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The short version

  • Proboscis bats employ ultrasonic frequencies to communicate without alerting predators or prey, a strategy known as acoustic crypsis.
  • The species produces at least 22 distinct syllable types, including complex songs during mating season and subtle social signals used while roosting.
  • Researchers found that most bat vocalizations exceed the hearing range of common predators like birds and spiders, which typically cannot detect sounds above 40 kilohertz.

A recent study published in the Annals of the New York Academy of Sciences offers new insights into how proboscis bats manage the inherent risks of their lifestyle. These tiny mammals, which weigh less than an adult house mouse, inhabit the rainforests of Central and South America. They spend their days roosting openly on tree trunks, a behavior that exposes them to significant danger from large birds and spiders. To survive this precarious existence, the bats rely on visual camouflage, with brown speckled wings and fur that blend seamlessly into bark.

However, remaining visually hidden is insufficient if the animals must communicate. Behavioral ecologist Lena Dressler of Berlin’s Natural History Museum notes that roosting outside requires ensuring detection avoidance even during social interactions. The research team discovered that proboscis bats also employ acoustic camouflage. They echolocate and exchange social signals at very high frequencies, keeping their chatter out of range for both potential predators and the insects they hunt.

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The investigation was led by Dressler, Martina Nagy, and Mirjam Knörnschild, who runs the laboratory where the work took place. Knörnschild initially believed proboscis bats were bioacoustically uninteresting because early observations occurred during summer months when vocal activity is low. The team later realized that these bats are highly vocal during mating seasons in spring and fall. This realization prompted a focused study on their communication patterns in Costa Rica.

Fieldwork presented significant challenges for the researchers stationed in Guanacaste and at the La Selva field station. Observing free-ranging bats required remaining perfectly still to avoid disturbing them. The environment was unpredictable, with other wildlife occasionally interfering with data collection. Dressler recounted an incident where monkeys threw mangoes at recording equipment, forcing a temporary retreat. Despite these obstacles, the team successfully captured extensive audio data using ultrasound microphones.

Analysis of the recordings revealed a complex vocal repertoire consisting of at least 22 distinct syllable types. A syllable is defined as the smallest sound unit surrounded by silence. One frequently used syllable, termed a 'hook,' features a frequency decrease at its end, creating a hook-like shape on spectrograms. Bats may produce these syllables individually or in series, and they also combine different syllables to form more complex vocalizations.

The study identified 17 different vocalization combinations. Some of these sounds fall within the human hearing range, particularly the elaborate songs produced by males during mating season. These songs are the longest recorded vocalizations and serve a sexually selected function, helping males attract mates. In contrast, other signals appear to be naturally selected traits designed to enhance survival rather than reproductive success.

One specific hook series, which is inaudible to humans, likely conveys social information such as individual identity and sex. Bats use these subtle vocalizations while engaging in self-care activities like grooming or swaying with the wind. By keeping these signals quiet and high-pitched, the bats disguise their presence. This behavior illustrates that communication strategies are not one-size-fits-all; signal properties depend on whether the goal is social bonding or predator avoidance.

The effectiveness of this acoustic strategy relies on the hearing limitations of local predators. Most birds and spiders cannot detect sounds above 40 kilohertz, whereas humans hear only up to about 20 kilohertz. Proboscis bats produce very few syllables below the 40-kilohertz threshold. Additionally, these high-frequency sounds do not travel far, further reducing the risk of detection. This combination of frequency and limited range allows the bats to maintain social connections while remaining acoustically invisible to threats.

The findings highlight the delicate balance animals must strike between visibility and invisibility. While sexually selected signals like mating songs are conspicuous and designed to reach distant receivers, socially selected signals for daily interaction are adapted to remain inconspicuous. This duality demonstrates that optimal communication depends entirely on the specific ecological pressures and behavioral needs of the species.

Future research may explore how these vocal strategies vary across different proboscis bat populations or in response to changing environmental conditions. Understanding these mechanisms provides broader insights into evolutionary adaptations for survival in predator-rich ecosystems. The study underscores the sophistication of bat communication, revealing that even the smallest mammals employ complex acoustic tactics to navigate dangerous environments.

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Go to the original reporting

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