Bats avoid overheating through intermittent flying

Abstract

Bats are remarkably versatile. Not only are they the sole mammal that has mastered the mystery of flight but they also have the ability to lower their body temperature to reduce their metabolic rate and conserve energy when the temperature drops. However, when bats are flying, their metabolic rate is considerably higher, causing their body temperature to rise. Yet, little is known about how bats prevent themselves from overheating during flight. Jinhong Luo from the Central China Normal University, China, Stefan Greif and Yossi Yovel from Tel Aviv University, Israel, and colleagues from Israel and Thailand, decided to monitor the body temperature of wild great roundleaf bats (Hipposideros armiger), which is one of the largest insect-eating bats, taking short intermittent breaks between flights while they hunt at night for moths, butterflies and beetles.The team went out to Satun cave in Thailand and caught 18 bats using hand nets. Then, they attached a small temperature and motion sensor equipped with a GPS tracker to the backs of 10 bats, gently inserting the tiny temperature sensor beneath the skin to simultaneously record their temperature as they flew at night. Ten days later, the team successfully relocated five of the original sensors by following the GPS signal, discovering that four had successfully recorded the bats’ body temperature, although only two of the motion sensors clearly showed their flight manoeuvres. In addition, the team tagged the eight remaining bats with the temperature and motion sensors alone and released the animals in a 3×2×2 m3 room, letting them fly freely to collect more detailed information about the mammal's flight manoeuvres and body temperature. Once the experiments were complete, the team removed the tags, and re-released the bats into the wild.After analysing the movements of the bats that had spent time in the laboratory, the team identified 55 flight bouts that lasted for about 48 s. Between these short flights, the bats would stop flying and take breaks on perches. The researchers then compared their activities with the skin temperature measurements and found that the bats that had experienced the fastest skin temperature rises had the highest skin temperatures, while the bats that had slower temperature rises ended up with cooler skins. The researchers then compared those findings with the situation of the bats in the wild and found a similar skin temperature pattern. Also, when the team checked the highest skin temperatures of both groups of bats, they reached approximately 40°C, fortunately well below the lethal level of 44–45°C.The researchers suspect that the short intermittent flights used by the hunting bats may help the animals to avoid potentially lethally high body temperatures. Additionally, when great roundleaf bats take breaks on a perch between foraging bouts, the bats probably reduce the amount of energy that they use, compared with the amount of energy they would consume if continually flying and searching for prey. Now that Luo, Greif and colleagues have successfully measured the skin temperature of wild flying great roundleaf bats, other researchers could use the same strategy to find out how bats that thrive on different diets, such as fruit or even scorpions, keep their temperature down when performing the most metabolically demanding activity known. Alternatively, scientists could look at the body temperature of smaller bats as they go about catching insects, to find out how body size impacts how they regulate their temperature while on the wing.