Most animal species that hibernate in winter do so by lowering their body temperature to near-freezing, but now scientists in Israel have found two species of bats that use a different, much more energy-efficient strategy to survive the cold.

Both species of bats from the Rhinopomatidae family find refuge in unusually warm caves in the Jordan Valley, where they are able to maintain a relatively balmy body temperature of 22 degrees Celsius throughout the winter, according to the research, which was conducted by scientists from the Zoology Department of Tel Aviv University and published last week in Proceedings B of the British Royal Society of Sciences.

“We found proof that mammals can hibernate in what we call ‘room temperature,'" said Dr. Eran Levin, a partner in the project who is now doing post-doctoral research at the University of Arizona. “This contrasts with common wisdom that says that to lower bodily functions very low, mammals have to reach the freezing point. We still don’t know how they do this,” he added.

The Jordan Valley caves favored by these bats have a temperature of over 20 degrees Celsius, due to the flow of warm, humid air through cracks deep underground. The bats enter the caves in October, and remain there for almost six months.

To understand what goes on during this time, a number of bats were caught in a net and released after having radio receivers attached to their bodies. One group of bats was caught and transferred to Prof. Noga Kronfeld-Schor’s lab at Tel Aviv University. There they were placed in an enclosure with conditions that simulated those found in the Jordan Valley caves. The researchers followed changes in the bats’ bodies over the winter, especially their metabolism. They were then released into the wild.

The findings show that bats of this family can hibernate for almost six months. They are not completely passive, but rather gradually lower their metabolic rate. In contrast, other species of bats in the same cave continued to leave it to search for food. The scientists observed that the Rhinopomatidae consumed their food supply ahead of time, with their body weight doubling before hibernation. They did this by eating ant queens, whose bodies are rich in fatty acids that apparently contribute to helping the bats’ bodies adapt to the heat conditions in the cave.

Photo by Dr. Eran Levin

The metabolic levels of the Rhinopomatidae decline to one percent compared to their fully waking state, and equal that of mammals that hibernate in cold regions of the world. The mammals in those regions have to wake up every few days to drink and eliminate waste. Every wake-up expends significant energy since the mammal must warm up from near-freezing to 36–38 degrees Celsius for a number of hours or days. In contrast, the Rhinopomatidae maintain a body temperature of 22 degrees, thanks to conditions in their cave.

“They can fly, talk to each other and crawl at this temperature,” Levin says, adding that this is how they save energy. They can also stop breathing for as much as half an hour, taking only short breaths during this time for durations of one minute, he adds. The Jordan Valley is the northern distribution boundary of the Rhinopomatidae. The warm caves allow them to survive the Israeli winter, because they cannot live in temperatures of less than 15 degrees Celsius.

Hibernation apparently has another advantage: it enables them to avoid being eaten or injured. The Tel Aviv U. research team urged that the caves, which are close to human habitation, be protected to ensure that the bats are not disturbed during hibernation.

Photo by Dr. Eran Levin