“No!” she cried. “Don’t sell your soul!”
Such was one reaction I received upon suggesting that I might pursue stable isotope analyses in the future. Alternative reactions have leaned more towards the ooh-that-could-be-interesting end of the spectrum. This division is probably due to the fact that stable isotope analysis – while not exactly the newest kid on the block – is certainly one of the latest “big things” in environmental archaeology. As a result, views can get polarized between those who think it’s the most exciting thing since radiocarbon dating, and those who think that more traditional studies are actually far more useful than this new-fangled wizardry which, in any case, sometimes tells us very little.
It’s a general policy of Farming Unearthed to be very agreeable – I simply don’t have a taste for fierce debate. So I was very happy to find an article in the new edition of Medieval Archaeology which uses stable isotope analysis as part of a suite of methods, in order to address a fairly straightforward question: how were pigs managed at Dudley Castle in the 14th century?
Hamilton & Thomas (2012) begin by outlining the whole question of pig management – very useful if, like me, you’ve never managed a pig. The basic dichotomy is between free-ranging (especially in woodland – “pannage”) and enclosed husbandry (i.e. foddering them in a sty). They argue, essentially, that pig husbandry at Dudley Castle, between the mid-13th and late 14th centuries, shifted from a pannage- to a sty-based system.
So what’s the evidence?
Exhibit A is the most “traditional” form of zooarchaeological evidence: basically, counting the number of bones identifiable to each species in an assemblage. In this case, the proportion of pig bones (relative to cattle and sheep) falls quite sharply between the mid-13th and late 14th centuries.
Exhibit B expands on this, to identify a general increase in the size of the pigs over this same period.
Exhibit C is another contemporary trend towards the slaughter of younger pigs (up to 3 years old) – as identified in dental and bone-fusion data (for a good introduction to the ageing of animal skeletons, try O’Connor 2000, pp.80-97).
Which, of course, leaves us with Exhibit D – the isotopes.
This kind of analysis requires good preservation of collagen, a protein component of bone, in order to examine the protein sources of the animal (or indeed human) in question. Hamilton & Thomas put it succinctly on page 237:
“The ratio of the heavier to the lighter isotope… [for carbon and nitrogren] …varies in characteristic ways across the food web, and is preserved and measurable in archaeological body tissues.”
As with abundance, size and age, the pig isotope ratios show clear differences between the mid-13th and late 14th centuries.
So what do all these changes suggest?
Well, the increased size of the pigs seems to suggest “a closer management of pig breeding” in some shape or form. The increase in the slaughter of younger pigs could, it is suggested, be a direct consequence of this: bigger pigs can be culled younger because they don’t take as long to fatten up. But also, some very young pigs present in the later phase are considered indicative of local breeding – on the basis that neonatal mortalities are unlikely to be present unless piglets were actually born at the site.
The carbon isotope ranges of the later phase are apparently considerably narrower than in the earlier phase, suggesting a more uniform diet for the pigs (the young piglets’ isotopes show a suckling signature, which is rather neat). Meanwhile, the nitrogen values decrease over time. Hamilton & Thomas consider three different explanations for this, and favour an increase in legumes in the pigs’ diet – since leguminous plants, as nitrogen fixers, could result in lower nitrogen values.
The overall decline in the relative abundance of pigs is rather more ambiguous, but would at least be consistent with a shift from extensive pig husbandry (lots of pigs, little effort) to intensive pig management (fewer pigs, more effort) – an intensive system which involved closer localised management of breeding (= bigger & younger pigs) and also controlled foddering with a strong leguminous element, as the Dudley Castle folk cast their pulses before swine (= lower nitrogen values).
As always, I’ve simplified what is actually a highly detailed and well-argued paper. But even so, I hope that this review demonstrates the way in which “traditional” and “novel” methods can be combined to produce a pretty convincing model. Hopefully we’ll see more work like this in the future!
Hamilton, J. and Thomas, R. 2012. “Pannage, pulses and pigs: isotopic and zooarchaeological evidence for changing pig management practices in later medieval England”, Medieval Archaeology 56, pp.234-259.
O’Connor, T. 2000. The Archaeology of Animal Bones (Stroud).