sentinel animal

noun

an animal that is placed in a given environment and then monitored to find out if an infectious disease or other harmful agent is present in that environment

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012