Ask biologists why predators don't wipe out all prey, and the answer is often that there's an arms race going on between predators and prey, with each side constantly evolving new ways to deceive the other. This assumption is especially common for bats and their prey, insects. Fifty million years ago, the first bats evolved the ability to echolocate so they could hunt in the dark, and in response, some insects evolved ears sensitive to ultrasound so they could hear and hide from the bats.
Researchers have questioned the idea of an arms race between bats and insects, arguing that bats' sound waves were inherited from quieter bat ancestors rather than being a direct adaptation to insect hearing.
However, if there is an ongoing arms race, bats should also respond, says Lasse Jakobsen, a biologist and associate professor at the University of Southern Denmark. He is a bat expert and co-author of a new study published in Current Biology. In the study, he and his colleagues questioned the evolutionary arms race between bats and insects.
Other authors include Daniel Lewanzik and Holger R. Goerlitz of the Max Planck Institute for Biointelligence, and John M. Ratcliffe and Erik Etzler of the University of Toronto.
The main argument in favor of the arms race hypothesis is that some bats' calls when hunting are not as loud as others and therefore not easily heard by insects. These bats are the western broad-eared bat (Barbastella barbastellus), and they are about 20 decibels quieter than other flying insect-eating bats, which means the sound pressure they emit is 10 times lower.
"This bat has historically been thought of as a 'fight back' bat against insects," said Lasse Jakobsen.
However, something puzzled him and his colleagues: Throughout the bat's closest relatives, almost no other members catch insects in the air. Instead, they eat insects on surfaces such as leaves and twigs, and these species are both quieter than those that prey on flying insects.
In the bat research community, bats that catch insects in the air are called eagle bats, while bats that feed on insects from the surface are called gleaning bats. The Western broad-eared bat is a species of hawk bat.
"If most members of the Western broad-eared bat family are hawkbats, then it's likely that their ancestors were also hawkbats," Lasse-Jacobson said.
Therefore, it is unlikely that the ancestor of the Western broad-eared bat family was a vocal species, or that it evolved into a low-singing species in order to adapt to insect hearing.
"When a species evolves in a new direction, it does not have a free choice. For example, the ancestors of mammals did not have feathers, which was a condition, so their descendants never evolved feathered wings. Instead, they found another solution for flight: modified skin between their fingers," explains Lasse-Jacobson.
Yet if the western broad-eared bat didn't evolve the ability to hunt more quietly in the air during an arms race between insects and bats, then where did it come from?
"It's not an evolved ability. It just couldn't make a call louder than itself because it was probably morphologically constrained as a descendant of an eagle bat. But it found a niche where it could use a low-amplitude call. It was an evolutionary coincidence; it kind of fell into this little world where there was something to eat."
This "little world" is home to flying nocturnal insects that can hear sounds and are therefore good at avoiding nocturnal bats. But their hearing is not strong enough to catch bats, so they end up becoming prey.
The reason for the morphological restriction must be found in how bats produce sounds. Most bats use their mouths to make their calls, which allows them to produce loud sounds. Many gleaning bats use their noses to make sounds, which makes their calls 20 decibels quieter.
"So the reason why bats are so quiet today is not a sign of an arms race between bats and insects, but simply a sign that their offspring cannot sing as loudly as other bats," Lasse-Jacobson said.
Insects that fly at night include moths, beetles, and mosquitoes, and many moths can hear when bats are approaching. Before bats appeared about 50 million years ago, nocturnal insects had no significant enemies; today, only bats hunt insects at night.