Listening to NPR radio last night, as I drove home from Grand Valley, I heard "The Ninety Second Naturalist," a program hosted by Thayne Maynard from the Cincinnati Zoo. He was telling the most fascinating story. Farmers in Costa Rico have been complaining that their cattle are being attacked more often by vampire bats than in the past. In an attempt to determine if this was true, scientists devised this interesting experiment.
The diet of cattle differs greatly from the diets of tapir and peccaries, the native rain forest mammals that were the vampires' traditional food source. This leads to different carbon isotopes in the breath of a bat that has fed on a cow versus a bat that has fed on native wildlife. By analyzing vampire bat breath, the scientists concluded that bats are dining on cattle more often than on tapirs or peccaries. Read more here and here.
One thing I haven't found out yet: How do you get a vampire bat to take a Breathalyzer test?