By
Dana Beach
Fellow, Institute for Mind and Biology
Back when she was a PhD student at Princeton, Cara Brook somehow managed to convince her committee members to let her travel to Madagascar and catch “mega bats.” As you might imagine, this is no easy feat, especially for three-foot-long, tree-dwelling bats like the Madagascar flying fox (Pteropus rufus) she was hoping to snare. Luckily, she found a network of skilled Malagasy bat trappers who taught her how to shoot a four-foot-long slingshot over trees, set up a pulley system, hoist nets into the canopy, and lower the nets down upon an eventual bat capture.
Prior to meeting Brook, the trappers were hunting bats for food, a critical nutrition source for the population of Madagascar and across Asia and Africa, with over half of the “mega bat” species being hunted for food. But this has consequences beyond endangering bat populations.
SARS-CoV2, the virus that causes COVID-19, has been speculated to have found its way to humans through eating infected bats. In fact, Brook chose to study bats for their unique zoonotic potential, or the likelihood of viruses that animals carry spilling over to humans. With COVID-19 and other zoonotic viruses, spillover happens when humans come into close contact with new and novel viruses. Eating virus hosts like bats is as close contact as it gets, and unfortunately bats are particularly pertinent zoonotic hosts due their unique position as the only mammals that fly.
What does flying have to do with it?
Flight is extremely metabolically expensive. The highest a human can elevate their base metabolic rate is about two-fold; for other mammals like rodents, this can increase to seven times their normal metabolism. But a bat in flight elevates its metabolic rate 15-fold, off the charts in terms of energy expenditure. Usually, there’s a trade-off between high metabolic rate and lifespan due to increased stress and inflammation placed on the animal. However, bats have been known to live up to 40 years, the longest lived of any mammal for their body size.
“It’s kind of like a yin-yang,” said Brook, who is now an Assistant Professor of Ecology and Evolution at the University of Chicago. “Bats have developed hyper-efficient innate immune responses that appear to be able to control viral load to low levels. But at the same time, they have all these anti-inflammatory mechanisms that allow them to avoid immunopathology.”
Typically, the presence of a virus triggers an immune response. But bats have developed an immune system that is constantly primed as if they are already infected, regardless of whether a virus is actually present, allowing them to not get sick from infection. In other animals, this would cause widespread inflammation due to the amount of work needed to mount this type of immune response. Think of how exhausted you feel after catching the flu—the immune system of bats is doing this work nonstop!
This inflammation should cause bats to have a short lifespan, but they have another adaptation to negate this, which, as Brook and her team showed in a 2020 study, may have evolved to mitigate the metabolic damage induced during flight. Bats have dampened inflammatory mechanisms, including the loss of certain genes that cause inflammation. This allows them to live longer, but it also makes them a friendlier place for viruses to roost. All of this means that viruses can evolve in bat hosts to higher growth rates and higher density, while not killing off the bats themselves. And when these more potent viruses spill over to other mammals like humans, they are way more severe or “virulent.” Viruses known to have spilled over from bats include Rabies, Henipaviruses, Ebola, and SARS (the category that COVID-19 falls into), just to name a few.
COVID-19 was actually more of an exception for a zoonotic virus transmitted from bats. For all viruses, there’s a tradeoff between virulence and transmissibility. For example, a more virulent disease will make its host sicker, killing them before they are able to spread it to many other hosts. On the other hand, less virulent diseases can spread to many more hosts by making their initial host less sick.
In another study earlier this year, Brook’s team showed that bat-born zoonoses tend to land firmly in the high-virulence camp due to bats acting as a growth reservoir for diseases, with fatality rates for the infected individual around 90%. Luckily, this high virulence and low transmission means that, while the bat-borne virus might be deadly for the people in direct contact, the global infection and death rate will not be high. COVID-19, on the other hand, had a fatality rate more like 1%, landing on the high-transmissibility side of things. Though COVID-19 infected many more hosts and led to a much greater number of total global deaths through its high transmissibility, we should not expect most bat-borne zoonoses to act this way.
Bats and humans are inextricably linked
As a conservation biologist, Brook thinks the distinction between virulence and transmissibility is important. Since bats host virulent viruses, bat-borne zoonoses don’t tend to cause the most deaths in the human population the way more transmissible viruses like COVID-19 did. But this does open the question of how to limit virus transmission between bats and humans. Bats shed more virus under conditions of stress, such as during food shortages or reproduction, thus increasing the potential for human contact with the virus. This makes the argument that if we improve the bats’ environment, such as limiting logging in areas with tree-dwelling bats, we reduce the potential for virus transmission to humans. A win for bats, and a win for humans.
“I didn’t go into this field wanting to make bats sound terrible. I think that they’re fascinating. I enjoy my work and want them to continue persisting,” Brook said.
The Pteropus rufus flying fox is in significant population decline, and both habitat conservation and human hunting are playing a role. Bat hunting is legal in Madagascar during a designated season, which unfortunately happens to be in the Malagasy winter when food is scarce (for both bats and humans) and when this bat species is gestating. Due to both conditions of stress, bats shed virus the most during this winter period. This also means that not only are hunters drastically dampening the population further by killing pregnant females, but that people are greatly increasing the likelihood of coming into contact with bat-borne viruses that are being expelled under both conditions of stress. One suggestion might be to switch bat hunting to another season or eliminated altogether. But what impact would this have on the Malagasy population?
The human element of the environment
After college, Brook went to Madagascar through the World Wildlife Fund and was hooked. She was always fascinated by being outside and loves conducting scientific research, but she also knew something was missing. “I was quite certain that I wanted to do research,” she said, “but I also wanted it to be linked with international development and work in a place where we can employ local people and train local students.”
This emphasis on human development is not something that comes out in peer-reviewed papers, but it is a crucial aspect of what Brook’s lab does. Not only do they employ former bat-hunters to be research assistants and collaborate with Malagasy labs, but they also teach modeling workshops every year for Malagasy students. Bats are just a part of the larger picture of ecology, epidemiology, and conservation biology that interests her lab, but the relationship of these bats with the human aspect of the environment is what keeps Brook coming back for more.
She knows that to solve the issue of bat hunting, they can’t just take away a vital food source from a starving population during a season in which food is scarce. So, she and her colleagues in Madagascar, including Chris Golden from Harvard University and Brian Fisher from the California Academy of Sciences, are looking at how to bring alternative sources of protein to the Malagasy people, including implementing poultry farming and developing a cricket-based protein powder. These researchers know that conservation doesn’t happen in a vacuum, and there’s a real human impact of implementing policy changes like this.
Brook’s research brings to light how humans are intrinsically connected to all aspects of our environment, and how we can form mutually beneficial relationships with the creatures and systems we share our world with. Studying bats provides a unique opportunity to understand how viruses evolve, how the environment plays a role, and the human impact on these factors.
Back when Brook and her postdoc Christian Ranaivoson received a Bill and Melinda Gates Foundation grant in 2019 for sequencing potential zoonoses, they couldn’t keep from laughing. “We started out catching bats, look what we’re doing now!”
