Introduction

The cave Myotis (Myotis velifer) is a medium-sized insectivorous bat (35 to 46 mm, ~15 g), which is distributed throughout the United States in Kansas to southern Nevada, and southeastern California, extending to Mexico and Honduras (^{Fitch et al. 1981}). This species is typically a cave dweller that is often found in mines, tunnels, and occasionally in buildings or under bridges (^{Hayward 1970}; ^{Davis and Cockrum 1963}). Its diet is mainly composed of beetles (Coleoptera) and moths (Lepidoptera; ^{Kunz 1974}). Myotis velifer regulates population growth of many insect herbivores, and therefore, is of great importance for conservation of mountain ecosystems (^{Böhm et al. 2011}).

Colonies of M. velifer reach sizes from 1,000 to 5,000 individuals in Arizona, Oklahoma, Kansas, and Texas, where populations appear to reside permanently (^{Tinkle and Patterson 1965}; ^{Fitch et al. 1981}; ^{Caire and Loucks 2010}). In these sites, bats hibernate during winter (^{Hayward 1970}; ^{Samanie and Caire 2007}). Hibernation may last up to six to eight months, allowing bats to save energy when food availability and ambient temperature are low (^{Lyman 1982}). In Mexico, M. velifer form colonies in caves of 100 to more than 10,000 individuals along the country (^{Arita 1993}). 50 years ago, ^{Villa (1967)} reported individuals of M. velifer migrating to high altitudes to the mountains in order to hibernate. However, there is no detailed information about the places used by this species to hibernate in the country, neither of the environmental conditions faced by bats in the hibernaculum. Our main goal was to describe and characterize a hibernation colony of M. velifer in a tunnel located in La Malinche National Park (LMNP), a mountain ecosystem in central Mexico.

La Malinche National Park is located in the Trans Mexican Volcanic Belt (19° 06’ 04” and 19° 20’ 06 “ N; -97° 55 ‘ 41” and -98° 10’ 52” W). The park is home for 11 bat species, seven of them belonging to the family Vespertilionidae (^{Ayala-Berdon et al. 2017}). Here, we report for the first time a colony of M. velifer using a tunnel while hibernating in central Mexico. The hibernaculum is located in the East slope of the main runoff of the park (19° 14’ 19” N, -98° 1’ 10” W) at 3,600 m. Surrounding vegetation is composed of dense patches of Abies forest and high mountain grasses (^{Villers et al. 2006}). We visited the tunnel in the middle of each month, every 30 days, from July 2015 to March 2016. The tunnel is 23 m long, 1.8 m high and 1.6 m wide. In each visit, we looked for bats in the interior of the tunnel and made a one-time measurement of the ambient temperature and relative humidity. We visited the tunnel around midday each time. All measurements were done in the same place in the middle of the colony, 12 m away from the entrance. During the first visit to the tunnel, we randomly captured and measured (i. e., sex, length of the forearm, body mass, and reproductive condition) seven individuals (Table 1). Species identification was conducted following ^{Medellín et al. (2008)}. Body mass was estimated by weighing the bats with an electronic balance to the nearest 0.1 g (Ohaus®, Parsippany-Troy Hills, New Jersey). After all measurements were completed, to avoid bats flee from the cave and be predated, we transferred individuals to laboratory conditions (19 ºC, 70 % humidity) to La Malinche Biological Station, 5 km away from the tunnel. We retained bats until sunset, and released them near the tunnel that same night. Bats were captured under permission of the Department of Wildlife management (SEMARNAT, permit number 07019 granted to our institution).

In the tunnel, mean ambient temperature was 7.5 ± 1.5 ºC, while mean ambient relative humidity was 70 ± 8.3 %, along the eight months sampled. These conditions are similar of those reported as preferred by M. velifer hibernating in other hibernacula. In Texas, Indiana and Kansas, M. velifer hibernates in caves with temperatures ranging from 1 to 10 ºC and a relative humidity from 70 to 100% (^{Tinkle and Patterson 1965}; ^{Webb et al. 1996}). In mid September 2015, we found 14 individuals of M. velifer hibernating in clusters of different sizes. All clusters were composed by M. velifer, and we only observed one individual of Corynorhinus mexicana resting alone and sharing the tunnel with our study species. Clustering has been previously reported for the species as a way to reduce the energetic expenditures caused by low ambient temperatures (^{Tinkle and Patterson 1965}). By mid January, the colony reached 81 individuals. We are confident that bats were hibernating, because they were unresponsive to our presence and most individuals remained until mid-February. Bats were not disturbed in subsequent visits to the tunnel. M. velifer has been reported hibernating at other latitudes during the same six-month period and in the same environmental conditions (^{Fitch et al. 1981}).

Hibernation is an important physiological process that determines the survival of bats during winter, when food is scarce and the ambient temperature is low (^{Lyman 1982}; ^{Speakman and Thomas 2003}). This new record exemplifies the specificity of hibernaculum used by bats in central Mexico. In the last decade, LMNP has suffered a huge anthropogenic pressure (i. e., illegal logging, grazing and firing; ^{Chávez et al. 1990}; ^{Espejel 1996}). To date, its vegetation has been reduced by 77 % since the reserve was decreed (^{Díaz Ojeda 1992}; ^{Vargas-Márquez 1997}). Currently, characterizing the environmental requirements of bats to hibernate is of relevance to promote specific conservation strategies in the country for this group of vertebrates.