With the introduction of large herds of livestock into their native environments of Central and South America, these bats don't need to hurry to catch the cattle from which they extract perhaps a tablespoon of blood at a time for sustenance. They feed while their prey are sleeping, spending perhaps 10 minutes drinking from the small cuts they make. However, running may help them avoid being stepped on, Riskin suggests. More likely, the researchers say, the ability to run evolved long ago, when vampire bats had to prey on faster South American athletes such as the agouti, a rodent about the size of a hare, which might wake up and take a swipe at the nocturnal visitor. It remains unclear exactly what the native prey were before the introduction of cattle, he adds.
The Cornell zoologists plan to go a step further and compare the vampire bat to another species of bat endemic to New Zealand. Riskin explains, "The vampire bat is good on the ground because when you feed on the hoof of an animal that weighs 14,000 times more than you do, it's dangerous." Also, there are and always have been a variety of predators in the Western Hemisphere environment. In contrast, Riskin says, the bats and birds of New Zealand had no predators to keep them flying. Many of the birds became flightless walkers, and the bats walk more than others, as flight is energetically very expensive.
"Vampire bats in Latin America are good on the ground because it's a very dangerous place, and bats in New Zealand are good on the ground because it's a very safe place," Riskin says. While New Zealand bats definitely don't run -- they failed the treadmill test -- Riskin wants to compare the walking gaits of the two species.