Common or Soprano: Which Bat Is It?

Pipistrelle in Flight

I recently saw a Pipistrelle bat fly past my window, its erratic flying pattern indicating it was feeding. Now I don't claim to have excellent eyesight, butI knew it a Pipistrelle as I know there is a roost nearby. Beyond this however, I could not say what species it was. Ireland has three species of Pipistrelle: Nathusius’ Pipistrelle (Pipistrellus nathusii), the Common Pipistrelle (P. pipistrellus) and the Soprano Pipistrelle (P. pygmaeus). The only confirmed breeding colony for P. nathusii in Ireland was recorded in 1997 near Lough Neagh and while there has been recordings in other counties, no records exist for the bat as far south as where I live (1). Its therefore extremely unlikely that is was P. nathusii that I saw. So that leaves P. pipistrellus or P. pygmaeus. However if I had spotted this bat pre-1999, I would have been sure that it was P. pipistrellus. Prior to this, P. pygmaeus was not recognised as a species in its own right. The two species are morphologically identical, measuring about 4 cm long with a 20 cm wingspan. Their difference lies in the frequency used by both bats during echo location (2). P. pipistrellus uses a call of 45 kHz, with P. pygmaeus using one of 55kHz. While differences were noted within the P. pipistrellus species as early as 1865 (2), it wasn't until the 1980s that the use of bat detectors revealed differences in the echo locating frequency of the two species. Further investigation showed that the two species occurred in close association however they did not share roosts. Conclusive support for the two-species theory was provided in differences in the cytochrome b genes.

References:

1. Russ et al., 1998. Journal of Zoology, London 245 pp. 345-349
2. Hulva et al., 2004. Molecular Phylogenetics and Evolution 32 pp. 1023-1035
3. Barrat et al., 1997. Nature 387pp. 138-139