Stopping the slaughter of the bats
As our canoe eases away from the muddy shore of the Rock River in southern Wisconsin, three guys languidly fishing on shore give us a glance and a “howdy.”
Above the water, a thick swarm of swallows hunts insects in the fading light. Then they fly toward their roosts and are quickly replaced by bats equally hungry for insect protein.
Globally, about 1,200 species of bats — the only mammals that really fly — play essential roles in pollination, seed dispersal and insect control.
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As we head toward the Illinois border, we see a few bats in the fading light. We hear many more from an electronic device that translates the bats’ sonar into something we can hear. From the center of the canoe, the machine snaps like a ’50s-movie Geiger counter in a heap of uranium ore.
One thing bats don’t do is get tangled in the hair, says J. Paul White, a field biologist with the Wisconsin Department of Natural Resources. “That’s an old wives’ tale. Bats definitely will fly very close and almost hit you, but I have been in areas with 50,000 to 80,000 bats … when they are swarming and mating, and I’ve never once been hit by a bat. They are actually feeding on insects that are attracted to you. They are doing you a service.”
Sonar, or “echolocation,” allows the bats to “see” in the dark, but this high-frequency sound is not audible to the human ear until it’s translated by an instrument like the one held in place by bungee cords.
Confirmed and suspected cases of white-nose syndrome
It’s a pleasant twilight for a canoe trip, but our goal is serious: taking a census of Wisconsin bats before white-nose syndrome reaches the state. Just six years after it invaded North America, this fungal disease has already killed an estimated 5.5 million bats in Eastern North America. Outbreaks have already been seen in 22 states and five Canadian provinces.
White-nose “has caused a massive die-off of unprecedented proportions in North America,” says Winifred Frick, a bat researcher at the University of California at Santa Cruz. “It’s a major conservation crisis.”
It’s probably only a matter of time before bats in Wisconsin start dying in the caves and mines where they hibernate over the winter.
But that’s later. As darkness falls, White interprets the staccato beat: “Little brown, big brown, another little brown.”
Little brown bats are the most common bat in the state. Elsewhere, they have been among the biggest victims of the alien fungus.
Uncountable bugs smack our faces as we paddle south toward the Illinois border, and we can’t know which insects the bats are eating. But bats, the only true flying mammal, need to score consistently. A little brown bat, the most common species in Wisconsin, needs to eat 600 to 1,000 mosquito-sized morsels per hour to obtain the energy for its airborne lifestyle.
A monumental outbreak
Because the white-nose fungus grows better in cool conditions, it’s a disease of hibernation, when the bat’s body temperature is low, and its immunity is compromised.
Despite “a staggering wave of mass mortality among all six species of hibernating bats that occur in north-eastern North America,”1 nobody has seen the disease in Wisconsin. Still, it has reached northern Illinois, only 100 miles away, and this summer, a research project led by Frick found the fungus – although not necessarily the disease — on bats at two Minnesota caves.
Fungal spores can travel with the wind and on migrating bats, so the fungus could arrive any day. A just-published study of bats in Canada over 21 years found that the bats that changed their summer or winter location from one year to the next moved an median distance of 315 kilometers, and 20 percent of the movements were greater than 500 kilometers.2
Data like this, and maps like the one above, have convinced White, along with a cadre of professional and volunteer bat-lovers in Wisconsin and other states, to frantically gather data about the airborne mammals in what we could call “salvage biology.” The goals are to fill biological black holes regarding:
Where do the bats live, and how common are they in different habitats?
Where do migratory bats spend the winter?
In what caves and mines do the non-migratory species hibernate? If and when white-nose syndrome strikes Wisconsin, conservation biologists hope that knowing where they lived before the great dying began will aid restoration efforts.
How does white-nose kill?
Biologists have been scrambling to understand white-nose syndrome since it was first noticed in 2006. In 2009, a new fungus named Geomyces destructans was identified as the pathogen.
Although the “white nose” around the muzzle gave a name to the condition, the disease actually seems to kill by damaging the wings. Bat wings are unusual; rich in blood vessels and only a few cells thick, they exchange water, oxygen and carbon dioxide with the atmosphere, much like the lungs. “Bats don’t just use their wings to fly around,” says Craig Willis, associate professor of biology at the University of Winnipeg, Canada.
The fungus injures wing tissue, and the bats “lose fluid through the lesions, like a victim of severe burns,” Willis says.
As the blood volume is reduced, the blood becomes syrupy, impairing oxygen transport. Lactic acid, a byproduct of metabolism, builds up, and the bats breathe heavily to get rid of carbon dioxide. further fueling dehydration, Willis says.
White-nose syndrome in wings of little brown bats
A 2013 study by Willis and colleagues3 found a loss of electrolytes in the blood. The bats looked “like someone at end of a triathlon who has just drunk water, not Gatorade,” Willis says.
The result is thirst, which could explain why sick bats wake during hibernation. “Evaporative water loss in torpor is a strong trigger for arousal,” says Willis, “and bats with the fungus warm up way too often, especially as they get sicker.”
If dehydration has you thinking about guzzlin’ Gatorade, the finding does point toward supplementing electrolytes. To test this, Willis plans to infect bats and give them access to a pediatric electrolyte supplement. “They can wake up and drink if they want, and we’ll monitor by video, so we will know if they are drinking or not,” Willis says.
Keeping bat caves healthy
In the absence of any treatment to date, the battle against white-nose has focused on a standby of epidemics: containing the pathogen. Simply studying bats can be hazardous to their health, says White. “While we are monitoring caves, we have the potential to transfer the fungus ourselves, so we go through strict decontamination, following Fish and Wildlife Service recommendations to make sure we not bringing anything from cave one to two.”
How strict? The biologists strip down to their underwear, even in winter, when they change clothes between caves. “We must treat every site as if it is infected,” White says.
Cave explorers may well have moved G destructans to North America in the first place. White-nose syndrome was unknown until it was detected in Howes Cavern in New York State in 2006, likely after hitchhiking on the gear of cavers who had previously visited caves in Europe.
“People enjoy caving partly for the experience of running into a bat, so they tend to cooperate” with efforts to stop the spread of white-nose, White says. “We’ve been trying make it as easy as possible for them, but still have made it clear that if they are going into caves, they must adopt decontamination measures.”
The fungus in Europe: Why no disease?
Europe, the source of G destructans, looms large in the discussion of white-nose syndrome. European strains have much greater genetic variation than American ones, suggesting that the fungus has lived much longer in Europe.
While both the European and North American strains of the fungus cause significant disease in North American bats, the European fungus does not cause severe disease in European bats, Willis says.
Here are some hypothetical reasons for the disparity:
Food: European bats may eat during hibernation. In the United Kingdom, Willis says, it’s often warm enough to hunt on a winter night, “so maybe European bats have more opportunity to forage than North American bats.”
Behavior: The fungus may, through evolution, have changed how the bats act. “We know that bats in both places cuddle” to stay warm, Willis says. “The hardest hit bats in North America form really big clusters, tens of thousands on the ceiling of caves, but we don’t tend to see that in Europe.” Smaller clusters could reduce disease transmission.
Immunity: After long exposure, the European bats’ immune systems may defend against G destructans.
Keeping an eye (or an ear!) on the bats
As the threat of extinction looms, friends of the bats are working nights to learn more about flying mammals, White says. In Wisconsin, volunteers are using more than 30 of the gadgets that translate the high-pitched echolocation sounds to a lower pitch that is suited to the human ear. On these bat walks, as on that trip down the Rock River, the instruments can “open their eyes and ears to a whole different world,” White says.
To date, Wisconsin has had almost 3,000 monitoring trips — by foot, boat or car. “There is a national plan for 20-plus states, to gather baseline data, and look at what happens” after white-nose syndrome arrives, White says. In Wisconsin, “We might hear 30 bats in an hour; in the East, maybe two bats. In a bizarre way, white-nose syndrome has been good for the bats. We have in poured a lot of money; at the start we were devoid of a lot of basic life-history data.”
All that time and money still leaves questions to be answered. For example:
Why exactly is G destructans so lethal? Scientists have found bats carrying benign fungi that are closely related to the white-nose strain. Comparing the genomes may identify exactly which genes on G destructans cause the disease. “Determining what makes it such a pathogen should open an avenue to understand why it is killing, and then what you can do to intervene,” says Jeff Lorch, a researcher at the U.S. Geological Survey’s National Wildlife Health Center and the University of Wisconsin-Madison.
Can a biological agent control G destructans? Competing organisms help prevent disease by crowding out pathogens from the skin and gut, and some researchers are starting to test whether that strategy might help with G destructans. But Lorch warns about possible complications. “Biocontrol has potential, but you have to find the right agent, and it has to be able colonize a bat at an abundance and for a long enough period of time that it can inhibit the pathogen.” Organisms that work in the lab often fail in the field, and if a candidate microbe is found in caves, why did it not protect bats in the first place? More in this video.
Does an infected cave ever become safe for bats? Maybe not, says Lorch, who collaborated on a study that found G destructans in the soil two years after the bats died. Fungi can often live indefinitely in the soil, which make them different from other pathogens. A virus has a selfish interest in not killing every potential host — because the virus needs host cells to reproduce. Control for fungal disease is a lot more complicated, especially if there is soil or another “environmental reservoir,” Lorch says. “We are not just talking about treating the host; we have to worry that the pathogen is always around, so the animals can be re-exposed.”
Will some bat species go extinct? It’s a real possibility, says Frick of the University of California. “Definitely, at least regional extirpation is of great concern. The little brown and northern long-eared have been hammered, are at risk of extinction. The Chytrid fungus has already caused extinctions in at least 20 species of frogs.
Why do some bats survive?
Researchers who dwell on white-nose are understandably concerned, but here’s what some consider a bright spot: The disease is not always 100 percent fatal. Perhaps the initial infection is stimulating an immune response that enables the host to survive a second exposure.
That, after all, is the basis for vaccination and adaptive immunity.
Acquired immunity, Willis says, “is a key aspect of understanding any infectious disease.” To find out if the few bats that outlast an outbreak have immunity, Frick and Willis will compare them to unexposed bats after both are deliberately exposed to G destructans.
Since white-nose syndrome is a slow-growing disease of hibernation, the results are likely to come in early spring at the earliest, Frick says.
Finding immunity would boost morale among the legion of scientists who are combating white-nose syndrome, but “A lot of work suggests bats don’t have much immune response during hibernation,” Lorch cautions. “Even if they were previously exposed, and had antibodies, it’s not clear whether they could mount an immune response during the second exposure.”
The immune system is fairly inactive at the cool body temperature of hibernation, Lorch notes, which may be one reason why G destructans affects hibernating bats. “We have seen caves where it takes a few years after the disease arrives for the bat population to crash, which could argue against the idea” that immunity will help upon a second exposure. “However, there does seem to be some type of resistance in some bats, whether it be behavioral or physiological, and this is very promising.”
What’s a bat worth?
A little brown bat — the most common species in Wisconsin — eat four to eight grams of insects per day in the summer,4, equal to roughly 1,000 mosquito sized insects.
The bat’s insect control is worth money, according to a 2006 study5 of a cotton crop in Southwest Texas. In a crop worth $4.6 to $6.4 million, the bats had “an annual value of $741,000 per year, with a range of $121,000–$1,725,000.”
The bats were most valuable early in the year, before farmers would see enough cotton bollworms to justify spraying insecticide. By delaying spraying, the bats were reducing the ecological and economic impact of pesticide spraying, the authors wrote.
Extrapolating that number6 to the entire U.S. agricultural base produces a benefit worth at least $3.7 billion per year.
But if you’re not a farmer, why fret about bats when so many other worries are afoot? “Part of it is a curiosity about nature,” says Willis. “It’s the only mammal that can fly under its own power, and that’s led to this fabulous ecological radiation, with 1,200-plus species throughout the world, in just about every ecological niche. They eat everything and sometimes have complex social lives. And they undergo some of longest migrations in the mammal world, akin to what the bison used to do on the Great Plains or the wildebeest in Africa.”
Frick answers the question differently. “Bats are an integral component of ecosystems. They have an intrinsic right to exist. We as a society decided 40 years ago, with the Endangered Species Act, that we want to keep species around, that we value the diversity of the wildlife around us.”
– David J. Tenenbaum
Terry Devitt, editor; S.V. Medaris, designer/illustrator; Yilang Peng, project assistant; David J. Tenenbaum, feature writer; Amy Toburen, content development executive
- White-nose syndrome in bats: illuminating the darkness, Paul M Cryan et al, BMC Biology 2013, 11:47 ↩
- Long-distance movements of little brown bats (Myotis lucifugus), K. J. O. Norquay et al, Journal of Mammalogy, 94(2):000–000, 2013 ↩
- Warnecke L, et al, 2013. Pathophysiology of white-nose syndrome in bats: a mechanistic model linking wing damage to mortality. Biological Lett 9: 20130177 ↩
- Economic Importance of Bats in Agriculture, Justin G. Boyles, et al, Science vol 332 1 APRIL 2011 ↩
- Economic value of the pest control service provided by Brazilian free-tailed bats in south-central Texas, Cutler J Cleveland et al, Front Ecol Environ 2006; 4(5): 238–243 ↩
- Economic Importance of Bats in Agriculture, Justin G. Boyles et al, Science 1 April 2011 ↩
- Texas: Mexican free-tail bat research ↩
- In the field: Texas bat research inspires military technology design ↩
- [Video] Meet the World’s Biggest Bat ↩
- Building a bat cave to stop a killer ↩
- Bat-killing fungus continues to spread west through the US ↩
- Die-off of bats could hurt area crops ↩
- Bad news for bats: White-nose syndrome approaches Wisconsin’s border ↩
- [Video] Shedding light on Chronic Wasting Disease ↩