Some slime mold we found in the woods, with tiny rodent footprints on it!
Most of the world’s parasitic organisms are highly limited in the number of host species they can feed upon; often only very similar animals found in the same range, and more commonly just one single species.
This awesome post on daily parasite details an experiment in which a species of flea, known to infest several types of rodent, was lab-reared on two types of host to see if each population would grow more specialized.
It only took six generations before the fleas from one host had a much, much harder time surviving on the other!
Nutria and soft-shelled turtle photographed by xor