Fruitflies (Drosophila melanogaster) can smell the difference between hydrogen and its heavier counterpart, deuterium, according to recent research, which they say offers support for a controversial hypothesis of how olfaction works - namely that odorants are identified not according to molecular shape, but by their atomic vibrations.

The flies can also be conditioned by electric-shock treatment to exhibit aversion to either form of the molecule, and the researchers say that shows they can clearly distinguish between them. 

Critical of the idea?  Co-authors Luca Turin of MIT addresses some of it in the comments on the Nature News piece by Philip Ball.  

Citation: Franco, M. I., Turin, L., Mershin, A.&Skoulakis, E. M. C. Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1012293108 (2011)