When winter approaches and the temperatures drop, curling up and hibernating seems like a blissful way to get through the dreary months. However, hibernation isn’t all it’s cracked up to be, with deep slumber being rudely punctuated by periods of arousal. These awakenings are energetically demanding, requiring the snoozing animals to raise their body temperatures and their metabolic rate to prepare for short bouts of activity. During the cold months, hibernating bats are only awake 5–10% of the time, but an incredible 85% of their winter energy expenditure goes towards powering these arousals. If these moments of alertness are so energetically draining, then what is the purpose of them? For Pipistrellus kulii bats it seems that these arousals are necessary for thirst-quenching drinks of water, according to a new study by Miriam Ben-Hamo and her colleagues from the Ben-Gurion University of the Negev, Israel (p. 573).
To determine what caused arousals in these bats, Ben-Hamo captured 25 bats in the surrounding Israeli desert before bringing them back to the university. Once there, the team slowly coaxed the bats into hibernation by mimicking wintery conditions, lowering the temperatures and decreasing daylight hours as well as providing enough food for them to fatten up before their hibernal sleep. Working quietly, the team of scientists then observed the bats, recording how long they hibernated before awakening and how frequently these arousals occurred. They also measured how much carbon dioxide they produced as an indicator of metabolic rate, and how much water they lost by evaporation under different humidity conditions.
The team found that changes in metabolic rate did not affect hibernation patterns. However, they did notice that the more water the bats lost through evaporation the less time they spent in deep slumber. These perspiring bats also awoke more frequently, and because waking up requires so much energy they also had lower body masses. Metabolic water production is not sufficient to replace evaporative water loss, so the team thinks that it is likely that the bats are waking up for a quick sip of water to rehydrate before drifting back to sleep.