These Small, Stealthy Bats Use High-Frequency Vocalizations to Keep Their Chatter Out of the Range of Predators and Prey

The tiny proboscis bat sits outside on tree trunks during the day and eats insects like moths and mosquitoes, keeping the bug population in check. But settling out in the open is a dangerous lifestyle that leaves these bats—which are even smaller than an adult house mouse—at risk of being eaten by the large birds and spiders in the rainforests of Central and South America. Brown speckled wings and fur that blend in with tree bark allow proboscis bats to hide in plain sight, for example.

A study that Dressler and colleagues published in July in the Annals of the New York Academy of Sciences now shows that not only do proboscis bats demonstrate visual camouflage, but they also hide vocally. They echolocate and communicate at very high frequencies, so prey can’t hear them approaching until it’s too late and predators often don’t notice them. Mirjam Knörnschild, a behavioral ecologist and evolutionary ethologist who runs the laboratory where Dressler works at Berlin’s Natural History Museum, wasn’t impressed with proboscis bats when she first observed them.

The problem was that Knörnschild had initially observed them at the “wrong” time of year—the summer months. When the team realized that the bats actually vocalize a lot, especially during mating season, which is a few months in spring and again in fall, they wanted to find out everything they could about proboscis bat communication. So, Dressler and behavioral ecologist Martina Nagy, co-lead scientist and postdoctoral fellow at the Natural History Museum, flew to Costa Rica.

Both researchers observed and recorded the vocal repertoire of free-ranging proboscis bats in Guanacaste. Nagy conducted additional studies at the La Selva field station, established by the Organization for Tropical Studies. Hanging out with bats was not always easy.

The researchers had to stay very still, and unwanted visitors would stop by. In the study, the team found that proboscis bats vocalize with at least 22 distinct types of syllables, defined as the smallest unit of a sound surrounded by silence. A syllable that the bats frequently used is called a “hook” because it decreases in frequency at the end, giving it a hook-like shape on a spectrogram.

The bats produced some syllables once, as a monosyllabic vocalization, but they also produced the same syllable multiple times in a series. Additionally, the bats made complex vocalizations, combining different syllables. Overall, the team recorded 17 different vocalizations, or combinations of syllables.