Newswise — Bats have long intrigued humans. In a variety of cultures, they embody malevolent symbolism, including darkness, death, foreboding, and evil spirits. In others, they’re benevolent flyers who bestow good fortune. Bats themselves also come in a variety of forms and shapes. The miniscule “bumblebee bat,” ranks among the world’s smallest mammals. Flying foxes, which eat mostly fruit and other vegetation, can have wingspans reaching up to 6 feet long. The clear-winged wooly bat may be one of the strangest to look at. Its wings are nearly transparent, and the muscles, circulatory system, and bones are clearly visible through the translucent, almost-paper-thin skin.

“Bat wings are freaky modified mammalian forelimbs,” says Brown University PhD student Andrea Rummel. Although they contain the same complement of muscles that we have in our arms – biceps, triceps, etc. – bats lack the fat that insulates other mammalian limbs. Essentially, you can see right through their thin skin right to individual muscle fibers. Muscle function declines in colder temperatures, but bat wings beat 10 times a second, even in the cold. 

How do bat muscles work so well with exposure the elements, and no insulation? This conundrum sparked interest in whether bat wing muscles are specialized to low temperatures. Rummel, along with Drs. Sharon Swartz and Richard Marsh of Brown University, discovered that bat muscles have specialized functions that allow them to fly quickly, despite extreme temperature changes.

To study these muscles in flight, the researchers trained bats to fly inside a wind tunnel, an aerial treadmill for bats, while hooked up to devices measuring temperature in different wing regions. “This was the more challenging part. Bats are more like cats than dogs [because] they do whatever they feel like doing.” When Rummel put the bats inside the wind tunnel, they would just fly as fast as possible to hold onto one end of the tunnel. To get the bats to fly through the tunnel, the researchers tried enticing them with a hanging piece of fruit in the middle. But, the bats didn't take the bait. The solution? One researcher stood at either end waving to the bats, which, wanting to keep their distance from the humans, stayed in flight inside the wind tunnel.

Dissected muscles were also examined, and the researchers compared muscles close the bat’s body, the pectoralis muscle, and one further out in the colder areas of the wing. The temperature difference between these two muscles during flight reaches 12°C, with the pectoralis muscles staying close to the core body temperature of the bats.

What the researchers found was that these muscles have very different responses to the change in temperature. The wing muscles further out on the wings, which get the coldest during flight, function well over a broad range of temperatures. The pectoralis, in contrast, functions poorly at the colder temperatures.

This kind of functional specialization in mammalian muscles is critical for understanding how bats are able to maintain fast wing movements during flight. If the wing tip muscles worked well only at body temperature (like the pectoralis), then they would move much more slowly at the colder temperatures. This means that, as you move out along the wing, as muscles get colder and colder, the muscles would move at different speeds and interfere with the wing beat.

The impact of muscle physiology of locomotor function has long been studied in ectotherms like lizards and frogs, which don’t physiologically regulate their own body temperature in the manner of birds and mammals. “Seeing these adaptations would make sense in bats whose wing muscles experience very varied temperature ranges,” explains Rummel. Her experiments show that an endotherm, an animal that physiologically regulates core temperature, can actually have very specialized muscles for coping with extreme temperature difference experienced within one limb.

Rummel will present this research at the 2019 annual Society for Integrative and Comparative Biology meeting in Tampa, Florida.