Dispersal and group formation dynamics in a rare and endangered temperate 1 forest bat (

25 For elusive mammals like bats, colonization of new areas and colony formation are poorly 26 understood, as is their relationship with the genetic structure of populations. Understanding 27 dispersal and group formation behaviors is critical not only for a better comprehension of 28 mammalian social dynamics, but also for guiding conservation efforts of rare and endangered 29 species. Using nuclear and mitochondrial markers, we studied patterns of genetic diversity 30 and differentiation among and within breeding colonies of giant noctule bats (Nyctalus 31 lasiopterus), their relation to a new colony still in formation, and the impact of this ongoing 32 process on the region-wide genetic makeup. Nuclear differentiation among colonies was 33 relatively low and mostly non-significant. Mitochondrial variation followed this pattern, 34 contrasting with findings for other temperate bat species. Our results suggest that this may 35 indicate a recent population expansion. On average, female giant noctules were not more 36 closely related to other colony members than to foreign individuals. This was also true for 37 members of the newly forming colony and those of another, older group sampled shortly after 38 its formation, suggesting that contrary to findings for other temperate bats, giant noctule 39 colonies are not founded by relatives. However, mother-daughter pairs were found in the 40 same populations more often than expected under random dispersal. Given this indication of 41 philopatry, the lack of mitochondrial differentiation among most colonies in the region is 42 probably due to the combination of a recent population expansion and group formation 43