White-nose syndrome is an introduced fungal disease that has reduced the size of hibernating populations of little brown bats Myotis lucifugus by 90% across much of eastern North America since 2007. Herein, we report the recapture of eight banded little brown bats, all males, with minimum ages of 18.6–25.6 y. The recaptures occurred during winter 2019–2020, at a hibernaculum in Michigan where white-nose syndrome likely has been present since 2013–2014, indicating that these old and apparently healthy males are in their seventh season of exposure to the disease. Hence, our data suggest that a long life in little brown bats and existence of white-nose syndrome are not necessarily incompatible.
Despite the small size of bats, these mammals have exceptional life spans (Wilkinson and Adams 2019; Lagunas-Rangel 2020). Thirty-three species have a maximum recorded longevity of 20 or more years, and 13 of these species are small-bodied animals (6–20 g) within the genus Myotis. White et al. (2019) recently reported five records of little brown bats M. lucifugus from Wisconsin with minimum ages between 18 and 32 y. However, each of these bats was last seen in 2016, which was the first year that an introduced disease, white-nose syndrome, was detected in their hibernacula. This disease, caused by the psychrophilic fungus Pseudogymnoascus destructans, has reduced most populations of bats that hibernate underground in eastern North America by about 90% (Turner et al. 2011; Frick et al. 2015), and because of the malady, White et al. (2019:N45) suggested that “longevity records of this magnitude may never be recorded again.” However, in this note, we provide eight records of little brown bats living more than 18 y, even though these animals apparently have been exposed to the fungus for the past seven winters.
Our study site was the hollow concrete spillway of Tippy Dam, a hydroelectric facility on the Manistee River, near Wellston, Manistee County, Michigan (44°13′5.8794″N, 85°57′35.64″W). Approximately 20,000–24,000 bats currently hibernate in the spillway, which is one of just three hibernacula for bats in the Lower Peninsula of Michigan, the nearest of which is a concrete tunnel sheltering only about 100 animals and located 230 km northeast of Wellston (Kurta 2008; Slider and Kurta 2011). Historically about 97% of bats overwintering at the dam were little brown bats, and most of the remainder were northern long-eared bats Myotis septentrionalis (Kurta et al. 1997). Although northern long-eared bats apparently have disappeared from the spillway after introduction of P. destructans, the number of little brown bats appears stable.
Placing a metal band on the forearm of a bat is standard technique for identifying individuals, tracking migratory movements, determining survivorship, and helping establish longevity (Ellison 2008; Kunz and Weise 2009; Rockey et al. 2013). To discover the summering grounds of animals that overwintered at Tippy Dam, biologists from Eastern Michigan University banded 5,524 bats between February 1995 and September 2008. Most (94%) banded animals were little brown bats, and 72% of these were male; a preponderance of males is typical of bats at northern hibernacula (Humphrey and Cope 1976; Keen and Hitchcock 1980). Each bat received a lipped band (Baker et al. 2001) that was 2.9 mm in diameter and manufactured by Lambournes or Porzana (UK). Each band was stamped with a unique four-digit number and the letters “EMU YPSI MI,” indicating the university (EMU), as well as the city (Ypsilanti) and state (Michigan) where the school is located.
During the hibernating season of 2019–2020, we began a study of thermoregulatory patterns of bats overwintering in the spillway to help determine why this particular population of little brown bats has not been devastated by white-nose syndrome. We entered the spillway on three dates (6 December 2019, 14 February 2020, and 6 May 2020) to conduct that project, but during each visit, we also briefly looked for any banded bats that might be present on the interior walls and ceilings. Banded individuals were removed from their roosting location by hand or with the aid of a long-handled net and examined for the presence of obvious abnormalities, such as frostbite damage, wing tears, open wounds, or emaciated appearance.
For each recovered animal, we calculated elapsed time between banding and recapture using an online program (https://www.calculator.net/date). Age of bats at the time of banding was unknown. To calculate minimum age at the time of recapture, we assumed that all marked bats were young-of-the-year when first caught and that all were born on 1 July, at the end of the preceding maternity season (Kurta 1980).
We recaptured eight marked individuals, 0.145% of the total banded, and all were male little brown bats (Table 1). One band was chipped along its bottom edge so that only the first two digits (3 and 1) were legible. Nevertheless, all bands numbered from 3100 to 3199 were applied on the same day, allowing us to determine time since banding for this individual, as well as the others. Minimum ages that were calculated for the eight bats ranged from 18.6 to 25.6 y.
During each of our visits, many bats roosting in the spillway displayed white patches of apparent fungal growth on the ears, snout, forearms, or feet, suggesting an active infection with P. destructans. However, all banded bats appeared to be in good physical condition, barring some tooth wear; for example, upper and lower incisors and the lower premolars were worn to the gums in bat #4840. We detected no white patches on the eight recovered animals and no obvious fungal damage to their wings, such as tears, holes, or depigmented areas (Reichard and Kunz 2009).
Two correlates of extreme longevity in bats are the use of hibernation and the habit of overwintering in caves or cavelike locations, especially at high latitudes where winters are long (Wilkinson and Adams 2019). The resulting prolonged period of inactivity in a protected site presumably reduces extrinsic mortality factors such as accidents and predation. At Tippy Dam, a torpid population begins building in mid- to late September, and thousands of bats, mostly males, are still present 7.5 mo later in early May (Kurta et al. 1997). Although our data are not sufficient to estimate average annual survival rates at Tippy Dam, 0.145% of the 5,524 bats that were initially banded lived at least another 18 y—an extraordinary life span for a mammal weighing only 7–10 g (Kurta 2008). Our data are similar to those obtained for little brown bats in northern Wisconsin, where 0.158% of 3,173 marked individuals survived 18 y or more after banding (White et al. 2019).
The little brown bats that we recovered were all males. Similarly, the five old bats found by White et al. (2019) in Wisconsin were male, as was the oldest little brown bat on record, a 34-y-old individual from New York reported by Davis and Hitchcock (1995). Although recapture of predominantly old males at Tippy Dam and other sites may reflect the greater number of males that were originally banded, our results are consistent with long-term survivorship studies indicating that males live longer than females in northern populations of insectivorous bats (Keen and Hitchcock 1980; Hitchcock et al. 1984).
Presence of white-nose syndrome at Tippy Dam was first documented during a routine survey by the senior author in winter 2014–2015 (Kurta and Smith 2014). Evidence included visible fungal growth on many bats, histopathology performed on two dead animals that were collected (D. O'Brien, Michigan Department of Natural Resources, personal communication), and identification of deoxyribonucleic acid (DNA) from P. destructans on swab samples (W. Frick, University of California Santa Cruz, personal communication). Fungal DNA was detected on 18 of 19 bats and on 9 of 12 samples obtained from the walls of the spillway. Discovery of P. destructans on the walls of a hibernaculum often lags 1 y behind detection on the bats themselves (Verant et al. 2018), so it seems likely that the fungus was present in Tippy Dam during winter 2013–2014. Although a survey of the spillway did not occur during the hibernating season of 2013–2014, infected bats were found elsewhere in the state during that winter (Kurta and Smith 2014).
Our data complement those obtained from bats on their summer range, which indicate that some little brown bats survive multiple years despite presence of white-nose syndrome in regional hibernacula. For example, Reichard et al. (2014) and Dobony and Johnson (2019) banded over 2,600 animals at maternity sites in New York and New England after arrival of white-nose syndrome, and they recaptured multiple individuals in subsequent years on the same summering grounds. However, the maximum interval between marking and recapture was about 6 y for five adults, all of which were female. As in our study, the ages of these five bats were not known at the time of original capture. If each animal had been banded as an adult in its second summer, minimum age at recapture would be approximately 7 y—far less than the minimum age of the eight males from Tippy Dam (Table 1).
Neither Reichard et al. (2014) nor Dobony and Johnson (2019) knew where their animals hibernated, but we are confident that our adult males overwintered each year at Tippy Dam. Little brown bats seldom change hibernacula (Humphrey and Cope 1976; Norquay et al. 2013), and this part of Michigan is covered in thick glacial till so that there are no caves, rock crevices, or other alternative hibernation sites within hundreds of kilometers of the dam (Kurta 2008; Slider and Kurta 2011). The fungus remains viable in a hibernaculum throughout summer, even in the absence of bats (Reynolds et al. 2015) and at temperatures of 24°C or higher (Campbell et al. 2020), and no evidence suggests that a site that harbors P. destructans can temporarily become free of the fungus. Hence, it is likely that our banded individuals were surviving during their seventh winter in a site that continually harbored P. destructans.
Tippy Dam is unusual. For example, the substrate is concrete and not natural rock, and the internal ambient temperature is quite cold (0.5–2°C) in midwinter but unusually warm (>20°C) in summer (Kurta et al. 1997). In addition, the assemblage of bacteria living on bats in the spillway is significantly different from the microbiome present on bats in the Upper Peninsula or Kentucky (George 2017). These physical and biological differences are well documented, but whether or how these differences affect survival in the face of white-nose syndrome is unknown (Cheng et al. 2019; Lilley et al. 2019; Auteri and Knowles 2020). Nevertheless, our data from Tippy Dam show that even bats that have lived for more than 2 decades may persist, despite multiyear exposure to the fungus, suggesting that a long life in little brown bats and presence of white-nose syndrome are not necessarily incompatible.
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Reference S1.Ellison, LE. 2008. Summary and analysis of the U.S. Government bat banding program: U.S. Geological Survey Open-File Report 2008-1363:1-117.
Found at DOI: https://doi.org/10.3996/JFWM-20-039.S1 (1.36 MB PDF).
We thank Consumers Energy Corporation for allowing access to the spillway of Tippy Dam and arranging support personnel. Funding was provided by a grant from the U.S. Fish and Wildlife Service (F20AP00002) and a contract with the East Lansing Field Office of the U.S. Fish and Wildlife Service (140F0319P0104). We also thank W. F. Frick and D. O'Brien for arranging analysis of swab samples and histopathology of animals, respectively, that the senior author collected in 2014–2015.
Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Citation: Kurta A, Foster RW, Daly BA, Wilson AK, Slider RM, Rockey CD, Rockey JM, Long BL, Auteri GG, Collins JD, White JD, Kaarakka HM, Redell JA, Reeder DM. 2020. Exceptional longevity in little brown bats still occurs, despite presence of white-nose syndrome. Journal of Fish and Wildlife Management 11(2):583–587; e1944-687X. https://doi.org/10.3996/JFWM-20-039
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.