How Disturbance Harms Hibernating Bats

How Disturbance Harms Hibernating Bats
By Merlin Tuttle

More than 30 years ago, biologists began to realize that bat populations were declining at alarming rates where they were disturbed during hibernation. It is normal for bats to periodically arouse from hibernation. However, additional human-caused arousals can cause bats to exhaust limited fat stores, forcing them to starve before spring.1-2

Little brown myotis (Myotis lucifugus) hibernating in an abandoned mine in Wisconsin. At such northern latitudes, they may hibernate for as much as 7 months, living off of stored fat reserves. Their fur is covered in condensed moisture droplets.

Controlled laboratory studies have documented the extremely high cost of arousals from hibernation and the severe impact of disturbance. Thomas et al.3 found that each arousal of a little brown myotis (Myotis lucifugus) cost 108 mg of fat, an amount equivalent to that required for 68 days of torpor. To measure the cost of arousals forced by human disturbance, Thomas4 studied little brown myotis equipped with temperature-sensitive radio transmitters in a Canadian mine. They aroused naturally at roughly 15-day intervals, followed by an average of five hours of elevated body temperature. However, following arousals caused by handling, they maintained elevated body temperatures for 14 hours. Also, following such arousals, the time spent in hibernation prior to the next arousal was significantly shortened, from the normal 15 days to just five, an added cost equivalent to that required to support 3.3 natural arousals or 50 days of uninterrupted hibernation.

In a second study Thomas5 installed passive, infrared motion detectors in a mine to determine the extent to which hibernating little brown myotis are sensitive to nontactile stimuli. The motion detectors tracked bat activity following passage of one or two humans, equipped with dim headlamps, scanning solitary and clustered bats as typically done during population counts. In striking contrast to previous beliefs, he found that flight activity increased dramatically after a human intrusion, with peak activity sometimes occurring as much as 7.5 hours later.

Thomas concluded that there is a “cascade effect” in which only a few bats initially respond. Then males attempt to copulate with still torpid females, arousing them, followed by even more arousals being triggered as these bats fly around vocalizing and disturbing still more when they attempt to reinsert themselves into clusters. He found increased activity for up to 24 hours.

He further noted that juveniles normally arrive with lower fat reserves, and suffer high first-winter mortality, implying little energetic flexibility. Well before the advent of WNS, he cautioned that “To avoid increased mortality due to the premature depletion of fat reserves, human visits to hibernacula should be kept to a minimum.”5

In my experience, disturbance appears to cost even more than can be documented by the increased frequency or length of arousals. In southern caves or mines that are too warm for hibernation without entry of cold, winter air, disturbance often forces bats to move into warmer, inner roosts where metabolic rates rise. That saves the cost of extra arousals, but may substantially increase metabolic rates, still reducing the odds of survival till spring.

Aware of such problems, recovery teams for both gray (Myotis grisescens) and Indiana (Myotis sodalis) myotis scheduled winter counts to occur no more than once every two years. Based on subsequent observations, I now suspect that both species would have benefited substantially from being surveyed at no more than three-year intervals.

A tightly packed cluster of endangered Indiana myotis hibernating in a Kentucky cave.

Furthermore, having now seen some surprising natural closures and openings of caves and mines, I would urge installation of temperature measuring dataloggers at roosts to track potentially threatening changes. These should be downloaded and gates checked for vandalism each summer. Without such careful monitoring, undetected changes have the potential to reverse decades of progress within just one or two years.

Illustrating the apparent importance of protection from disturbance, the hibernating gray myotis population of Pearson Cave, in Tennessee, remained at approximately 100,000 for some 30 years prior to installation of a protective gate in 1989. By the next survey in 1992, it had grown to 175,000. Then it was not surveyed again till 2002. In that 10 years, free of any disturbance, most of the bats relocated from inner roosts with mid-winter temperatures of 7 – 9 º C to outer ones at 0 – 2 º C, likely resulting in substantial energy savings. When next surveyed, the population had jumped to more than 365,000, the fastest known recovery of a hibernating population.

There are several sources of potential confusion for inexperienced observers. At a few tour caves, where managers ensure complete protection from vandalism, some bats have learned to ignore human passage, something they can’t afford to do elsewhere. Also, disturbed populations may take from one to seven or more hours to fully arouse, following a disturbance. Others whose energy reserves are already dangerously depleted, may opt to ignore disturbance in a desperate effort to survive. Finally, small remnant groups that occupy ideal nursery roosts near enough to avoid the cost of migration, may simply bear the cost of a limited number of extra arousals. That does not mean that bat numbers couldn’t be greatly increased with protection and/or restoration of ideal conditions as we demonstrated at Pearson Cave.

At a time when WNS is forcing increased arousals and high mortality due to premature exhaustion of limited fat reserves, every possible precaution must be taken to minimize disturbance and restore the best possible hibernation conditions.

A tri-colored bat (Perimyotis subflavus) hibernting within 10 feet of passing tourists in a Texas cave where this exceptionally tolerant species is strictly protected and apparently has learned to ignore passing tours. This one is covered in condensed moisture.



  1. Daan, S. 1973. Activity during natural hibernation in three species of vespertilionid bats. Netherlands J. Zool., 23:1-77.
  2. Brady, J., T. Kunz, M.D. Tuttle, and D. Wilson. 1982. Gray bat recovery plan. U.S. Fish and Wildlife Service.
  3. Thomas, D.W., M. Dorais, and J. Bergeron. 1990. Winter energy budgets and cost of arousals for hibernating little brown bats, Myotis lucifugus. J. Mamm., 71(3):475-479.
  4. Thomas, D.W. 1995a. The physiological ecology of hibernation in vespertilionid bats. Symp. Zool. Soc. London, 67:233-244.
  5. Thomas, D.W. 1995b. Hibernating bats are sensitive to nontactile human disturbance. J. Mamm., 76(3):940-946.