Migrating silver-haired bats fine-tune torpor to save energy

Abstract

Ultra-distance marathon runners are almost a race apart. Covering thousands of miles over weeks and even months, these extreme human athletes require mental stamina and physical endurance. Yet, many migratory species take these distances in their stride. ‘Some studies suggest that silver-haired bats spend their winters in southern USA and their summers in the Canadian boreal forest’, says Dylan Baloun, currently a graduate student at the University of Saskatchewan, Canada. But the bats that undertake these odysseys are at a natural disadvantage. ‘They are nocturnal, so daytime stopovers are mandatory’, says Baloun, who explains that stopovers can be extremely costly for some species – totalling ∼70% of the entire migration cost for some birds. But, the mini mammals have a nifty energy-saving trick up their sleeves. ‘Bats use torpor’, says Baloun, explaining that they drop their body temperature while they rest to conserve energy. However, no one had directly measured how much energy the torpid mammals conserve while hanging out during the day and how the roost temperature affects their energy saving. So, Baloun and PI Chris Guglielmo from the University of Western Ontario, Canada, packed up their lab and headed out to the Long Point Bird Observatory to catch bats in the act.‘It is difficult to bring and maintain state-of-the-art physiological equipment into the field, even with a mobile laboratory trailer to maintain a stable climate’, says Baloun, who ventured out into the near-by woods at night to retrieve silver-haired bats during their spring and autumn migrations. Back in the temporary lab, the duo weighed and scanned the animals using magnetic resonance to measure how much fat and muscle the animals were carrying. Then, after attaching a thermometer to each bat to track its body temperature, they snuggled the individual animals inside soft cloth bags where they could slip into torpor and slumber away at roost temperatures of 10, 17.0 or 25°C. Finally, when the bats were preparing to depart again during the following dusk, the duo repeated the body measurements before allowing the animals to continue on their way.Calculating how much fat and muscle the bats lost during their daytime stopover and converting the loss into the amount of energy consumed during their slumber, the duo could see that the animals used essentially the same amount of energy over the course of the day, regardless of the temperature in their roost. The team then calculated the total amount of torpor experienced by each animal and it was clear that the bats were fine tuning how cold they became and for how long, with the result that they all consumed similar amounts of energy during their stopover regardless of the air temperature. And when the duo calculated how much energy the bats consumed during their break, they were impressed to see that it was as low as 15–20% of the total migration energy cost. ‘By entering a state of torpor, bats save energy for future migratory and potentially reproductive costs’, says Baloun.However, the team also realised that some of the bats were conserving energy and using torpor in slightly different ways, with bats that carried more weight using less torpor at the warmer temperatures, while the heavier females consumed more energy during spring stopovers than the lighter males. ‘Body mass was a major determinant in how bats used torpor’, says Baloun.So, migrating bats have the ability to dramatically reduce their energy costs during enforced daytime stopovers and the study adds another piece to the puzzle of how these tiny mammals pull off extreme feats of endurance, at which most of us can only marvel.