IESRE Pyranometers and Big Brown Bats

What possible connection could there be between bats and pyranometers? Well, scientists who study tree-roosting bats have long assumed that they choose their roosting sites in tree cavities based on the amount of solar radiation the site receives -- the more solar radiation, the warmer the tree cavity.

This long-held assumption has been challenged by research done by University of Regina graduate student Kristin Bondo, who recently completed her study of the roosting behavior of big brown bats (Eptesicus fuscus) in the Cypress Hills Interprovincial Park in Saskatchewan, Canada.

For this project, Ms. Bondo needed lots of pyranometers in order to monitor solar radiation striking trees from the east, south, and west. So, she turned to IESRE's very inexpensive pyranometer design. Working from a kit supplied by IESRE, she and her university's electronics shop built 25 pyranometers for this project. They are shown here being calibrated at the Cypress Hills research station. A pyranometer from Apogee Instruments, visible on the upper right-hand corner of the table, is used as the calibration reference. The pyranometers are connected to four-channel Onset Computer Corporation U12-series data loggers.

The Cypress Hills bats use cavities created by sapsuckers and flickers high up on the trunks of aspen trees. The first chore is to trap some bats and mount miniature radio transmitters on their backs, in order to track their movements. When bats have settled into their cavities for the day, researchers climb the tree and fasten a net in front of the cavity entrance. Then, when a bat tries to leave they may be able to catch it. The net can't be left in front of the cavity or, of course, the bats wouldn't fly back into the same cavity. This means a LOT of climbing up trees that are often not very healthy, and this is dangerous work that requires special equipment and training. The research site is remote and many miles from medical facilities, so it is no place for a tree-climbing accident!

Bats are furry little mammals, and cute in their own way. But, they must be handled with great care (gloves are an absolute requirement) both for the safety of the researchers and to minimize the stress of capture and of having their fur shaved so the transmitter can be glued on their skin. These insect-eating bats have very sharp teeth. It is possible for humans to contract rabies from a bat bite, but rabid bats are actually very rare.

Ms. Bondo also measured the temperature inside tree cavities. The self-portrait here shows her next to an instrumented (and currently unused) cavity about 10 m above the ground. The wires leading into the cavity are for temperature loggers.

Here is what Ms. Bondo has to say about her research:

"So, from all of my bat studies I have done within the last three years, here is what I think it all means. Although trees receive varying amounts of solar radiation, bats are not switching roost trees every two days to switch between microclimates. There were no significant differences between mean temperature in the trees they used on a day-by-day basis or between the mean, max, min temperature or rate of change between used and available trees. Furthermore, trees on south, west, and east aspects did not differ significantly in temperature.

"Tree cavity temperature is actually most highly correlated by ambient temperature, which is a typical correlation at latitudes above 30 degrees. They are not roosting in trees receiving the highest amounts of solar radiation, as used and available trees did not differ significantly in solar radiation at the cavity entrance or total solar radiation reaching the tree. Trees receiving the highest amounts of solar radiation did not reach the highest tempeatures inside, so orientation of the hole, canopy surrounding the tree, tree circumference at breast height (about 1.5 m), and decay stage most likely influenced small variatons in tree cavity temperatures.

"The one thing different about the trees bats use versus the ones they don't use is that the ones they use contain large cavities. This could be due to small temperature advantages, but also because large tree cavities can house a lot of individuals. They most likely adjust the microclimate inside a tree by adjusting group size. When a cavity is occupied by a group of bats, roost temperature can be raised by 7 degrees. They most likely don't associate with kin when forming sub-groups, because if they were to roost only with their closest kin, at most they would be able to roost with between 1-11 individuals, which is much a much smaller group than what is typically observed in our study area. Bats most likely switch roost trees every two days to avoid roosting in a tree with a high build-up of ectoparasites, guano, and even perhaps ammonia [from their own urine]. They may switch roost-mates every time they switch roost trees to engage in a social network, very similiar to the most basic networks people form, where a weak link between sub-groups allows for the maximization of sharing new information, especally about roost trees (which are constantly falling down)."

Ms. Bondo also observed that, contrary to what has been commonly believed, bats hardly ever sit at roost entrances to "bask" in sunshine falling on the tree.

Do not try this kind of work on your own! Any project involving physical contact with wild animals is potentially dangerous both to humans and to the animals, and should be conducted only by people with appropriate expertise, experience, and equipment.