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. 2013 Dec 11;8(12):e81638. doi: 10.1371/journal.pone.0081638

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© 2013 Becker et al

This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

PMC Copyright notice

Worldwide sample density, including all measurements, illustrated as a heatmap (© OpenStreetMap contributors for map data, used and redistributed under the CC-BY-SA licence[26]).