Field workers using heterodyne detectors noticed that British pipistrelles peaked either near 45 kilohertz or near 55 kilohertz, with few individuals in between. The obvious explanations, age or sex or habitat, did not account for the gap.
Genetic work in the 1990s confirmed two distinct species with no evidence of interbreeding: Pipistrellus pipistrellus, the 45 kilohertz caller, and Pipistrellus pygmaeus, the soprano pipistrelle at around 55 kilohertz. They differ also in social calls, in wing venation detail, and in habitat preference, with the soprano more associated with riparian woodland and a diet skewed towards aquatic-emerging flies. Physical separation in the hand is subtle and unreliable for many individuals.
The episode is a useful correction to the assumption that Europe's mammals were fully catalogued by the twentieth century. It also shows how a measuring instrument can create a discovery: the species boundary was audible before it was visible, and the split was only possible once cheap detectors put ultrasound in the hands of ordinary surveyors.