First forays into μ-CT scanning

Now I’m just using this blog to validate the work I do each day…

And on this particular day I’ve been messing around with a CT scan of a spider jaw (chelicera). As I might’ve mentioned previously, during the summer, the male sheet-web spiders (who have massive jaws – far bigger than those of the females) go wandering in search of females/their webs.

Lads and ladies (Cambridgea foliata)
Lads and ladies (Cambridgea foliata)

When they find one, the male sits in the hub of the web and will stay there all night. If the male wanders into a web with another male already resident, they will fight. What determines the outcome is the differential in “resource holding potential”. RHP is combination of an individual’s body size, weapon size, physiological condition and previous experience. The individual with the higher RHP is more likely to win.

I’m looking at the evolution of exaggerated male weaponry and it has been suggested that extreme male weapons evolve to help males win these competitions i.e. contribute significantly to RHP. The idea is that a bigger weapon makes for a better fighter or that a large weapon makes a male look like a better fighter so other males are less likely to approach. Both processes would result in a male who is able to monopolise the female.

The problem is that weapon size is highly correlated with body size so it’s been very difficult to determine statistically whether larger weapons actually make any meaningful contribution over and above that of a larger body size. Try to imagine a giant bare-fisted boxer who had massive hands which were big relative to the boxer himself. Like, hands bigger than own face. One could argue that bigger hands/fist could help him to win a fight but how do you actually prove that? You would have to separate the effect of his hands from the effect of just being a big person. You can’t consider the two traits in isolation because that would require removing his hands which only shows that you need hands to box effectively and that a disembodied pair of hands is…just that.


Jedis and Thing are the only exceptions

There are a couple ways that people have tried to circumvent this when studying animals. One is that people have used force transducers to get direct measurements of bite force or nipping force, depending on the type of weapon. Another avenue is doing finite element analysis on the structures. Some biologists already use finite element analysis to look at loading on things like leg bones in different species of moa and have used various traits to estimate bite strength from crocodile skulls. Recently someone has done this sort of analysis on spider fangs.

One of the first steps to doing such an analysis is doing a micro-CT scan (computed tomography) of the weapons and the musculature. I went along for my first CT training session a couple weeks ago. I was told to bring a sample to scan so I decided to take along part of a male Cambridgea foliata. Unfortunately I was an eejit who decided to dissect off the jaw to scan rather than taking the whole head. The result is that you can’t see the full length of the muscles controlling fang movement but the scan I got has given me the chance to play around with different bits of software and generally get a first look into the structure.

The end result is an 8 second video which I’m more proud of then I really should be. *tadaa* *spirit fingers*