In the tropical forests of Central and South America, tiny proboscis bats (Rhynchonycteris naso) spend their days pressed against tree trunks. Their mottled fur and wings blend in with the bark, but their open habitat makes them vulnerable to birds and spiders. To avoid attracting attention, these bats not only camouflage themselves visually but also communicate and navigate using sounds that remain beyond the hearing of most predators and prey.
A new study published in Annals of the New York Academy of Sciences shows that proboscis bats use frequencies above 40 kilohertz — the threshold beyond which their natural enemies usually cannot hear. Humans perceive sounds only up to 20 kilohertz, so many of the bats' signals are indistinguishable to us. Scientists from the Museum of Natural History in Berlin and the Smithsonian Tropical Research Institute recorded in Costa Rica at least 22 types of syllables and 17 combinations, among which high "hooks" predominate — series of sounds carrying information about the sex and individuality of the individual.
The bats "talk" especially actively during the mating season. Males perform complex songs, but these too mostly remain in the ultrasonic range. Their echolocation signals turned out to be higher than those of closely related species of the same size. Calculations showed: a moth is able to hear an approaching bat only at a distance of less than three meters, which gives the insect a minimum of time to escape. Thus, high frequencies simultaneously help them hunt in stealth mode and reduce the risk of being detected themselves.
The researchers note that social signals and echolocation apparently evolved together: the same organs produce and perceive both types of sounds. Selection aimed at stealth could have influenced both. At the same time, the range of such signals is small — they do not carry far, which further reduces the likelihood of interception.
Unlike many other bats, proboscis bats prefer open places, so acoustic camouflage becomes especially important for them. The data indicate that only a few types of syllables fall within the range audible to predators, while most do not. This allows them to maintain social ties without sacrificing safety.
However, the future of the species raises questions. Climate change and possible super-El Niño events could disrupt the usual precipitation regime in the region, which has already led to significant deaths of other bat species. Scientists continue to study the song repertoire and the conditions under which the bats switch to more noticeable signals.
Observations of proboscis bats are a reminder: even in a world where survival depends on the ability to remain unnoticed, nature finds ways to maintain connection between individuals without attracting unnecessary attention.
