The panda and polar bear datasets should have been included in the data section rather than hidden in the URLs section. Production removed the DOIs and used (now dead) URLs instead, but for the working links and insight see the following blog: http://blogs.biomedcentral.com/gigablog/2012/12/21/promoting-datacitation-in-nature/

More on the backstory and other papers using and citing this data before the Cell publication in ths blog posting: http://blogs.biomedcentral.com/gigablog/2014/05/14/the-latest-weapon-in-publishing-data-the-polar-bear/