Bats: Innovative Survivors
By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester
The bats at Tryon Creek State Natural Area (TCSNA) are one of our least seen, yet most interesting, animals. Their lifestyle is so different than ours that it’s no surprise our paths rarely cross. Interestingly, 25% of all species of mammals in the world are bats. The lifestyle of bats is very challenging, and they use some very unusual strategies to be successful.
Bats are the only flying mammals. As flying mammals, bats use enormous amounts of energy. Bats in some parts of the world eat fruit, drink nectar, eat pollen (and some even drink blood), but our TCSNA bats eat insects exclusively. This food is only seasonally available. Bats do not store food like some squirrels. And some of our bats are among the smallest mammals.
How do bats make it?
One of the challenges facing bats is that some of them are very small. Our little brown bat (Myotis lucifugus) weighs ¼ to ½ an ounce. Our native Douglas squirrel (Tamiasciurus douglasii) weighs approximately 20 times as much. The weight of an animal is very important because small bodies have a much larger surface area per unit of weight and lose heat a lot more rapidly. To demonstrate this, I ran an experiment.
Pictured below are two plastic containers which are filled with water dyed blue to make them more visible. The thickness of the walls of these two containers is identical.
I simultaneously filled them both with hot (132° Fahrenheit) water. I put on their lids and placed them in the freezer. After 40 minutes I took their temperatures, which are shown below.
The water temperature in the smaller container fell 70° F, while than the water temperature in the large container only fell 48° F. This is because the ratio of the surface area of the container to the volume of the container is 3.0 for the small container and 2.6 for the larger container. The larger relative surface area of the small container caused it to cool more quickly. Likewise, the small bats get colder much faster than larger mammals.
The way animals keep warm is to “burn” food. To save energy, when bats rest they let their body temperature drop to within a degree or two of their environment. When the bats let their body temperature drop they enter a state called “torpor.” This is an inactive state that is much deeper than human sleep. Some scientists believe torpor is different from hibernation only in depth of rest. Bats oftentimes enter torpor on a daily basis for a few hours. During torpor or hibernation, the bat uses less than 1/40 as much energy as it does when it is active. However, when the air temperature gets to just a few degrees above freezing, the bat will arouse itself and exercise muscles sufficiently to warm itself up.
Where do bats rest?
When not flying around feeding, the bats are in “roosts.” Most commonly we think of them roosting somewhere during the day while they’re waiting for their night-time feeding frenzy. These are called “day roosts.” These roosts keep them warm, and hide them from predators.
However, it turns out that bats only feed for a few hours each night. Bats can sleep up to 19 hours a day! The big feeding frenzy for bats is at dusk for a couple of hours, after which the bats rest a bit. At least for many bats, there is a second “meal” in the pre-dawn hours. During their inactive night hours they stay in a separate “night roost”. People who study bats think this might be to keep the day roosts more secret by reducing the fecal deposits near their day roost.
The day and night roosts can be as simple as hiding in an attic or under a bridge. In the more natural world, bats might roost behind a chunk of loose tree bark, or in a hollow tree, such as these examples at TCSNA. The hollow in the tree below is 10 inches deep, plenty of room for some little brown bats!
Bark crevices are also good roosting spots. Bats prefer Douglas-fir (Pseudotsuga menziesii) over pines (Pinus spp.) or true-firs (Abies spp.), probably because of the bark. Older Douglas-fir have a greater tendency to develop nicer protective bark crevices than most other species.
Winter Homes: Hibernacula
Bats’ third home is their winter home where they hibernate during the insects’ “off-season.” A Portland area survey showed a sharp decline in the number of active bats between August 6th and September 23rd. This was probably due to declining insect numbers and cooler weather, a time when bats might start thinking about hibernation. Bats oftentimes hibernate in caves, or for some species of bats, barns, sheds or attics. These places are called “hibernacula.”
Regrettably, the location of the overwintering hibernacula for the bats inhabiting TCSNA in the summertime are unknown. The best general information we have is from a study* of wintertime collections at museums, and a survey of 650 caves plus 70 buildings that occurred in Oregon and Washington from 1982 through 1989. Combining the results for the 1980s study and some museum collection records, the closest overwintering location for little brown bats was in Washington County near Sherwood. For long-legged myotis (Myotis volans), the closest overwintering individuals were located in Gladstone. The closest winter home of big brown bats (Eptesicus fuscus) was near Corvallis. The closest wintering locations of the California myotis (Myotis californicus) were near Gladstone and Vernonia.
It is important to note that this nearly decades-long study only found a total of only 174 overwintering bats. With the exception of the long-legged myotis, all the overwintering bats were found either singly or in groups of less than ten. This is in sharp contrast to some other areas of the country where overwintering bats occur in groups of hundreds to million in a single cave. However, it does demonstrate that at least some of our bats spend the winter in this area.
While long winter hibernations are the rule, California myotis and silver-haired bats (Lasionycteris noctivagans) (both species reported at TCSNA) have been observed feeding between November and February near Olympia, Washington.
Picking out a roost
Several reports indicate that the absence of suitable day roosts is an important factor limiting the bat population. Long-legged myotis, a species reported at TCSNA, was the subject of a study in the Oregon Cascades east of Springfield. Sixteen radio-tagged female bats were tracked for 8 days. In this period, the bats used a total of 41 different day roosts scattered over 12 square miles of forest. Of these, 1 roost was in a rock crevice, 4 were in green trees, and 36 were on, or in, snags (a dead, or nearly dead, standing tree).
This study also found that different bats from a common night roost frequently used different day roosts. An individual bat might use the same day roost repeatedly, or switch day roosts every couple of days.
Two of the key factors in selecting roosts seemed to be the presence of bark on the snag, and a snag being taller than the trees immediately around it. The tall snags towering above the rest of the forest are in fall sun all day, and thus are probably warmer places for the bats to roost.
Different species of our bats here at TCSNA have different companionship preferences. While the little brown bat might roost in colonies up to several thousand, silver-haired bats are solitary roosters, except when a female is with her young.
What do bats eat?
The bats at TCSNA all eat insects. What kind depends upon the bat, of course, but bat food includes flies, midges, leaf-hoppers, gnats, moths, caddisflies, beetles, mayflies, wasps and mosquitos. Adult bats can eat up to 1000 to 1200 insects per hour. That’s about 1/3 to ½ of the bat’s body weight. This is the equivalent to an insect every 3 seconds! Yikes! That’s a lot of chasing, but bats sometimes fly into swarms of insects which probably helps a lot. Interestingly, bats can catch insects in their tail or wings, not just their mouths.
When I first read this, I thought that adding that much body weight each night would make the bat too heavy to fly. Never fear, the bats have solved this problem. Bats can complete the processing of “food to feces” in 35-54 minutes!
Life cycle of bats
Males inseminate the females in the fall. However, the semen is held within the female, and the eggs are not fertilized until spring. The babies are born in June or July. Baby bats learn to fly within 3 weeks. Little brown bats live an average of 6 to 7 years, but the record is 31 years. Other species live an average of up to 20 years.
Bats are amazing creatures that play a vital role at TCSNA. As our forest naturally ages, there should be a greater number of snags providing more roosts. This could mean the bat population grows. Hopefully, these fascinating little critters will be with us for many generations to come.
*Perkins, J. Mark, John M. Barss and Joshua Peterson. 1990. Winter Records of Bats in Oregon and Washington. Northwestern Naturalist 71:59-62.