Twinkle, twinkle, little bat!
How I wonder what you’re at!
Up above the world you fly,
Like a tea tray in the sky.
Lewis Carroll, Alice in Wonderland
Replace “tea tray” with “seed disperser” in the quote above, and we have an appropriate ode to the fruit bat. A stalwart among fruit bats is the straw-colored fruit bat (Eidolon helvum), which can disperse seeds over long distances and live in colonies of a few thousand to several million bats. Once widely distributed in Africa, its declining populations have placed this species in the “near threatened” category on the IUCN Red List of threatened species.
Already faced with challenges, such as habitat loss, bushmeat hunting, and bad press due to associations with infectious diseases, the ongoing COVID-19 pandemic has also contributed to the misguided persecution of bats in parts of the world.
Tanzania is one of the countries that the straw-colored fruit bat calls home, and while their movements had been studied in other countries, little was known about their flying activities in this country. We therefore set out to place GPS tags on these tree-roosting bats from two colonies in Tanzania: one in the Morogoro Municipal, and the second in the Kilombero District near the Udzungwa Mountains.
Given that we didn’t know their migratory patterns there, we played it safe and used two types of tags: high resolution solar e-obs loggers with local data download, and lower resolution satellite tags which enabled us to collect GPS fixes remotely. The former gave us large amounts of GPS fixes from tags that recharged when exposed to sun rays, and the latter provided us with information even if a bat didn’t fly back to the roost to enable us to download tag-data.
One of the 25 tagged bats, K5310, flew upwards of 96 km on two successive nights, and although the average nightly flight distance was 25.25 km, this particular bat covered 55.83 km in one hour itself. A unique ability of the e-obs tags was the capture of acceleration data. This enabled us to classify the GPS fixes into flying or not-flying activities, and subsequently fixes associated with foraging areas/feeding roost locations.
You can find a video here on the Bat K5309’s tracks.
Based on this information, we could ascertain that the majority (46%) of these foraging-activity locations of bats tagged in Kilombero were in or within 100 m of urban built-up areas, while 32% of these foraging-associated fixes were in protected areas (Udzungwa Mountains National Park and Mikumi National Park). The movement patterns of these bats also showed that while they individually foraged in different locations, they tended to visit the same areas on subsequent nights.
As the natural food sources for bats decrease due to habitat loss due to humans, it is unsurprising to observe them foraging on crops grown by humans. Our study period coincided with the mango harvesting season, and conversations with community members as well as GPS data pointed out that these bats enjoyed feasting on mangoes. The movements of one bat led us to the horticulture garden in Sokoine University of Agriculture, Morogoro, where we set up camera traps and identified species frequenting this foraging site.
Out of all the species observed via camera-trap images, vervet monkeys were seen directly handling fruit discarded by bats. Fruits contaminated with viruses shed via oral secretions might be a possible route for disease transmission, and this pathway warrants further investigation. Currently, there no known human outbreaks associated with bats in Tanzania.
The data above demonstrate how far Eidolon helvum bats can fly as they forage each night, and indirectly highlight the large distances across which they can disperse seeds. Faced with increasing land degradation and forest fragmentation, our world needs more of these mammals to connect fragmented landscapes and regenerate forests in degraded landscapes.
Even though bats are associated with viral zoonoses, such as Hendra, Nipah, Marburg, Ebola, and SARS-related coronaviruses, they do not harbor more human-infecting viruses than other mammals when species diversity is taken into consideration. Furthermore, the drivers for virus spillover to humans are mainly human activities associated with wildlife exploitation and losses in wildlife habitat quality.
Prior to conducting this study as a PhD student, I did not realize how multifaceted and wonderful bats actually are. It’s easy to be in awe of charismatic wildlife, such as tigers, pandas, and elephants, even though being charismatic does not protect against extinction.
Bats, however, are unique with respect to how much they contribute to our ecosystems, even as they are generally overlooked and vilified by humans for outcomes resulting from human behavior itself.
Ultimately, their vital ecosystem services should be only one of the reasons for their conservation, as they are fascinating creations in their own right.