Illustrated field keys to the bats ( Mammalia : Chiroptera ) of Taiwan

Bats of Taiwan, comprising 38 species in total, are biogeographically unique (11 endemic species and 16 endemic subspecies), taxonomically diverse (six families), and play crucial ecosystem roles as seed dispersers or insect suppressors. Many of these bat species were, nevertheless, scientifically described from or newly recorded in Taiwan during the last 20 years, rendering limited knowledge hitherto gained regarding their fundamental ecology and population statuses. To aid ecological research and thus benefit conservation of Taiwan’s bat diversity, we constructed illustrated field keys to all 38 species.


INTRODUCTION
Bat diversity in Taiwan is impressive since the island covers only around 36,000km 2 , but harbours 38 species (Wu 2007;Cheng et al. 2017b), which is more than twice the species richness in the United Kingdom. Taiwan's rich bat diversity is also characterized by high endemism, with 11 species found exclusively in this area and another six representing geographically unique races. Situated between the Palearctic and the Indomalayan realms, Taiwan has received fauna and flora that incurred following several routes from nearby mainland regions (He et al. 2018). Indeed, Taiwan bat fauna include genera with predominantly Palearctic distributions (e.g., Plecotus, Barbastella), and also shares genera with the Indochinese (e.g., Kerivoula, Harpiola) and the Sunda Shelf (e.g., Pteropus) bioregions of Indomalaya. While a few species such as the Ryukyu Flying Fox (Pteropus dasymallus) probably dispersed along the islands of the Philippine Arc and the Ryukyu Arc, the others presumably immigrated via the exposed continental shelf during the Pleistocene glacial drops in sea level (Li et al. 2006;Kuo et al. 2014). For the latter group, reformation of the Taiwan Strait during the interglacials blocked former genetic exchanges between Taiwan and the continent, and thus promoted endemism in Taiwan through allopatric divergence (Kuo et al. 2015).
Six of the Old World's 12 bat families (including the recently recognized family Rhinonycteridae) are recorded in Taiwan, namely Pteropodidae, Hipposideridae, Rhinolophidae, Molossidae, Miniopteridae, and Vespertilionidae. Taiwanese vespertilionids contribute three quarters (29 species) of the total richness, whereas only one to three species represent each of the other five families. Within Vespertilionidae, Vespertilioninae is the largest subfamily (13 species, nine genera), followed by Myotinae (nine species, two genera), Murininae (six species, three genera) and Kerivoulinae (one species, one genus). Myotis represents the most speciose genus of all, with eight species in total, followed by Murina (four species) and Pipistrellus (four species). The other genera are only recorded by one or two species each (Cheng et al. 2017b).
Bats are ecologically and economically significant for many natural and human-dominated habitats (Maas et al. 2015). The Formosan Flying Fox feeds mostly on small fruits of native tree species and plays a crucial role in forest regeneration on small islands of Taiwan (Chen et al. 2017). On the main island, except for few records of the flying fox, the bat fauna is exclusively composed of insectivorous species that provide services of insect pest suppression in agricultural systems and help to maintain forest ecosystem stability through consumption of arthropods. Nevertheless, the diversity of bats and the associated ecosystem functions are threatened by increasing urbanization and agriculture as well as roost disturbance (Cheng et al. 2017b). Nine species are considered threatened or Near Threatened, and another 13 as Data Deficient by either the IUCN Red List (IUCN 2017) or the nation (Cheng et al. 2017a), or with statuses not yet assessed (Table 1).
Despite ongoing conservation assessments, our understanding of the natural history and distribution of most of the bat species in Taiwan as well as understanding the threats to their survival are still limited. Largely the knowledge about Taiwanese bats is restricted to some of the most common species and the protected species. The poor understanding of most bat species is partially due to the fact that many of them have come to be known only in the last 20 years. Lin et al. (1997) described 22 species in the first edition of "Bats of Taiwan". Since then, eight new species from genera Thainycteris, Harpiola, Kerivoula, Murina, and Myotis have been described (Csorba & Lee 1999;Kuo et al. 2006Kuo et al. , 2009Kuo et al. , 2017Ruedi et al. 2015), and three Pipistrellus species were reported in Wu's (2007) thesis as putative new species to be formally described. In addition, new range extensions have been made for species of Cynopterus, Hypsugo, Myotis, and Rousettus on Taiwan's main island and/or in its nearby islets (see Cheng et al. 2017b for these new records; Myotis pilosus was recorded by the Bat Association of Taiwan in February 2018). The dramatic increase in the number of bat species of Taiwan warrants the need of an effective tool for species identification, especially for the multi-species vespertilionid genera Murina, Myotis, and Pipistrellus. Here, we present the first bilingual (English and Chinese) illustrated keys to the 38 Taiwanese bat species. Our goal is to provide a user-friendly tool for inexperienced researchers to identify species in the field without sacrificing animals. Such a tool is fundamental and vital for the further development of ecological research and conservation carrying capacity in Taiwan. Measurements and descriptions were taken from captured individuals in the field (from >60 localities across whole Taiwan), from specimens deposited at the Zoological Museum of the National Taiwan University (Taipei, Taiwan), the National Museum of Natural Science (Taichung, Taiwan), and from literature (Kuo et al. 2006(Kuo et al. , 2009(Kuo et al. , 2017Huang et al. 2008;Ruedi et al. 2015). For species rarely recorded in Taiwan, data from additional conspecific samples from the continent (Francis 2008;Kruskop 2013;Smith & Xie 2013) were also used to avoid underestimation of intraspecific variation. Measurements were taken only from adults in order to control the age effect. Six external measurements were used: forearm length (FA), from the end of the elbow to the end of the wrist with the wing folded; hind foot length (HF), from the base of calcar to the tip of the toe, excluding the claw; penis length (PL), the whole length of the penis excluding the short proximal portion that is perpendicular to the rest; tibia length (TIB), from the knee joint to the ankle; weight (W), body mass of the live bat; (see Appendix 1 for measuring of FA, PL and TIB). Since our data are collected by different researchers and sources, all measurements in this paper are rounded to the first decimal place to minimize authors' biases.

RESULTS
Herein we provided Illustrated keys to all 38 bat species in Taiwan, serving to aid researchers easily and accurately identifying these bats in the field. An English version of the identification keys is given below, whereas a Chinese version is provided as a supplementary material (Appendix 2).

Picture credits
All pictures are credited to the authors (JCCH, YYH, and HCK, see details below), except the following items are used with permission from the original photographers. Image 2 of Pteropus dasymallus formosus courtesy of Han-Chun Lee. Image 44 of Vespertilio sinensis courtesy of Yu-Cheng Chang, and Image 45 of Hypsugo pulveratus courtesy of Ching-Fong Lin.