To study bats, we first catch them with mist-nets, which are made of a fine nylon mesh that looks like a really fine fish-net stocking. The idea is to put nets where bats aren’t paying close attention to where they’re going (because they’re too busy feeding, for example). This way it’s harder for the bats to notice the nets with their echolocation (aka sonar). The bats then get tangled in the nets and we carefully remove them.
Each of the bats we catch is weighed and their forearms are measured to examine their size and overall body condition. Fatter bats are happier bats, and are in better shape for winter hibernation.
To understand more about the population, we record the sex of each bat we catch. It’s pretty easy to tell males and females apart, just like in humans 😉 Male testes become enlarged close to mating season, which takes place just before hibernation in the fall.
We also age bats. You might be surprised to learn that some bats can live to be 20 or more years. In fact, the record is 44 years! One way we figure out their age is by looking at their knuckles; juveniles (born this year) have only cartilage in their knuckles that is gradually replaced by bone as they grow over the summer. The cartilage will appear as a ‘gap’ when light shines through the knuckles.
The amount of tooth wear on bats also gives us a rough estimate of age. Older bats have more tooth wear, which is caused by years of munching on crunchy insects.
Each bat also gets a microchip (or PIT-tag), just like many pets have these days. The chip is 12mm long (about the size of a grain of rice); the chip is injected under the skin between the shoulder blades. Each chip has its own unique barcode. When we recapture a bat, we can scan it with a reader much like scanning groceries at the checkout. These chips let us identify individual bats, which lets us look at population size and track the health of each bat over the summer.
The microchip IDs can then be used to look at social structure. You see, big brown bats living in natural roosts (where they rest during the day in trees and rock crevices) move every two or three days, but they don’t all move at the same time or to the same roost. By putting up a scanner at roost exits, we can record who comes and goes out of the roost, as well as when they come and go. This tells us which bats spend more time together in the same roost so we can figure out if they have ‘best friends’ of sorts. This sort of information can then tell us how information might be shared among group members.
A few of these bats are outfitted with radio-transmitters. Just like different radio channels that you can tune in to listen to your favourite song, each transmitter has its own unique frequency.
We then use a receiver to tune into that frequency and an antenna to track the radio signals to locate the bats’ roosts and their social groups.
We also track the bats at night to figure out where they are feeding, and when they go back to bed.
We also use bat detectors to figure out where bats are feeding. Bat detectors record the echolocation calls (aka biosonar) that bats make and turn them into sounds that humans can hear. Each bat species makes different sounds; using special programs, we can also look at the sounds the bats make to figure out what kinds of bats are feeding in the area.