- Bats in parts of Tamil Nadu are facing a decline in population due to urbanisation, the use of bright lights, temple renovations and tree removals.
- The districts of Tirunelveli, Tuticorin and Tenkasi in particular are home to bat species that provide various ecological benefits, such as pest control and pollination and are also of important social significance.
- Bats in the region use crevices and abandoned rooms in old temples as roosting sites. However, renovation of old temples, including cementing crevices and adding artificial lights, is not conducive for the bats to roost or forage.
- Experts say that misconceptions about bats and zoonotic diseases are further harming the transitional human-bat symbiotic relationship which could lead to detrimental effects on the ecosystem.
When the novel Vavval Desam (The World of Bats) by Cho Dharman was released in 2022, it drew much interest in Tamil literary circles for its eerie portrayal of bats. “While other species walk upright, bats hang upside down. When the world is asleep through the night, they roam. While most species choose airy spaces to reside in, bats opt for abandoned places and old buildings. I wanted to capture this contradictory nature in my novel. So, I chose bats as a symbol,” says the Sahitya Akademi-winning novelist, Dharman. The author is based in a region in South India where bats thrive in antiquated buildings, abandoned houses, and quaint temple structures.
The districts of Tirunelveli, Tuticorin and Tenkasi, in particular, which lie along the banks of Tamil Nadu’s only perennial river, Thamirabarani, are known to be home to several unique bat species. The region hosts two major categories of bats — insectivores that subsist on insects and frugivores that rely on fruits. The predominant species found are Hipposideros speoris, Rousettus leschenaultii, Taphozous melanopogon, Megaderma lyra, Tadarida aegyptiaca, Pipistrellus, and Rhinopoma hardwickii. Of these, Hipposideros speoris is the most prevalent, making up nearly half of the population.
Their chief roosting sites in this landscape are old temples and open trees.
The nocturnal species, however, is facing survival challenges in this area, with rapidly increasing urbanisation, bright lights, temple renovations, tree removals and a weakening human-bat symbiotic relationship. Some subspecies have even vanished.
“We have morphometric data [of bats] from the past 25 years,” said Paramananda Swamidoss, a bat researcher who heads the Zoology department at St. John’s College in Palayamkottai, talking about the information on size and form of the organism. “There are no physical changes. But the population has declined by about 30% to 40% [over this period]. For instance, an evening bat named Pipistrellus dormeri, which would emerge from its roosting sites early in the evenings and return late the following morning, has disappeared from this region since 1997, perhaps because the species couldn’t adapt [to these changes],” he said.
He explained that this species can live only within wall crevices, which were previously prevalent in houses in the region. “Nowadays, that’s not the case. We couldn’t find this species even in villages. We don’t know whether it exists here anymore,” he added.
Another emerging factor impacting bats is the installation of wind turbines for renewable energy. This region of Tamil Nadu, which sits at the foot of the Western Ghats, is a coveted location for wind energy, with thousands of turbines being built on vacant lands. “Renewable energy is important. [But] there should be regulations on the number of wind turbines that need to be set up. When we speak to local folks where the turbines are erected, [they report that] bats are killed by the rotating blades of the turbines,” said M. Mathivanan, a scientist with the non-profit research organisation Ashoka Trust for Research in Ecology and Environment (ATREE).
As temples renovate, bats disappear
Cave-dwelling by nature, bat species in this landscape made old temples their home, as natural caves disappeared over centuries. However, among the key reasons for the diminishing bat numbers in the region now, is the renovation of old temples that dot the banks of river Thamirabarani. Research studies show that painting temple walls and cementing the crevices in the structures have driven away bats, such as the species Egyptian free-tailed bat and mouse-tailed bat, from their traditional roosting sites or microhabitats.
“Bats prefer darkness. They cannot tolerate daylight. But as people have set up artificial and bright lights inside the temples, in places where bats roost, they have inadvertently chased them out,” said Mathivanan of ATREE. “False vampire bats stay only in temple towers or abandoned rooms. But these days, even if there are more rooms in temples, people put up bright lights thus leaving no space for bats,” he said.
Mathivanan’s team has been studying bat behaviour for over a decade in three districts in this landscape and found that the renovation of temples has had a negative impact on the bats’ habitats. The 2022 study concluded that “retaining undisturbed dark rooms with small exits in temples and other dimly lit areas and having natural areas around temples are vital for bat conservation.”
An earlier study reiterates that darkness and reduced human disturbance, after hours at temples, facilitates more bat-visits that show up to forage. The study, conducted between February 2012 and January 2013, studied the foraging behaviour of the Cynopterus sphinx species in and around the Murugan temple, Thirupparankundram on the outskirts of Madurai city in Tamil Nadu. It found that the greater short nosed fruit bat, which usually foraged on fruits, nectar and leaves on trees in urban and wild areas, was now a regular visitor at the temple for the readily-available bananas.
Individuals of C. sphinx began to arrive in the temple complex at 8:30 pm at night. “The highest number of bats were seen between 22:00hr and 22:30hr, one hour after the temple lights were switched off,” noted the study, which documented the bats foraging on ripe bananas either in shops, on remains of offerings or bits dropped by monkeys in the temple. “Temples may prove to be promising sites for bat conservation in India,” the decade-old study has said.
Bright lights impact bat flights
When lights go off, bat activity is on. But what happens when lights, markers of modern infrastructure, shine bright through the night?
Artificial lighting at night affects behaviour in nocturnal species, such as bats, finds a 2022 research paper that studied the cave-roosting Rousettus leschenaultii species and its emergence and return in two sites – a roost in an undisturbed, naturally-lit agricultural well and an artificially-lit roost in a temple in a semi-urban site in southern India. The study also compared emergence times at five colonies of the tree-roosting Pteropus giganteus (currently P. medius) that were exposed to different intensities of artificial light. Since the bats rely on vision and light-based cues, the artificial lighting at roost sites could have consequences on their behaviour, activity and the ecosystem services they provide, noted the study.
The study found that emergence-return flights at the naturally-lit R. leschenaultii roost occurred significantly earlier than at the artificially-lit roost. “Peak flight activity across nights varied more in the naturally-lit than at the artificially-lit roost. Nightly flight durations (interval between peak emergence and peak return times) varied more in the naturally-lit roost, while mean flight durations were similar between these roosts. In P. giganteus, emergence was significantly earlier in the highly light-polluted roost than in the other roosts,” said the study. These modified flight activities could have potential consequences on the physiology and ecology of fruit bats and requires further study, it said.
Baheerathan M., a bat researcher at the Indian Institute of Science Education and Research Thriuvananthapuram and co-author of this study on the effect of artificial lights on frugivorous bats, said the excessive use of artificial lighting during the night can have a species-specific impact on bats.
He said, “The impact of artificial lighting on bats is a very less explored avenue in India and we are just starting to understand it. Globally, artificial lights have been known to have species-specific effects on bats. My study in fruit bats of Tamil Nadu also shows a similar trend. Some species are positively affected and some are negatively affected by lighting.” The researcher added that among the two species he studied in the nearby district in Madurai, one species of fruit bats was positively impacted while the other one was negatively affected.
Holy temples, unholy bats
Temple authorities and staff, however, often view the bats as a nuisance, particularly because of the stench from bats’ faeces.
“They are locally called ‘poop-bats’ because they make the place filthy. Annually, we paint and whitewash the temple during Thaipusam [a Tamil festival occasion],” said Hariharapalan, the person in charge of the popular temple in Thirupudaimaruthur, near Tirunelveli.
He added that they are setting up measures to prevent bats from roosting in the temple. “We’ve now set up safety nets. Though somehow, bats find a way. If we had looked behind the temple a few years ago, there would be numerous bats. But once we started using bright lights, their numbers decreased,” he said.
Another local staff member who manages a temple-aligned mutt, a structure at least 500 years old, said they spray pesticides to drive away the bats. “This place is quite comfortable for the bats as no one comes here at night,” said Annamalai, who has been managing the facility for five years. “Entire walls, pillars, and even clothes would get dirty because of bat faeces. Sometimes we create smoke or spray pesticides. The bats leave for two days but always return.”
The misunderstood bats are also sometimes considered an “evil symbol” in this region. Recent rumours and misconceptions about bats spreading zoonotic diseases are worsening matters. These impinge on the traditional human-bat relationship and have a detrimental effect, say experts.
At the same time, there are pockets within this landscape where the social significance and ecological benefits of bats is recognised. A hill tribe in the Western Ghats, for example, celebrates an annual bat festival in secret to express gratitude to the species, reveals author Dharman, who chooses not to reveal the tribe’s identity for their privacy. “It takes place on a particular night during the year. All the tribes in this region gather on level ground. They’re entirely nude except for a black cloth tied to their hands. Throughout the night, they act like bats. They hang in the trees and jump like bats,” added Dharman, who says he had attended the festival but did not divulge more details.
Beyond its communal relevance, historically, the nocturnal species has maintained a symbiotic relationship with the farming community as well. Bats assist in pollination, spread germinating seeds through their travels, devour unwanted pests such as insects and spiders and enrich the soil with their mineral-rich faeces, called guano. Previous studies have shown their ecological benefits to farmers such as increasing crop yields and pest control. Yet, their numbers are worryingly on the decline in the agriculturally dominant topography.
Banner Photo: A bat roost in a temple. Photo by S. Lokesh/Mongabay.