Natures Wonders: a bat migration (Introduction)

by David Turell , Friday, November 29, 2024, 21:02 (14 days ago) @ David Turell

Fruit bats descend in Zambia:

https://www.smithsonianmag.com/travel/the-worlds-largest-mammal-migration-is-taking-pla...

"Straw-colored fruit bats (Eidolon helvum) arrive at Kasanka, in Zambia’s Central Province, from various corners of the country and continent. They settle into the cool, swampy forest waiting for nightfall so they can arise en masse and turn the surrounding farmlands and woods into an all-you-can-eat fruit buffet. At dawn, the nocturnal mammals return to the forest to sleep off their meals,

"Witnessing the bats in flight is a mind-blowing display of nature that truly has to be seen to be believed.

"This is the Largest Mammal Migration in the World.

"The so-called megabats, which look like flying foxes, fly in from the Democratic Republic of Congo, parts of Zambia, and sometimes Tanzania and Malawi, coming each year to fill the woods with their chirps and hang off the trees like icicles. They typically start drifting into the park around October and leave in January, though local guides say this year the first bats came on September 26, perhaps due to the country’s ongoing drought or the ripple effects of climate change.

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"The straw-colored fruit bat is the only long-distance migrating fruit bat on the continent, and it’s believed that bats from several different colonies meet at Kasanka. As for why the bats come to Kasanka specifically every year, it’s a bit of a mystery. Scientists know that these straw-colored fruit bats aren’t congregating in Kasanka to breed, as they likely breed in Mozambique and Tanzania.

“'In terms of understanding the reasons for migration—the tendencies for migration for bats—we don’t have a whole lot of background in it, but we believe they come for the food,” says Teague O’Mara, an expert on bat movement and behavior and the director of conservation evidence at Bat Conservation International.

***

"But if they’re here for the buffet, O’Mara isn’t sure why the bats come to Kasanka specifically from October to January, because the fruit is ripe and available for eating at other times of the year, too.

"Straw-colored fruit bats play a vital role in the ecosystem. “They’re the best seed dispersers,” says O’Mara. “They really are Africa’s gardeners.” Since the bats love fruit, they eat as much as they can, digest the seeds and then leave them in their wake as they fly. “They’re depositing seeds in places that no other seed disperser will go. They have the potential to restore a lot of economically important timber species,” he adds. And yet several factors are contributing to a decline in their population, which numbers somewhere in the millions. Their habitat is being threatened by deforestation, with humans cutting down their roosts for farms and houses. Zambia reportedly has one of the highest rates of deforestation around the globe, which harms the bat population. People have also been known to eat the bats. They are currently listed as a near threatened species by the International Union for Conservation of Nature.

"The loss of straw-colored fruit bats would have both ecological and financial implications. That’s because a colony of 100,000 or so bats can have a real-world financial impact. That one colony “can put about $800,000 worth of economic value into the economy every year,” says O’Mara. “And that’s just by doing what they naturally do.”

***

"One of the reasons that scientists haven’t been able to track the migratory patterns of the bats closely is due to a basic tech problem. “We don’t have GPS tags or other tags that can last an entire year,” O’Mara says.

"Another mystery is why the bats don’t live in Kasanka year-round. “We see, particularly in places where there’s less seasonality across Equatorial Africa and Accra [Ghana], that there are populations that have big influxes of migratory animals, but they’ll have a stable population throughout the rest of the year,” says O’Mara. “That doesn’t happen in Kasanka. Once January hits, all the bats are gone.”

"Scientists, however, are slightly more confident about where bats go when they leave the national park. “We’ve seen them going into Congo. We’ve seen them going out into Malawi and start heading north into Tanzania. And then we’ve tracked one that has gone through Congo and up into South Sudan,” says O’Mara. On their travels, the bats are mostly looking for fruit and for each other. “They’re highly colonial,” he says. “They love to be around each other. They always tend to roost in really big aggregations, at least as far as the ones that we can find.”

***

"They determined that around a million bats were roosting in Kasanka at peak season in November 2019. O’Mara and the guides at the park think that number is far too low. “Just use your eyes,” says Siame, who watches the bats twice a day nearly every day when they are in the park. “That is way more than a million bats.” The guides believe the true number of bats is on the higher end of what even O’Mara cited, which was a number between 10 million and 12 million bats."

Comment: this looks like a learned behavior, but if the fruit is there year round why do they leave? Perhaps it is all instinct, but the studies are too early to give us an answer.