First record of a Nathusius’ pipistrelle (Pipistrellus nathusii) overwintering at a latitude above 60°N

Highly mobile species are considered to be the first to respond to climate change by transforming their ranges of distribution. There is evidence suggesting that Pipistrellus nathusii, a long-distance migrant, is expanding both its reproduction and overwintering ranges to the North. We recorded the echolocation calls of bats at 16 sites in South-Western Finland on two consecutive winters, and detected calls of P. nathusii at one of the sites throughout the latter winter. To our knowledge, this is the northernmost observation of an overwintering P. nathusii, and further evidence that the species is already responding to climate change.

Climate change is already affecting the range limits and phenology of organisms across a plethora of taxa (Parmesan and Yohe 2003;Hickling et al. 2006;Tingley et al. 2009).
According to a review by Thomas (2010), it is possible that more than half of observed animal range boundaries have shown a response to climate change. In most cases, range expansion has occurred on the edge of their of their distribution range with lower ambient temperatures, which appears to take place more rapidly than local extinctions at the side with higher ambient temperatures (Hickling et al. 2006;Thomas et al. 2006;Brommer et al. 2012). While it is difficult to differentiate changes caused by climate change from those resulting from habitat loss or other local changes, or an increase in sampling effort, there is some evidence that bats are also already responding to the changing climate through range expansion (Lundy et al. 2010;Sherwin et al. 2013). In Europe, Pipistrellus kuhlii Ancillotto et al. 2016) and Hypsugo savii (Lehotská and Lehotský 2006;Uhrin et al. 2016) have shown a remarkable increase in their geographical range, whereas in North-America, Willis and Brigham (2003) reported the tree roosting Lasiurus borealis in Southwestern Saskatchewan, Canada, 300 km from the nearest previous observation.
Pipistrellus nathusii is a long-distance migrant, with distances of up to 1905 km between its breeding and overwintering areas (Pētersons 2004). Highly mobile species, such as P. nathusii, are most likely to be the first to respond to a changing climate by shifting their ranges towards the poles or to higher latitudes (Warren et al. 2001;Brommer et al. 2012). Indeed, Lundy et al. (2010) showed that P. nathusii has already adapted its distribution range in Europe. Breeding colonies, and a likely increase in the amount of individuals, have been documented in the British Isles (Russ et al. 2001;Matthews et al. 2018), Northern Italy (Martinoli et al. 2000) and on the Iberian Peninsula (Flaquer et al. 2005).
Winter is often a critical time for animals inhabiting high latitudes. Therefore, for many relatively sedentary species, such as most hibernating mammals, climate change can have unfavorable effects (Lane et al. 2012). Bats, however, are mobile and more capable of shifting their overwintering ranges as a response. Indeed, Humphries et al. (2002) suggest that the hibernation ranges of North American bats will shift towards the north because of climate change. Furthermore, according to Newson et al. (2009) While climate change may lead to loss in biodiversity especially in Southern Europe, many studies predict that species richness in the Northern Europe and Scandinavia will increase (Levinsky et al. 2007;Rebelo et al. 2010). According to Mikkonen et al. (2015), the mean annual temperatures in Finland have risen 2.3 ± 0.4 °C from the mid-19th century. The highest increases in temperature have taken place over the winter months, most notably in December when the monthly mean temperature has risen 4.8 °C. Furthermore, the spring months have also warmed more than the annual average.
Until recently, P. nathusii has been considered a vagrant in Finland during migration periods in late May and late August-early October (Rydell et al. 2014;Ijäs et al. 2017).  (Table 1; Figure 2). In addition to being the coldest month of that winter with a mean temperature of -4.4 C°, January was also the only month without any recordings of the species. Our results provide further evidence on the hibernation range expansion of P. nathusii. In the summer of 2017, we captured two P. nathusii weanlings in Turku, approximately 10 km from Härmälä cleft, indicating that the species is also breeding in the area. Our observations in Finland are consistent with other reports on the exceptional breeding and overwintering range expansion of P.

The first breeding colony was found in Southern
nathusii. Furthermore, our finding indicates that acoustic surveillance of hibernation sites can yield valuable information on more elusive bat species. Our results suggest that P. nathusii has joined the seven species of bats found to hibernate above 60°N in Fennoscandia.