Assortative roosting in the two phonic types of Pipistrellus pipistrellus during the mating season

Abstract

The European pipistrelle, Pipistrellus pipistrellus, consists of two phonic types that echolocate with frequencies of maximum energy (FMAXE) averaging 46 kHz and 55 kHz (named the 45 kHz and 55 kHz phonic types for simplicity). These two phonic types occur in sympatry over much of Britain. Between 1993 and 1995 mating groups of P. pipistrellus were located in bat boxes between August and October, the main mating period for these bats. Mating groups comprised a single male and up to three females. Broadband, time-expanded recordings of search-phase echolocation calls were made from bats immediately after release. The average FMAXE of each bat was calculated: bats with an FMAXE less than 49 kHz were classed as 45 kHz phonic types and bats with an FMAXE greater than 52 kHz were classed as 55 kHz phonic types. A total of 16, 45 kHz groups and ten 55 kHz groups were located. No mating group ever contained more than one phonic type, although both types were known to be present at several of the study sites. Males emitted calls of higher frequency than females for both phonic types. The study suggests that the two phonic types are reproductively isolated and supports the hypothesis that P. pipistrellus should be considered as two cryptic sibling species.