Bats in the belfry: friends or foes?
1. National Centre for Biological Sciences, TIFR, Bengaluru, Karnataka 560065
2. Ashoka Trust for Research in Ecology & the Environment (ATREE), Royal Enclave, Sriramapura, Jakkur Post, Bengaluru, Karnataka 560 064
3.Ecosystem Services Group, National Mission on Biodiversity and Human Well-being
It is dusk in the second most populous country in the world, India. As a billion people return to their homes from farms, factories, offices and forests, tens of millions of bats, belonging to over 125 distinct species1 emerge from their roosts in trees, caves, rock ledges, temples and buildings. Through the night they devour insects in farms, fields, forests, grasslands and around our homes, including many agricultural pests and those that transmit diseases to humans such as mosquitoes2. Some bats sip nectar, pollinate flowers, eat fruits, and spread the seeds of many economically and ecologically important tree species such as Mahua, wild Jamun, wild figs and cashew as they fly2.
Unfortunately, despite the critical roles that bats play in ecosystems, they remain amongst the most misunderstood of all animals. They are often considered ill-omens and harbingers of bad luck, and often vilified and persecuted by the public at large and in stories and legends. This is particularly true of the current pandemic as well. With scientific evidence mounting that the SARS-CoV2 virus that causes COVID-19 originated in bats (potentially passing through an intermediate host such as pangolins before infecting humans) there have been growing fears of further transmission of the disease from bats to humans. In fact, bat borne viruses rarely are transmitted directly to humans. As these fears grow and rumors spread, there have been calls for the extermination of bats around human neighborhoods and for fruit trees where they roost to be cut down3.4. Is such a move justified and what purpose will it serve? Here, we try to bring some balance into the narrative about bats, dispel myths surrounding them and discuss their importance to well-functioning ecosystems and for human well-being.
Why are bats special?
Bats are the only mammal capable of sustained flight. They are a very species rich group, with over 1200 species worldwide2, and more species being discovered every year. After rodents, they are the second most diverse mammalian group in the world, and make up ~20% of all mammal species. In other words, one out of every 5 mammal species in the world is a bat! They range in size from the bumblebee bat – the world’s smallest mammal that weighs about 2 grams – to flying foxes that weigh 1.5 Kg, have wingspans of up to 2 m, and can fly over 500 km in two nights5. Most bat species (>70%) are insectivorous. The remaining feed largely on fruits and nectar (fruit bats), while three species of bats native to the Americas are known to consume the blood of animals. India has an estimated 128 species of bats, of which twelve are fruit bats.
Bats are found in a diverse range of habitats from high altitudes to tropical evergreen forests, from deserts to urban settings. They typically roost in caves, tree hollows, plant foliage, roofs of homes, or temples. While some bats are solitary, many are social and live in large colonies. The largest known insectivorous bat colony in the world is in Bracken Cave, Texas, USA where an estimated 20 million Mexican free-tailed bats (Tadarida brasiliensis) roost, making this the largest congregation of mammals anywhere in the world. To put it in perspective, that is about the total human population of New Delhi living in a single cave.
Why should we care about bats?
Bats play many ecologically and economically important roles in ecosystems and provide several benefits and services to humans in the form of pest control, seed dispersal, pollination, fertilizer provisioning and food security. Insectivorous bats prey on many agriculturally important pests and those that transmit diseases to humans (e.g. mosquitoes), typically consuming about a half to two-thirds of their body weight each night2, thus providing critical services in terms of reduced crop damage and reduced need for pesticides. In Thailand, one study showed that pest-control by bats saved over $1 million dollars of rice, or the equivalent of meals for 26,000 people per year6. Globally, the value of pest control services provided by bats is estimated to range from $54 billion and $1 trillion7.
Many species of fruit bats act as pollinators and seed dispersers, in turn contributing to the production of several products that humans depend on including timber, food, fibre, animal fodder, fresh fruit, dyes and medicines2. It is estimated that the production of about a third of the world’s food, including 87 of the 113 leading food crops, depends on pollination carried out by insects, bats and birds, worth about USD200 billion per year8. The three common fruit bat species in India - Pteropus giganteus, Rousettus leschenaulti, and Cynopterus sphinx - are estimated to aid in the pollination and seed dispersal of more than 114 species of plants9, many of which have important ecological, economic and medicinal significance including wild varieties of bananas, guava, cashew, mango, figs, mahua and other fruits. Fruit bat roosts are also a convenient source of quality seeds for farmers growing tree-crops such as cashew in India. Fruit bats can also, however, be pests on agricultural fruit crops, although they tend to feed primarily on overripe fruits. Nevertheless, without the contributions of bats to our lives, we would have significantly less food and significantly more insect pests.
Bat droppings (guano) mined from caves is also widely used as a fertilizer for agricultural crops due to its high concentrations of nitrogen and phosphorous.
Emerging infectious diseases, bats and human health
We are all familiar with infectious diseases such as tuberculosis and the common flu that have afflicted humans for centuries. However, there are other infectious diseases such as SARS, HIV, Ebola, Zika and Covid-19 that have only recently appeared in human populations. Emerging infectious diseases is the term used to define such previously undetected or unrecognized infectious agents. Many of these novel emerging infectious diseases typically tend to ‘emerge’ from other animals, and ‘spill-over’ to humans. When spill-overs occur they can sometimes cause outbreaks and pandemics, of the kind we are experiencing now. Interestingly, many of these spill-overs have occurred in the last 20-30 years. Why do spill-overs happen and what can we do to prevent it?
Spill-overs - the transmission of pathogens from their natural host or reservoirs to novel hosts such as humans – are unusual and rare events10, and tend to occur when there is increased contact between humans and wild hosts. Studies that have looked to assess conditions across spill-over events suggest that they are more likely to occur in areas that support high human population densities and mammal species richness, and where human-caused changes to the landscape, such as deforestation or hunting, are high11. Extensive land-use change, as we have witnessed the world over these last 20-30 years, has allowed for increased contact between host species such as bats, and people, and thus, for spill-over events to occur.
Bats, in particular, have been linked to many recent emerging infectious diseases. They are known to harbour a high diversity of viruses, many of which we know almost nothing about and the vast majority of which do not infect humans12. However, they are also known or suspected to be the natural reservoirs for many novel and recently emerged disease viruses such as Nipah, Hendra, Marburg, Ebola and the coronaviruses that cause severe acute respiratory syndrome (SARS-CoV-1 and SARS-CoV-2). How did these viruses come to infect humans? In a few rare cases, the viruses are believed to have been transmitted directly from bats to humans, as in the case of Nipah outbreaks in Bangladesh where the infections are believed to have resulted from humans consuming fermented date palm sap that was contaminated with bat urine, saliva or excreta13,14. Direct transfer of viruses can also happen when fluids are exchanged with bats, such as when bats bite us for example (rabies virus) or when we eat bats (e.g. the 2007 Ebola outbreak in the Democratic Republic of Congo which has been linked to consumption of freshly killed bats by humans15). Interestingly, it is not just humans, but also other species such as gorillas and chimpanzees that can contract Ebola by eating fruits that have been sampled first by bats14. However, more often than not, these viruses are not transmitted directly from bats to humans, but indirectly through another intermediate host. In the most recent cases of diseases like SARS, MERS and CoVid-19, the virus is believed to have first moved from bats to an intermediate host, such as civets, camels and pangolins, before infecting humans.
If they harbour so many viruses, do bats get sick? Surprisingly, they don't seem to! Scientists are still trying to understand why this is the case, but the answer seems to be linked to their ability to fly16,17,18,19. Flying is very stressful business that result in toxic by-products that can damage cell contents. Bats have over time evolved mechanisms to avoid this, which also appear to help them combat infections and control virus propagation. Interestingly, they do not overreact to viral infections and are able to limit excessive inflammatory responses which are largely responsible for the damage observed in infected humans, and also in multiple chronic age-related diseases. In other words, in gaining the ability to fly long distances, bats have also inherited an immune system that protects them from viruses. The same immune system also makes them age slower, and live longer. They are amongst the longest lived mammals for their body size! There is much we can learn from bats then, about how to stave of diseases!
Should we be worried about bats during the current crisis?
There are growing concerns amongst the public during the current pandemic, maybe understandably so, about the presence of bats around human settlements and the threats they pose. It is important to understand that in the ongoing crisis, it is us humans that are transmitting the COVID-19 disease virus to other humans, and not bats. In fact, in the United States, they have stopped all research on wild bat populations because they are worried about humans infecting bats. Attempting to clear colonies and exterminate bats is not a very good idea overall – for one, it will only increase the chances of human-bat contact which can increase exposure to other pathogens. It will additionally serve to stress bats out, which has been shown to cause them to shed greater numbers of viruses. A better option would be for us to let them be and continue to coexist with them as we have done for centuries. By taking a few sensible precautions that minimize our direct interactions with bats - such as avoiding handling or eating bats, and not eating fallen fruits gnawed by bats or fruits likely to be contaminated by bat fluids – we can greatly reduce the chances of spillover. In the longer term, we should work towards restricting and reversing land-use change practices that are bringing us in greater contact with, and increasingly stressing out, animals that may harbour ‘emerging infections’. This will require a global commitment to reduction of habitat loss and the preservation and restoration of our natural habitats and biodiversity. A world with fewer bats around us will be one that suffers greater crop losses to agricultural pests, witnesses increased incidences of other diseases such as those transmitted by mosquitoes, one without mahua and tequila, and generally a less interesting world overall.
We thank Prof. Satyajit Mayor, Dr. Ian Mendenhall, Dr. Ravi Chellam and Kadambari Deshpande for their comments, inputs and suggestions.
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