A Mexican free-tailed bat hunts a corn earworm moth.

Using high-speed video equipment that has become affordable for scientists in the past five years, Breuer and Swartz created the movies as a research tool to investigate the flight mechanics of bats. They coax the bats to fly inside wind tunnels using food rewards.

Lesser dog-faced fruit bat.

The videos reveal the intricacies of how bats use their wings.

“One thing we found is that the movements of the wing are extremely complex compared to birds and insects,” said Swartz. “They have many, many joints they can control independently.”

Their complex wings enable bats to make sharp turns that birds can’t manage.

“They don’t necessarily turn fast,” said Swartz, “but they can turn on a dime.”

Bat makes a sharp turn.

Bat wings are also made out of soft skin – which Swartz calls “compliant.” This gives bats yet another advantage over birds and insects.

“If your wing is made of compliant skin,” she said, “then it billows and that gives you extra lift.”

Not all bat wings are the same, though. Mexican free-tailed bats, which are abundant in California’s Central Valley, have stiffer wings than species like the lesser dog-faced fruit bat found in Southeast Asia.

“As we’ve studied the Mexican free-tailed bats, it turns out that their wing motions look very much like birds’,” said Swartz. “The wing stays pretty rigid, even though they have the same joints than the fruit bats.”

Mexican free-tailed bat. Credit: Nick Hristov and Tatjana Hubel, Brown University

Swartz believes the differences in the wing capabilities of Mexican free-tailed bats and lesser dog-faced fruit bats are related to the differences in their diets. The Mexican free-tailed bats eat mainly insects, while the lesser dog-faced fruit bats eat, well, mainly fruit. Fruit-eating bats need to move through their habitat slower than insect-eating bats, Swartz said. And because they’re moving more slowly, it’s harder for them to maintain lift.

“It might be that the animals that need to fly more slowly need the three-dimensional complexity to make up for that,” she said.

Watch more videos from the Swartz and Breuer labs.

Watch QUEST TV's story on Northern California's bats

QUEST on KQED Public Media.

37.76355 -122.458

So we visited zookeeper Andrea Dougall at the Oakland Zoo to learn about their Malayan and Island Flying Fox. Both are a type of fruit bat, and I couldn’t readily see the difference between them. There are many fascinating things that Andrea taught us about these bats that we couldn't fit into our two minute segment (and honestly, this producer wouldn't mind making a half hour special on these critters!). For instance, they have a lot of blood vessels in their wing tissue, so they make excellent thermo-regulators. If the bat is cold, he wraps himself up in his wings so that the heat from his blood vessels can keep him warm. Likewise, when it's hot out the bats flap their wings to cool off.

When Andrea told us that bats are the only mammals that can have sustained flight by flapping their wings, someone said "but what about the flying squirrel?" Nope– they glide.

These bats don't actually swallow the fruit that they eat, instead they chew it into small pieces, push it up against the roof of their mouth to ring out the juice, which they then swallow, and spit out the leftovers. This is something that Andrea reminded me of when I told her I'd like to take one of these cute critters home as a pet… the amount of rotten fruit pulp that you have to pick up is really unappealing. Plus, of course, it would be illegal.

Perhaps the most intriguing thing to me is the simple fact that these animals spend all of their time hanging upside down. I asked Andrea about that too– how is it possible that they wouldn't experience some sort of leg fatigue and let go of their grip? She told me what's in the scientific literature on other kinds of bats (and we're assuming it applies to fruit bats as well). The deal is that the tendon of the muscle that flexes the claw passes through a tough sheath that consists of 19-50 rings, oriented at an angle so that the inside surface is ridged. So there's some ratchet-action going on in the sheath that holds the claw in a grasping position even after the muscle has relaxed, and it's the tension on that tendon from the body weight that holds the ratchet in place. When the bat wants to move, the tension is released and therefore the claw releases its hold. So basically, the clenched position is the "at rest" position, and the releasing of the foot is the part that takes energy.

If you haven't yet, I highly suggest you make a trip over to the Oakland Zoo to see these highly captivating animals for yourself.

Watch the Cool Critters: Fruit Bats television story report online. Also, if you’d like to see close-up photos of these bats, please visit our photo set over on Flickr.

37.7772 -122.166595