In this particular motion, the bat is trying to gain altitude, so it’s not the most efficient flight configuration. The bat’s propulsive efficiency (as opposed to other kinds of biological efficiency) is only about 8 percent, Tafti says. But that’s OK because of what it’s doing. It’s akin to a jetliner using maximum power to take off, and leveling out when it reaches cruising altitude. “A lot of the power it is expending is going toward increasing its lift, and not thrust,” Tafti says.