Hot bats change torpor game rules

Abstract

In just the same way that turning off the furnace in your house in the middle of a cold winter night causes the property to cool, when hibernating mammals stop generating heat during winter to conserve energy, their temperature also drops to match that of their environment. But not all animals turn down the inner thermostat for an entire season. A shorter version of hibernation is torpor, where animals stop keeping their body warm for a few hours or days when it is cold or when food is scarce. However, most of our understanding of how animals use torpor has been gleaned from research done in colder regions, where there is little spare heat and an animal's body temperature drops as soon as they turn down their inner furnace. But what happens when animals resort to this form of energy conservation in warmer climes? Stephanie Reher and Kathrin Dausmann, from the University of Hamburg, Germany, recently studied a species of bat (Macronycteris commersoni) in Madagascar and found that when these bats slow down their rate of energy usage, their body temperature actually increases.Reher and Dausmann found this new flavour of torpor by measuring both the bats’ skin temperature and metabolism when the animals were resting during the day. Unexpectedly, their skin temperature increased when the bats reduced their metabolism. In other words, the bats had been actively using energy to stay cooler than the surrounding air temperatures, and as soon they stopped putting effort in, their bodies heated up. In fact, the air temperature in the dry forests of Madagascar is often hotter than the bat's body temperature, sometimes as high as 41°C, and the bats’ body temperatures even reached 42.9°C on one occasion. Reher and Dausmann think that this ‘hot torpor’ saves the bats energy and allows them to better tolerate hotter temperatures.In addition to this unusual form of torpor, the bats did a second unique thing on cooler days, around 34°C; some bats entered many micro-spells of torpor throughout their daytime resting period, almost like cat-napping when you are supposed to be listening to a presentation. During these micro-spells of torpor, the bats used less energy, essentially pausing their metabolism for only a few minutes at a time (3–53 min). Because these micro-spells were so short, the body temperature did not have time to reach that of the local air; it only changed a little, shifting toward the air temperature. The researchers think this penny-pinching attitude saves the bats energy over time, but allows them to remain alert to any threats while they are roosting, compared with traditional lengthier periods of torpor when bats are lethargic or even unconscious – imagine being woken from a very deep sleep.In addition, this discovery of a novel mode of torpor reveals the importance of studying species in the tropics, where high temperatures can cause major physiological differences relative to species that reside in cooler climes. This finding, that some species do not follow the traditional rules of torpor, may also suggest that the rules were not right to begin with. The fact that body temperatures can increase when the metabolism is reduced may force us to reconsider our previous understanding of torpor and rewrite what, until now, was an accepted rule of thermal physiology.