In the summer, after the sun sets in the city of Konstanz in southern Germany, the sky fills with bat boys. They are common noctules, or Nyctalus noctula: brown and furry bats that are smaller than a kiwi fruit and about a third of its heft... But the males remain, living in colonies of bachelors-perhaps the most literal instance of a man cave-where they rest, hunt, and produce as much sperm as they possibly can.
It is never easy to be a bat. The animals burn immense amounts of energy during flight, and once they find food, they burn through it quickly, putting them at risk of starvation...summer for male bats, which need even more energy and nutrients to produce massive amounts of sperm-so much sperm that their testicles can increase to up to eight percent of their body mass.
In a paper recently published in the journal Proceedings of the Royal Society B, a group of researchers attached custom-made heart-rate trackers that could be carried by a flying bat. They successfully measured the heart rates of male common noctule bats over the course of a few days in the spring and summer of 2020, revealing just how much energy a bachelor bat needs during this hectic testicular time.
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