Well, it's been a while since my last post, but much has happened since then.  Unfortunately I have now come to a bit of a halt with lab work due to a fractured shoulder blade following an unintentional gallop and fall off a horse. 

I have weighed and run nearly all the carapace and plastron samples for carbon and nitrogen analysis and the results seem promising.  These samples are of unknown provenance - they were re-exports from the Ukraine but that is all I know.  I am therefore using them purely to test variation within individual tortoises.  As I had hoped, my preliminary results suggest that there is no significant difference between scutes (the hexagonal sections that make up the shell), and no significant difference between the top (carapace) and underside (plastron) of the shell.  This means that I will be able to pool material from multiple scutes to get a better sample weight.   
So: no significant variation between the scutes.  There is however a fair amount of variation between the different ages of growth, particularly for nitrogen: the tissue the tortoise was born with (the juvenile scute) is more enriched in Nitrogen than other growth layers (figure 1).  Nitrogen becomes enriched in the heavier isotope as trophic level increases; a predator will be more enriched in 15N than it's prey.  My results are therefore not surprising as Horsfield's tortoises are capital breeders, which means they use stored resources for reproduction.  The tissues of the offspring are produced from metabolised fat and are thus effectively a trophic level higher than the tissues of the mother.  Had I not seen a difference between the juvenile and newer growth I would have been concerned about my sampling. 
Figure 1: Isotope signatures accross the scute. Left: nitrogen; right: carbon

Interestingly, there is also a significant change in the carbon signature across the scute (figure 1), which could be due to a change in diet or location, but I don't have enough information about the tortoises to know what the cause is.  Again, although I don't know the reason, there are also significant differences between individual tortoises in both carbon and nitrogen signatures (figure 2).  This might be because some are ranched and others wild-caught - the higher nitrogen signatures do belong to the smoother shelled tortoises (ranched tortoises tend to have lumpier shells than wild tortoises) but this is sheer speculation.
Figure 2: Nitrogen signatures of individual Uzbekistan re-exported Horsfield's tortoises


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