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Except where otherwise noted, this item's license is described as © The Author 2015. Published by Oxford University Press on behalf of Royal African Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http:// creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact journals. permissions@oup.com