The largest flying fox colony on earth

palm fruit bats

Palm fruit bats in Kasanka National Park in Zambia: The world’s largest colony of fruit bats is located there. © Christian Ziegler/ MPI for Animal Behavior

Once a year, a group of trees in Zambia's Kasanka National Park becomes the scene of an impressive natural spectacle: fruit bats from all over Africa gather in this little forest and form the largest colony of fruit bats in the world for three months. But how many of these bats, which are essential for African ecosystems, come together there has so far been a matter of debate. A team of biologists has now counted the flying foxes using an AI-supported method.

The palm fruit bat (Eidolon helvum) is one of the most common mammals in Africa. With its wingspan of up to 76 centimetres, this representative of the bats is hard to miss. The flying foxes mainly eat fruit and therefore play an important role as seed dispersers for many plants. They thus make a particularly strong contribution to the natural reforestation of deforested areas. This is all the more true as the fruit bats are seasonal migrants: they can travel more than two thousand kilometers to avoid the dry season.

Mass gathering in Kasanka National Park

"Palm fruit bats are Africa's secret gardeners," explains Dina Dechmann from the Max Planck Institute for Animal Behavior in Radolfzell. “They connect the continent in a way that no other seed spreader does. A loss of the species would be devastating for the ecosystem. So we urgently need to know if the population is declining.” One way to tell is to count the size of fruit bat colonies. Because at certain times of the year, these bats gather in large groups and then form a gigantic fruit bat colony for a few months. One of these colonies forms every November in a grove in Kasanka National Park in Zambia.

This annual gathering of fruit bats is considered to be the largest colony of fruit bats in Africa. But so far scientists have not been able to count the number of animals in this colony. Estimates vary between one and ten million. That is why Dechmann, first author Benjamin Koger from the MPI and their colleagues have now developed and tested an AI-supported, automated method for this. "We were concerned that automated counting would be prohibitively expensive because it takes sophisticated technology and a lot of equipment to monitor a colony as massive as Kazanka's," says Koger.

census for flying foxes

But as it turned out, very simple equipment was sufficient for the census of palm fruit bats: The team set up nine GoPro cameras around the colony at regular intervals and filmed the fruit bats when they left their sleeping places at dusk. Koger then trained deep learning models to recognize and count the animals in the videos. Comparisons with manual sample counts showed that the artificial intelligence managed to record around 95 percent of the flying foxes. The method provided reliable results even in the dark. "Our results show that the colony's flying foxes can be identified and counted very precisely with inexpensive cameras and our software. This is enormously important for the future surveillance of the colony,” says Koger.

To determine the total number of fruit bats, the biologists repeated their automated counting method for five nights. The result: the colony of fruit bats in Kazanka consists of 750,000 to 1,000,000 animals. This makes it the largest bat colony in the world in terms of biomass, the team reports. Nevertheless, these values ​​are significantly lower than earlier estimates. The researchers suspect that at the time of their counting, perhaps not all fruit bats had arrived from their migrations. However, it is important that the new method allows the count to be repeated at any time: "We now have an efficient and reliable technique for monitoring animals over a long period of time - a major step forward for the preservation of large populations," says Dechmann.

(Video: Wild Animals)

Kasanka Colony is essential

This is particularly important for the fruit bats, as the team explains. "With so many individuals, it's easy to think that the loss of a few animals isn't significant. But if we want to preserve the ecosystem services of fruit bats, we have to keep their populations at a high level," says Dechmann. And the Kasanka colony is not just one of many, but a collective colony of flying foxes from the entire subcontinent. "The loss of this colony would be devastating for the whole of Africa," emphasizes the biologist. Because this colony is threatened by agriculture and habitat loss, precise and repeated control is particularly urgent.

Source: Max Planck Institute for Animal Behavior; Specialist article: Ecosphere, doi: 10.1002/ecs2.4590

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