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  • Writer's pictureORC Intern

Seeing Sounds: Surveying Ongava’s bats

Of all mammal groups, bats are the second most diverse with a whooping total of 1400 species worldwide and no less than 34 species native to Namibia. They provide a lot of ecosystem services, including seed dispersal, pollination, control of agricultural pests and control of disease vectors of human health concern. Yet, bats are also historically understudied compared to other mammals. This is largely because they are not easy to identify when in flight coupled with their nocturnal lifestyle, nor are they easy to capture for a hands-on identification. However, the way bats navigate their surroundings and forage for food has given researchers a different opportunity to monitor and study them.

Bat echolocation

Most bats echolocate by making high frequency sounds in the larynx and then listening for the difference in sound waves that hit objects and those that dissipate into the environment. What’s more, bats emit echolocation calls that are usually specific to the species they belong to. Hence, an easy way to detect and identify the bat species present in an area is simply to deploy acoustic detectors that are capable of recording bat-emitted sounds.

This, in turn, allows us to inspect recorded sounds by means of sonograms, which are just visual representations of sound (see image below), with frequency on the y-axis and time on the x-axis, while intensity/energy (volume) represented by differences in colours. The higher the pitch of the sound, the higher up the y-axis it will be. Longer sounds will appear as longer lines along the x-axis. We convert sound to graphs for a couple of reasons. For bats, this is primarily because we can’t hear most bats. Human hearing is limited to a measly 20 kHz, while the average bat call reaches frequencies of around 60 kHz.

A sonogram of a bats detected at Roland’s waterhole on Ongava.

Using sonograms to inspect the frequency ranges and call shape can determine the species producing these calls by looking at the sonogram recorded. Each species has different minimum frequencies, maximum frequencies and different shapes.

A selection of bat sonograms that show the differences between the search calls of several species of bats. The lower the frequency of the call the more likely the bat is to occupy open space and the less nimble it will be, and the higher the frequency of the call the nimbler a flyer the bat is more likely to inhabit more cluttered areas. Interestingly, each search pulse is emitted on the downstroke of the bats wings to reduce the energetic cost of echolocation, so the faster a bat is flying the more pulses it emits.

Recording bat calls comes with its own unique set of challenges. Whereas bird calls are meant to communicate specific things (mating, food, danger, etc.) Echolocation is the bat trying to “see” the world around it, which results in a simpler call. Despite this relative simplicity, bats that occupy similar niches will likely have very similar calls, making species-specific identification more difficult. These calls are also often emitted at high frequencies (so that bats can get information faster) that don’t carry as far in the environment, making detection at a distance an issue as well.

This is a sonogram showing the same bat's echolocations when going through the searching, approachign and feeding stages. The search phase is characterized by less frequent, evenly spaced, consistent, echolocation pulses. The approach phase, where the bat advances on its prey, and closes the distance, marked by higher frequency of calls and the echolocation pulses being emitted more frequently. Then finally the feeding/capture face, defined by the highest density of pulses, plus a decrease in the frequency of each pulse.

Bats can also adjust the frequency of their echolocation pulses depending on the clutter of the landscape they’re in, the presence of other bats, and whether they are searching for, approaching, capturing or feeding on prey. This allows us to determine when a bat is feeding or simply flying around. Bats can also adjust for the Doppler effect (the change in the frequency of a sound due to the source moving towards or away from you) as they near their prey to increase chances of successfully capturing their prey.

Deploying a bat detector at Suiderkruis water hole.

On Ongava Game Reserve, we placed bat detectors at nine watering holes and at the entrances of three caves. Caves are densely populated roosting habitats during the day. Although, some bats can also roost in woody vegetation or in and around human-made structures. Bats in this environment are drawn to watering holes to feed on the insects that concentrate there and drink by skimming the water surface. A recent study from South Africa even found that bats even use swimming pools at lodges when other sources dry up.

Current range maps suggest that around 20 bat species could be occupying Ongava Game Reserve. Our initial aim is to simply determine how many of these species we are able to document using our acoustic recorders.

Additionally, there are plans to use mist nets to capture (and release) bats in the future to help gather more information and confirm species that might be more difficult to distinguish based on calls alone. Due to their plant-based diet, old world fruit bats no longer require echolocation and have lost their ability to echolocate, and would be missed entirely by acoustic surveys. With intensifying monitoring efforts other aspects of bat ecology can be investigated, such as demographics, habitat selection, seasonality of bat activity, or even the effect of lights on bat activity.

Author: Rhys Medcalfe, outreach intern from the University of Georgia.

Rhys's internship were supported by University of Georgia, United States Department of Agriculture, Georgia Department of Natural Resources

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