Two sub-tropical bats found in caves in Israel challenge key assumptions about hibernation, according to Professor Noga Kronfeld-Schor and doctoral student Dr. Eran Levin, from Tel Aviv University (TAU).
Rhinopoma microphyllum and the R. Cystops, two species of mouse-tailed bat, have been documented by the researchers hibernating in their country's Great Rift Valley caves -- doing so in consistently warm surroundings, at consistently warm body temperatures.
A variety of mammals dodge the cold of winter by hibernating anywhere from three to nine months out of the year. But the survival tactic is usually only seen in very cold temperatures, whereas the bats observed by the TAU scientists were hibernating in a cave in the Middle East, where the temperature was a consistent 68 degrees F.
What's more, the bats did something else distinctive: they managed the trick of hibernating while maintaining a high body temperature, something also against the grain for a hibernator. Typically a mammal's body temperature stays very low throughout hibernation, the better for the animal to expend as little energy as possible so they can lay low until warmer weather arrives.
The bats indeed used very little energy, but they did so even as their skin temperatures averaged about 71 degrees F.
"Hibernation in mammals is known to occur at much lower temperatures, allowing the animal to undergo many physiological changes, including decreased heart rate and body temperature," explained Kronfeld-Schor, chair of the department of zoology in TAU's faculty of life sciences.
"But we have found these bats maintain a high body temperature while lowering energy expenditure levels drastically," he said. "We hypothesize that these caves, which feature a constant high temperature during winter, enable these subtropical species to survive on the northernmost edge of their world distribution."
The two mouse-tailed bats studied by Kronfeld-Schor and Levin hibernated from October through February, existing in a barely conscious state, drawing breath only once every 15 to 30 minutes.
Even when other bat species in the same cave were active, these two bats took nothing to eat or drink, the scientists said. They also spent long periods holding back exhalation.
Before the pair's discovery, the temperate Middle East wasn't considered the most likely place to find mammals hibernating. Indeed, "until recently, it was believed that there was no mammalian hibernation in Israel, apart from hedgehogs," Kronfeld-Schor said.
"But this discovery leads us to believe there may be others we don't know about," he added. "Scientists haven't been looking for incidences of hibernation at warm temperatures. This is a new direction for us."
New also, the scientist noted, is that hibernating animals are capable of conserving energy without lowering their body temperatures. "These bats exhibited dramatic metabolic depression at warm body temperatures in the hottest caves in the desert," Kronfeld-Schor said.
Before hibernation, the bats shifted their food intake from unsaturated fats to saturated fats, adding 50 percent to their body mass before checking out. A strict diet of only queen ants with wings helped them beef up, the researchers said.
Kronfeld-Schor's and Levin's findings have recently been published in a paper in Proceedings of the Royal Society of London.