If there are 250,000 bats in a colony and each one eats 3375 mosquitoes in a night, how long will it take the bats to rid one Texan swamp of malaria? Charles Campbell, a doctor in San Antonio, did the sums and was convinced that bats could consume enough mosquitoes to eradicate the disease from the swamplands around the city. But how could he persuade some of the millions of bats that flew in from Mexico each spring to settle where the mosquitoes were worst?
Campbell鈥檚 answer was the bat tower, a high-rise home to tempt even the most discerning bat. And not only would the tower鈥檚 tenants transform the malarial mires into wholesome and habitable land, they would provide a steady income. Bat guano was a highly prized fertiliser, and if one bat produced 40 grams of guano a year, then a quarter of a million bats鈥�
IT SEEMED so simple. Mosquitoes spread malaria. Bats eat mosquitoes. Set the bats on the mosquitoes and the disease would disappear. Charles Campbell knew about malaria: he was a doctor. He also knew about bats, which he studied in his spare time. There was no doubt that they ate mosquitoes, and lots of them. He had poked through countless pellets of bat guano and totted up the fragments of wings, legs and other indigestible bits to come up with an estimated nightly toll.
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At the turn of the 20th century malaria was still rife in America鈥檚 southern states. Each year, there were hundreds of thousands of cases, costing the nation around $250 million. But while parts of Texas were plagued by mosquitoes, the state was also home to huge populations of bats. Late each February, a hundred million Mexican free-tailed bats fly north from Mexico to central Texas, forming immense colonies in caves, old barns and derelict buildings. Campbell鈥檚 plan was to install bats in purpose-built roosts close to places where mosquitoes bred.
In 1902, Campbell lined some wooden boxes with guano-coated cheesecloth and fixed them in old buildings, under country bridges and in trees near a cave where millions of bats roosted. After five years and no bats, Campbell conceded that the boxes were a dismal failure. Bats, he deduced, preferred larger homes.
Within months he had built a 10-metre tower at the local experimental farm. It cost him $500. Inside were sloping shelves for the bats to cling to, a large heap of guano to make them feel at home and 鈥渢hree perfectly good hams with a nice slice cut out of the side of each, exhibiting their splendid quality for the delectation of the intended guests鈥�. But there were no guests. Eventually Campbell resorted to kidnap, capturing 500 bats and incarcerating them in the tower, hoping their squeaks would attract others. By 1910, it was clear that bats were never going to move in. He dismantled the tower and sold the timber for $45.
Disappointed, Campbell left his practice and headed for the wilds of Texas to learn more about bats. He discovered that they don鈥檛 hunt immediately outside their roost but make a fast getaway to avoid predators that might be lying in wait for them. In April 1911, Campbell built his second tower on the shore of Mitchell鈥檚 Lake, just south of San Antonio.
鈥淣o swamp in the low lands could possibly be worse,鈥� he said. All San Antonio鈥檚 sewage flowed into the lake, and water seeping from it formed a huge shallow pool 鈥� a perfect breeding place for mosquitoes, and the right sort of distance from the tower. In summer great clouds of mosquitoes drove the tenant farmers from their fields around the lake and tormented their animals. Crops went to ruin. Cows stopped producing milk and hens gave up laying. Worse, this was the malaria season. Of the 87 people Campbell examined that spring, 78 had malaria. Each year two, three or sometimes four children died.
Three months later, Campbell returned to check on his tower. Just after dusk, a stream of bats emerged. It took five minutes for all of them to come out. This was promising, but the tower could hold many more bats than that 鈥� at least 250,000, perhaps even half a million. Campbell knew of two roosts nearby, one in a derelict ranch house, the other in a duck hunters鈥� shack. If he could evict the bats, maybe they would seek refuge in his tower.
Campbell had tried evicting bats before. Shouting, clapping, even hosing them down with water would dislodge them, but they always returned. He decided to try music. Campbell reasoned that because bats have sensitive hearing, tuned 鈥渢o detect the soft sonorous tones made by the vibrations of the wings of mosquitoes鈥�, they might be upset by less agreeable noises. 鈥淎 brass band suggested itself,鈥� he wrote later. From hundreds of recordings he picked the Mexico City Police Band鈥檚 rousing rendition of Cascade of Roses 鈥渙n account of the large number of reed instruments and some blatant high notes of cornets鈥�.
With the help of a friend with a phonograph, Campbell tested his bat-scarer at the abandoned ranch house. At 4 am the brassy tones of Mexico City鈥檚 finest began to belt out of the building. An hour later, the bats began to return from hunting. They circled the house a dozen times, then fled for good. A repeat performance emptied the duck hunters鈥� shack. But had the bats moved into the tower? They had. On Campbell鈥檚 next visit the evening stream of bats took two hours to come out.
By 1914, local duck hunters reported a huge reduction in mosquitoes. Campbell visited the farmers and their families again. None had malaria. The farmers told him the clouds of mosquitoes had gone. They could work after dusk and their animals were healthy. Campbell attributed the mosquitoes鈥� disappearance to his bats: 鈥淭here was nothing that could have brought about this modified condition except the great increase in the number of bats.鈥�
Impressed, the San Antonio city council made it an offence to kill a bat, and stumped up for a municipal bat roost. In 1917, bats became protected throughout Texas. More towers sprang up 鈥� and not just as a way to defeat malaria. From the start, Campbell had promoted the idea of bat towers as a nice little earner. He had weighed a free-tailed bat鈥檚 daily output and had done the sums. The Mitchell鈥檚 Lake tower produced about 2 tonnes a year of high-quality fertiliser that fetched twice the price of the stuff from caves.
If Campbell鈥檚 bats eradicated malaria, why isn鈥檛 he one of the state鈥檚 great heroes? Although malaria did disappear from San Antonio around this time, many doubt the bats had anything to do with it. 鈥淐ampbell assumed bats ate mosquitoes but his identification was suspect,鈥� says Thomas Kunz, a bat expert at Boston University. Last year, Ya-Fu Lee, a PhD student at Gary McCracken鈥檚 bat lab at the University of Tennessee, Knoxville, reported the results of the most extensive study so far of the free-tailed bat鈥檚 diet. On average, 33 per cent of the diet is moths, beetles make up 30 per cent and bugs 15 per cent. Dipterans 鈥� which include mosquitoes 鈥� make up just 2 per cent of the diet. Stomach analyses and lab tests of guano failed to turn up anything resembling mosquito remains. 鈥淚f the bats are eating mosquitoes then it must be a tiny part of the diet,鈥� says McCracken.
Merlin Tuttle, founder of Bat Conservation International in Austin, Texas, isn鈥檛 so sure. 鈥淭he bats鈥� main food is moths but they are highly opportunistic. If there were a large number of mosquitoes then they might eat them.鈥�
Today it鈥檚 impossible to test whether Campbell鈥檚 bats helped to eradicate malaria. The land has been drained, most of the mosquitoes have gone and there鈥檚 no longer malaria in the US. 鈥淭he disappearance of malaria may have been serendipitous,鈥� says Kunz. As Tuttle admits: 鈥淲e simply don鈥檛 know what happened.鈥�