Creature Feature: Greater Sac-winged bat

Imagine yourself on a walk through a Costa Rican rainforest at dawn. The sunlight starts to break through the canopy, all kinds of different bird species start their morning choruses, slowly awakening the other inhabitants of the jungle. Suddenly, there is another sound tuning in with all the different animal voices – it is a high-pitched chatter-like song descending from a large tree next to the trail you are walking on. But it does not exactly sound like a bird, does it? Following the sound, you discover a crevice on a large tree and find that the sound source is a small, brown furry ball, with two wavy, conspicuous white lines on the back, widely opening its mouth while singing from the bottom of its lungs, sitting next to other furry balls. Congratulations, you found a day roost of the Neotropical Greater Sac-winged bat, Saccopteryx bilineata!

Bats are special creatures in many ways. They are the only mammals that can actively fly and “see” the world with their ears by producing loud, high-pitched vocalizations – echolocation calls – and listening to the echoes bouncing back from objects or prey. Less well-known is the fact that bats are very social animals and also very communicative ones. Bats often live together in stable, perennial groups, repeatedly interacting with one another – a social lifestyle which requires vocal communication to mediate daily interactions.

Among the many bat species, Saccopteryx bilineata is an especially chatty one. Its unusually large vocal repertoire is composed of many distinct syllable types (a syllable is defined as a sound surrounded by silence) and two song types produced by males, the courtship and the territorial song [1]. During the day, this species roosts in well-lit tree crevices, hollow trees and outer walls of buildings to find protection from predators and inclement weather. Saccopteryx bilineata lives in mid-sized harems, made up of a territorial male and up to eight females roosting in his territory. All individuals in a day roost constitute a colony.

At their day roost, every colony member has its favorite spot to hang out, and members maintain an inter-individual distance of several centimeters. Usually, individuals can be identified by their preferred roosting location. The Greater Sac-winged bat can be found from Brazil to Mexico, including the islands of Trinidad and Tobago [2]. [Michael Stifter]

Male S. bilineata bats are very proficient singers. Every day, at dusk and dawn, territorial males produce the so-called territorial song, signalling to other males that the righteous owner of the territory is still thriving and ready to defend his home [1]. The males are also true gentlemen; every day when the females return to their day roost after foraging, each female is greeted by the territorial male. He will sing them a courtship song [1] and present them with a distinct behavioral display, the hover flight. When hovering, the bat remains at one place in the air, in this case in front of the female perching in the day roost. Males also use self-made perfume to charm females. They have sacs in their wing membrane (hence the name, Greater Sac-winged bat), which they fill every day with a mixture of urine and secretions from their glands [3]. This mixtureis subsequently fermented by bacteria in their sacs to create a unique perfume [3]. While hovering in front of a female, their wing sacs open and their individual odor is fanned in the direction of the courted bat.

Every year, around the beginning of May, life in day roosts starts to become more noisy and wild. Females give birth to a single pup. During their first days, pups constantly remain attached to the belly of their mothers, spending their time nursing and resting. After two and a half weeks, life starts to be more exciting for the little ones (and probably more tiring for their moms); the pups start to roost independently. This is the beginning of an extremely interesting vocal development phase: the babbling phase [4].

A pup sitting next to its mother (individually banded with plastic rings on its forearms) while babbling. Over the course of 7 weeks, bat pups engage in babbling daily. Babbling bouts have an average duration of 7 minutes but can last up to 43 minutes in total [4], which would probably be comparable to singing two operas one after another. [A. A. Fernandez]

Babbling in bat pups is reminiscent of the characteristic vocal practice behavior in six-month-old human infants when producing syllable sequences like “bababa,” “gagaga,” “dadada.” Although baby bats and human infants are so different, their babbling behavior is characterized by the same features such as rhythmicity and syllable repetition [4]. While babbling, the pups acquire and practice the syllables from the adult vocal repertoire. Especially noteworthy is that while babbling, the syllables of the territorial song are learned through imitating singing tutor males [5], very much like how young male birds learn their songs or human babies learn to speak. With increasing age, the youngsters become more agile – and bolder. They start to move around in the day roost, performing short flights and practicing the hovering flight [6]. Mothers also interact with their babbling pups, vocally and with behavioral displays. Bat moms even change their voice depending on whether they address their pups or other adult bats [7], a phenomenon reminiscent of what we call “motherese” in humans. Irrespective of the language to be learned, humans change their voice when talking to babies. Our pitch increases and we speak slower and in an exaggerated tone. This helps infants to pick up on our speech. We have yet to find out if and how bat “motherese” affects pup vocal development. At about 10 to 12 weeks old, the youngsters are ready for the next chapter of their lives; females disperse to find a new colony to settle in, while males mostly stay in their natal colony, waiting for a chance to obtain their own territory [8].

Bats, as creatures of the night, often get a bad rap. However, they are truly fascinating creatures that can help us to perceive the world in a different way by studying their behavior and communication. Therefore, if you are lucky enough to find a bat roost, stop, observe them and let yourself be immersed in their world.

Ahana Aurora Fernandez is a postdoctoral researcher in the Knörnschild Lab at the Museum of Natural History in Berlin, Germany. She studies social vocal communication in bats, in particular, vocal ontogenetic processes in the Greater Sac-winged bat.

References

[1] Behr, O. & von Helversen O. (2004) Bat serenades – complex courtship songs of the sac-winged bat (Saccopteryx bilineata). Behavioural Ecology and Sociobiology, 56(2): 106-115.

[2] Yancey, F. D., Goetze, J. R. & Jones, C. (1998) Saccopteryx bilineata. Mammalian Species, 581: 1-5.

[3] Voigt, C. C., Behr, O., Caspers, B., von Helversen, O., Knörnschild, M., Mayer, F. & Nagy, M. (2008). Songs, scents, and senses: sexual selection in the greater sac-winged bat, Saccopteryx bilineata. Journal of Mammalogy, 89(6): 1401-1410.

[4] Fernandez, A. A., Burchardt, L. S., Nagy, M. & Knörnschild, M. (2021). Babbling in a vocal learning bat resembles human infant babbling. Science, 373(6557): 923-926.

[5] Knörnschild, M., Nagy, M., Metz, M., Mayer, F. & von Helversen, O. (2010). Compex vocal imitation during ontogeny in a bat. Biology Letters, 6(2): 156-159.

[6] Strauss, M., von Helversen, O., & Knörnschild, M. (2010). The ontogeny of courtship behaviours in bat pups (Saccopteryx bilineata). Behaviour, 661-676.

[7] Fernandez, A. A. & Knörnschild, M. (2020). Pup directed vocalizations of adult females and males in a vocal learning bat. Frontiers in Ecology and Evolution, 8: 265. [8] Nagy, M., Heckel, G., Voigt, C. C. & Mayer, F. (2007). Female-biased dispersal and philopatric kin groups in a mammal with resource-defence polygyny. Proceedings of the Royal Society B: Biological Sciences, 274(1628): 3019-3025.

Main image: Mother-pup pair of the Greater Sac-winged bat. [Michael Stifter]

[Edited by Alexandra Dwulit.]