 This causes a problem for the human population because these colonies can get quite large, in fact much larger than the "Pipistrellus pipistrellus" colonies, which tend to be fewer than 200 bats.This large size of colony causes a nuisance for humans because of the smell.During early pregnancy, "Pipistrellus pygmaeus" emerges later from its roost than it does in late pregnancy or lactation.