In the world of predator and prey, there a constant push and pull, where both sides continuously evolve to outwit each other. For example, bats evolved echolocation abilities and some insects developed ultrasound-sensitive ears to evade them.
However, a recent study has brought forth questions regarding this evolutionary arms race theory between bats and insects.
The study was conducted by Professor Lasse Jakobsen, a bat expert at the University of Southern Denmark, along with researchers at the Max Planck Institute for Biological Intelligence and the University of Toronto.
Historically, the barbastelle bat (Barbastella barbastellus) is known to be quieter than other bats that hunt flying insects. Emitting sounds approximately 20 dB quieter means they produce a sound pressure that is 10 times lower.
This feature has led many to believe that the barbastelle evolved to be quieter as a retaliatory move against insects with ultrasound-sensitive ears, labeling it as the bat that “struck back.”
However, upon analyzing the barbastelle’s close relatives, the researchers made a surprising discovery. Most of the bat’s relatives are not in the business of catching flying insects. Instead, they feed on insects resting on surfaces like leaves and branches.
These species, known as “gleaning bats,’ are all quieter compared to those that hunt flying insects called “hawking bats.” The barbastelle, despite its quiet nature, is categorized as a hawking bat.
“If most of the barbastelle’s family are gleaners, then their ancestor was very likely also a gleaner,” said Jakobsen.
This means that it’s improbable for the ancestor of the barbastelle to have been a loud hawking bat that adapted into a whispering barbastelle due to insect evolution.
“A species does not have free choice when it evolves in a new direction. For example, it is a condition for mammals that their ancestor did not have feathers, so their descendants will never evolve a wing with feathers. Instead, they have found another solution for flying: modified skin between the fingers,” explained Jakobsen.
So, why is the barbastelle quieter when hunting in the air? “It is not an evolved ability. It just cannot produce louder calls than it does, because as a descendant of a gleaner it is probably morphologically limited,” said Jakobsen.
“But it has found a niche, where it can use its low amplitude calls. It is an evolutionary coincidence; it sort of fell into this niche, where there was something to eat.”
Interestingly, the prey in this niche consist of flying, nocturnal insects that, despite their ultrasound-sensitive hearing, fail to detect the barbastelle, making them easy targets.
Further insights revealed that many gleaning bats emit sound from their noses, leading to their calls being 20 dB lower, while most bats call out of their mouths, allowing for louder sounds.
This distinction clarifies that the barbastelle’s soft calls aren’t a result of an evolutionary arms race, but rather an inherent trait derived from its ancestors.
“So, the reason why the barbastelles are so quiet today is not an expression of an arms race between bats and insects, but rather simply an expression of the fact that it is descended from bats that cannot call as loudly as others,” said Jakobsen.
Image Credit: Sherri and Brock Fenton
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