Following Flying Fruit Foxes

The Swarm

Every evening, I sit reading on a west-facing deck, glancing up now and then to check the sunset, birds, weather. Just when temple bells ring out sundown, I often notice huge fruit bats passing in substantial numbers, flying east from daylight, heading out for nighttime forage. They emerge, I surmise, from massive Rajagiriya and Kotte marshland roosts, soar over Battaramulla, then funnel where I see them up close through the Defence Ministry exercise paddyfield, free of trees and buildings. Into the Malabe murk they proceed onward to where patches of favourite fruits proliferate.

Capable of logging 60 kilometres in a nightly forage, some may flow between riverbanks when they reach the Kelani, following it easily to Hanwella and beyond.

On an evening this past May, something astonishing drew my eye. First, I observed the seemingly normal bat emergence and dove back into my book. But when I glanced up again, an enormous swarm of fruit bats blackened the sky out there in the middle distance. For at least twenty minutes after I first noticed, they flew so thick I couldn’t pick out individual bats in mid-pack. Instead of bat silhouettes against greying cloud cover, the background was black with multitudinous bats. They proceeded to their usual paddyfield funnel a couple hundred metres from where I sat. Many flapped through the space in between, some passed over the house, and a few came spookily close to my deck before veering off. I must have seen tens of thousands of bats.

A comparable unforgettable moment came years ago at Kandy’s Mahaveli Reach hotel. As we sat riverside at dusk, a stupefying fruit bat swarm came flapping downstream, some just above the water, others a few metres up. In their tens of thousands, they swooped almost close enough to touch and took half an hour to pass, stragglers straining to catch up.

As many as 25,000 fruit bats snooze days away in Kandy’s Peradeniya botanical gardens, dripping upside down and close-packed in tall ancient trees just by the river. At dusk, they loosen claws on their branches and, dropping down into a glide, gain speed for powered flight. They can reach Victoria Reservoir easily in an evening’s forage. Word on the street holds that they flap back to the botanical gardens and other riverine roosts every dawn. This has supposedly been documented since the 1880s. They chatter, bicker and jostle for high positions because dominant bats up top will be safest from tree-climbing predators and will fly out earliest at dusk. Lower-downs must wait.

Based on such home-every-dawn lore, I half-expected the Swarm over Pelawatte to repeat the next evening. That did not happen. I saw a few hundred bats go by but nothing near the numbers and density of the Swarm. With that dawn-returnhome scenario still in mind, I wondered. Could they feel sated and prefer rest after gorging last night? Predict slim pickings after a huge harvest? Select foraging routes alternate to a picked-over buffet?

The second night after the Swarm, even fewer bats. On the next four nights, basically no bats at all! The Swarm began to look like a one-off flapdoodle. (Sorry!) It evidently sprang from some singular trigger. That would make sense since I never saw one here before.

The Swarm followed many weeks of intense drought in Colomboland. The drought broke into a downpour on the day before and the day of the Swarm. Drought retards fruit ripening. Trees conserve scarce water for essential metabolism, leaving less H2O to support fruit maturation. Conversely, downpour accelerates ripening. Flying foxes smell ripening fruit from many kilometres away. Voilà le Swarm!

Moreover, heavy cloudburst raindrops cause cracking and splitting in fruit. Split fruit yields accelerated fermentation due to increased surface area, microbial access through broken skin, and release of juice to microbial cultures. There’s no reason to think bats don’t enjoy the fermented stuff. Could the Swarm signify a gigantic race to Happy Hour?

Studies suggest that fruit bat navigation suffers little from inebriation, at alcohol levels far beyond what would disqualify human auto drivers. Alcohol tolerance may broaden bat dietary options, thereby conferring evolutionary advantage.

Fruit bats carry large brains for their body size with seemingly good memory. They forage sometimes in ‘trap-lining’ technique: re-visiting sites to check for ripeness. Their companionability shows ‘fission/fusion’ patterns akin to dolphins, where large groupings break into various smaller clusters that later merge back into bigger ones.

Aside from companionship, dense roosting fosters mutual warning of predators: owls, hawks, eagles, falcons, snakes, jungle cats and leopards.

Contrast between Kandy and Colomboland may illustrate different positions on a fruit bat spectrum from ‘roost fidelity’ to ‘nomadism.’ Kandyan bats display exceptional roost fidelity in their mass centralised dawn return to Peradeniya gardens. This does not mean that every bat departing the gardens at dusk returns there at dawn, only that a vast majority does, perhaps with some strays who weren’t there the day before.

Colomboland bats may lean more towards ‘nomadism,’ utilising various shifting, decentralised roosting sites near bounteous buffets in hinterlands. Why fly back to Colombo (especially if tipsy) when sitting right here on sumptuous smorgasbords? Fruit bat gregariousness may spawn ad hoc slumber parties: ‘Bunk down here together, shall us?… Sleep tight… Love you all…

The Bat

One of the largest bats on the planet, Lanka’s fruit bat goes by the name ‘Indian Flying Fox’ for its puppyish face: cute! As with many dog breeds, long snouts enhance sense of smell. It weighs up to 1.6 kilograms with wing spans reaching 1.5 metres. Such size and weight require huge energy outlay to stay aloft and push through air. To sustain that output, they must eat roughly a third of their body weight every day. That entails strenuous commuting: large lungs, enormous oxygen consumption and heartbeat rates staggeringly high.

They feed on flowers, pollen and nectar, along with a suite of fruits, notably mangos, guava, banana and seed pods. They especially love figs. In a pinch they will also choke down insects, leaves, bark, cones and twigs.

As fruit eaters, the bats foster seed dispersal through poo. Digestion and excretion come quickly so that they can continue flying without carrying excessive weight. Fig seeds may germinate better for passing through them, like seagrass seeds through sea cows or coffee beans through civet cats. As nectar sippers meanwhile, fruit bats pollinate flowers. These seed dispersal and pollination roles earn them status as ‘keystone species’ in forest maintenance and rejuvenation.

Don’t nominate them for sainthood though. They can devastate a fruit farm that hasn’t protected itself with netting over its canopy, and they may harbour diseases dangerously transferrable to humans.

Despite their close-packed outward flocking, they wind up foraging singly at harvest. Outsized eyes packed with light-sensitive rods give them superb night vision, which combines with dog-strength sense of smell for detecting what’s delectable. Close-set and forward-facing, those eyes provide excellent binocular vision for judging distance accurately.

Some scientists think they wield still another ocular talent: ultraviolet sight. Ripe fruit and flowers, both reflecting and absorbing ultraviolet light, may highlight themselves against backgrounds of foliage, attracting foragers and pollinators in dim dawn and dusk. Alternating reflection and absorption can create ‘bullseye’ patterns leading bats directly to ripe fruit and accessible nectar.

Finding an alluring tree, bats flap and glide around, triangulating on fruit and other foodstuffs within. Once inside, they clamber through branches, tugging them with specialised hook-shaped thumbs at the base of wings formed by webbing across their other phalanges. Bon appétit! Specialised feet lock onto branches reflexively with scant muscular exertion, which is also how they can slumber all day from branches, upside down.

Many readers will already know this next flying fox fun fact: like most fruit bats, they cannot echolocate.

The Past

Echolocation proves super useful in chasing down darting prey on the loose: insects for bats, fish for dolphins. But it confers no advantage in locating stationary food sources. Fruit bats use eyes and nose to find fruit and flowers.

Likewise, filter-feeding baleens like Lanka’s blue whale find large and momentarily stationary krill balls without echolocation and plunge right through them with jaws agape. They find their relatively stationary prey balls by tracking wind and current patterns to locate cold-water upwellings supporting krill blooms. They also listen for sounds of diving birds and ocean predators feeding on krill.

With fruit bats as with baleen whales, a question: did they forsake echolocation or did they never acquire it? Consensus on whales holds that echolocators and filter feeders separated early and then perfected their divergent feeding techniques. But a minority view holds that filter feeders evolved away from echolocators as climate changes favoured upwellings with small-creature blooms. They perfected filter feeding and stopped wasting energy on expensive echolocation apparatus.

With fruit bats, consensus favours the equivalent of the minority view on whales: echolocation first, evolution away from it later. Again, however, consensus encounters dissent.

Proto-bats emerged soon after the great dinosaur extinction some 66 million years ago (mya): great tree-climbers and sometimes gliders with rudimentary flight, eating insects and other small prey. They lacked enlarged inner ears (cochlea) and small middle ear bones needed for high-frequency echolocation. By 50 mya, however, at least some had acquired powered flight.

Recent fossil findings suggest that echolocation structures requisite for chasing bugs on the wing emerged only later. Early known fruit bats appear in the fossil record only around 20 mya. Maybe they emerged from echolocators sometime previously or right around then.

Or maybe they evolved back before 50 mya into a lineage separate and distinct from future echolocators, with their existence over the 30 million intervening years scientifically invisible now and perhaps forever.

If fruit bats indeed arose before that 30-million-year interval, their habitat disfavoured fossilisation. Fossilisation requires slow decomposition and quick burial by sediment, preventing oxygen-fuelled decay and promoting mineralisation of remains.

By contrast, in forests where fruit bats would have thrived, decomposition comes rapidly from bone-dissolving acidic soil and microbial digestion, along with scattering from scavengers and insects: all bad for leaving recognisable fossils.

If non-existent fossils cannot sort this controversy out, perhaps we can try embryology? A 2017 study finds that fruit bat embryos feature enlarged cochleas like echolocators and that this ‘vestigial developmental stage’ indicates ‘past echolocation capabilities.’

Unfortunately, this does not settle matters. A 2021 study concludes that fruit bat embryos show ear bone development closely akin to non-echolocating mammals like shrews. This indicates that today’s fruit bats never went through an echolocating stage before abandoning it.

Intuitively, this seems the more plausible position. When airborne bats emerged prior to echolocation, trees of the time harboured fruit and flowers. Why wouldn’t non-echolocating bats seek and thrive on them right then and thereafter through millions of generations until now?

The Future

In the overall Indian subcontinent, flying foxes rate as ‘near threatened,’ primarily due to habitat loss. One study projects a 25-30% population drop over a 2012-2037 period, which we now sit halfway through. Forest depletion and fragmentation stem from well-known drivers: conversion to cropland and plantations (rubber, palm oil); urban expansion; roads, dams and mining infrastructure; commercial and illicit logging; ongoing slash-and-burn agriculture.

Roosting and feeding sites both decline with all this, of course. Forest sustainability and regeneration suffer in turn from loss of fruit bat pollination and seed dispersal. Worrisome also is low and slow fruit bat reproduction: only one pup per year from fecund females. They reach sexual maturity slowly.

No storybook romance, mating typically involves the male chasing a reluctant female, cornering her, and holding her fast for a few mercifully brief moments. Both males and females wind up with multiple partners during mating season. This may diversify gene pools but does not raise birth rates.

Worldwide trends seem to match India’s grim picture. Out of nearly 60 flying fox species, more than half lie imperiled on the range from Near Threatened to Critically Endangered. Of some 190 species in the larger family of Old World Fruit Bats, over 70% count as Vulnerable, Endangered or Critically Endangered.

Now some good news: Lanka’s flying fox rates as Least Concern on conservation indexes. Nearly a quarter of Sri Lanka’s land lies protected. Though down from 50% in the 1950s, forest still covers roughly 30% of acreage. Total tree cover declined only 6% from 2001 to 2024, and humid primary forest declined only 0.2% between 2002 and 2025.

Long may Lanka nurture her forests!

Worldwide, meanwhile, conserving fruit bats means saving forests. Vice versa perhaps as well. Improvement may come by-and-by with reduced human numbers.

Human population will inevitably peak late in this century, so say the experts. Sri Lanka’s population will peak around 2050 and then subside. Many worry about disasters potentially flowing from reduced human numbers. For example: too many overtaxed young workers supporting too many over-aged pensioners, which will stifle economic growth essential to democracy.

We should not gainsay such worries. But from the standpoint of forests and their dependent species, nothing raises hope more than that human numbers will peak and thereafter shrink in coming decades. That’s at least one ground for conservation optimism.

Author, lawyer, ex-professor, Mark Hager lives in Pelawatte with his family.