I’m slightly late, but Nature recently reported that fruit flies can smell the difference between hydrogen and deuterium. This is really quite impressive. The chemistry of an element depends on its electrons, and the number of electrons depends on the charge of the nucleus. This depends on the number of protons. The neutrons in the nucleus help hold the nucleus together, but to a first approximation make no difference to the chemistry. All that differentiates hydrogen from deuterium is an extra neutron.
Biological systems are known to be able to differentiate between isotopes. For example, drink heavy water (water that substitutes deuterium for hydrogen) and you will eventually die – the chemical properties are different enough that life finds heavy water useless. Being able to smell the difference, even in small quantities, is surprising and will hopefully reveal something about how the sense of smell actually works.