - a few more data points may change everything. I agree with you that confidence intervals are probably the way to go. All the benefits of p-values but much more useful.