Researchers listen to bats' hearts in flight

  • Art
  • July 10, 2024

Researchers from Konstanz have measured the heart rates of bats in the wild over several days, including entire flights — the first time this has been done for a bat species. To record the heart rates of male red bats during flight, the scientists attached heart rate transmitters weighing less than one gram to the animals, which they then accompanied in an aircraft as the bats flew, sometimes for more than an hour, in search of food. Their results, published in Proceedings of the Royal Society Bshow how much energy bats consume per day and which energy-saving strategies they use to survive.

Researchers from the Max Planck Institute of Animal Behaviour (MPI-AB) and the University of Konstanz used a special method to study male red bats, which are found all over Europe. Their aim was to understand exactly how much energy bats use during the day and how this changes over the course of the year.

“Bats are fascinating animals that often share habitat with humans,” said Lara Keicher, the study’s lead author. “But bats are still shrouded in mystery. We don’t yet have clear answers to simple questions like: How much food do they need, and can they find enough to survive in different seasons?” To predict how bats will fare in a changing climate, Keicher said understanding their energy needs is key.

Bats with heart rate transmitters

To find out, the scientists fitted bats with tiny heart rate transmitters weighing just 0.8 grams. Just like in humans, heart rate can be used to determine energy expenditure. The transmitters, which the bats wore for just a few days, transmit an audio signal of the bats' heartbeats, which is then recorded by a radio receiver. However, this only works if the receiver is within a few hundred meters of the bats.

“During the day, it was no problem to record heartbeats without major interruptions, because bats rested in tree caves or bat boxes,” says Keicher, who conducted the study as part of her doctoral thesis at the University of Konstanz and the MPI-AB. At night, however, bats fly out to hunt insects and can cover several kilometers in a short time. In order to accompany the bats around the clock, including during their nocturnal flight, the researchers flew in a small plane to follow individuals for entire flights of more than an hour. “I know that we surprised the people of Konstanz when our small plane flew in circles over the island of Mainau late at night,” Keicher recalls.

Awake during the day

The team, which also included members from the Swiss Institute for Snow and Avalanche Research and the University of Freiburg, found that the bats' heart rate during flight is about 900 beats per minute. According to Keicher, who analyzed the signal, “it sounded to our ears like a single high-pitched tone.”

Using the unique heartbeat recordings, the scientists uncovered fascinating strategies bats use to budget their energy expenditure in different seasons. They found that male noctule bats use up to 42 percent more energy in the summer compared to the spring. This is largely because the bats enter a short-term hibernation known as “torpor” in the spring — an energy-saving state in which the heart rate can drop to as low as six beats per minute. “We found that bats in the spring could increase their heart rate when they woke up, reaching a peak rate of 900 beats per minute within a few minutes,” Keicher said.

The team was surprised to find that male bats didn’t use torpor at all in the summer. Keicher explained, “In the warmer months, when food is plentiful, males stay awake during the day to invest energy in producing sperm to prepare for mating in the fall.” To replenish the energy expended, males hunt twice as long in the summer as they do in the spring, eating up to 33 June beetles or more than 2,500 mosquitoes in a single night.

The results have provided insight into the energetic challenges of bats and their fascinating survival strategies. This insight will allow better predictions of how increasingly extreme temperature fluctuations or changes in food availability will affect the animals' lives and possibly threaten them.

The study's lead author, MPI-AB scientist Dina Dechmann, says: “All bat species are protected in Germany and some are threatened with extinction. Basic research that investigates the animals' behaviour and their adaptations to the environment can help us develop protective measures so that, for example, common nocturnal bats can continue to be seen in the night sky above Konstanz.”

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