for National Geographic News
A rare bat that roosts upright clings to slick leaves by secreting a "modified sweat" into pads on its wrists and ankles, a new study says.
Until now, scientists had thought that the bat—a Madagascar native dubbed Myzopoda aurita—used its pads like biological suction cups. Myzopoda means "sucker foot" in Latin.
But the new research shows that the bat is actually using wet adhesion, a sticky trick employed by certain insects and tree frogs.
"It's what allows flies to hang on to smooth surfaces like glass ceilings or wet paper to stick to your windshield. It's the surface tension of water," said study author Daniel Riskin, an evolutionary biologist at Brown University in Rhode Island.
Bats Can Cling to Holey Surfaces
Of the 1,200 or so species of bat worldwide, only 6 are known to roost upright—rather than the usual upside down position.
The sucker-footed bat does so while clinging to the broad, fanlike leaves of the traveler's palm, a tree native to Madagascar.
Researchers had long thought that Madagascar's bats were suctioning to the leaves just like their upright counterparts in Central and South America.
During recent field experiments, Riskin and his team tested this assumption by placing the Madagascar bats on metal plates that had evenly spaced holes, making it impossible to establish suction.
"To use suction, you have to create a sealed volume" with a partial vacuum, Riskin explained. "This changes the pressure inside the volume so that it's lower than outside. That's what pushes the surfaces together."
Surprisingly, the bats had no problem clinging to the perforated plates.
"The name of this bat is wrong," Riskin said. "It's not the sucker-footed bat, but the wet-adhesion bat. But it's been named in Latin, so you can't change it."
Heads-Down Not an Option
The bats likely evolved their upright-roosting stance because they tend to rest in new palm leaves, which are curled up like ice-cream cones.
Since the leaves' openings are above the bats, the animals roost upward to make exiting easier, Riskin said.
And thanks to evolution, it seems, the bats don't really have an option.
As part of their experiments, Riskin's team tested how easy it is to "peel" the bats off smooth surfaces.
Video: Sucker-footed bat being "peeled" off glass
The team found that the bats can be easily pushed upward and will start walking up the glass. But the animals can cling doggedly when pulled downward.
This is because the bats use special tendons in their wrists and ankles to decrease the wet pads' surface area, allowing them to detach.
Because the tendons work this way, roosting heads-down would make the bats more likely to fall.
The research is detailed in the Biological Journal of the Linnean Society and was partially funded by the National Geographic Society, which owns National Geographic News.