By Sean Henahan, Access Excellence

CHAMPAIGN, Ill. - Bats appear to use a built in barometer to determine the chances of a good night's bug hunting, according to a study by Ken Paige of the University of Illinois Institute for Environmental Studies.

Paige spent nearly a year monitoring the dining habits of a colony of eastern pipistrelles, a cave-dwelling bat commonly seen in western Illinois. He found that the bats emerged in larger numbers when barometric pressures dropped and insects were numerous. He noted that the number of insects declined dramatically and that fewer bats left their roost when air pressure increased. Indeed, barometric pressure alone explained 87 percent of the variation in bat activity, he noted:

"My research strongly suggests that the eastern pipistrelle uses barometric pressure as a cue for predicting the relative abundance of aerial insect prey outside their roost. Barometric pressure is the only physical environmental cue available to a bat roosting deep within a cave. All other variables such as light, temperature, relative humidity and wind currents are virtually constant. When air pressure is low, aerial insects are abundant, and bats respond by leaving the roost to forage."

The bat's built-in barometer is likely to be its Vitali organ, a middle-ear receptor that is thought to help birds measure air pressure. Bats are the only mammals to have such a sensory organ. Paige's bat-watching also showed that bats track barometric pressure metabolically. When the barometer fell, the bats slowed down their metabolisms, allowing them to conserve energy. By doing so, they can delay or eliminate the need for entering torpor, a sluggish state of dormancy that makes them susceptible to predators. In addition, the bats' tracking of metabolic pressure may function as a bet-hedging strategy, he said

"When pressure is low, insects are most abundant, except during heavy rain," he said. "Because it is unlikely that bats can detect the rain from deep inside a cave, they have to fly out of the roost to check. This is no problem, however, because at low pressure they already are running on an economy setting and will waste minimal energy if they find their trip to the outside is a waste of time."

Because they already are conserving energy, he added, they can extend the benefit of their previous meal. Thus barometric-pressure tracking can be viewed as an alternative evolutionary strategy to torpor, he explained.

This results of this research project appeared in the June issue of the British journal Functional Ecology.