Complaints by men about nagging wives are a popular theme in popular culture. If bats had standup comics, the males among them would probably also depict the females as howling tyrants who keep them under heel. Female bats, it turns out, scold the males regularly at high intensity, and apparently over the predictable issues: food, relationships, space.
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This finding emerges from a new study conducted by scientists from Tel Aviv University, which seeks to decipher the chirp-based communication among the winged mammals. The fact that vocal communication exists among bats is known. But what messages do the vocalizations in the bat colonies transmit? What are the bats “saying”? And can the sounds they make be considered a language?
The study, published this month in Scientific Reports, an online, open-access journal published by the same people as Nature magazine, was conducted by Mor Taub and Yosef Prat in the Bat Lab for Neuro-Ecology, part of the zoology department and the Sagol School of Neuroscience at Tel Aviv University. According to Dr. Yossi Yovel, who is the director of the lab, “Bats are very social mammals that live in colonies of thousands or even millions. Entering a cave that contains a colony of bats, one hears a cacophony of sounds and chirps. Because crowding in a colony is extreme, most of the interactions are along the lines of ‘Move! Don’t bother me!’”
How do the researchers know this? To monitor and analyze what goes on among these flying mammals, they focused on 22 Egyptian fruit bats equipped with special tracking devices, recording and filming them for nearly three months, 24 hours a day.
“From the hundreds of thousands of interactions that we observed among the bats, we selected 15,000 vocalizations in which we knew for certain who the ‘speakers’ were and the context of the vocalizing,” Yovel explains.
The 15,000 interactions were the raw materials for an in-depth analysis. “We used algorithms that serve for vocal identification in telephone applications,” Yovel says. “We used the computer to examine the vocal spectrum of the different bats by analyzing sound waves. We found that a great deal of information can be extracted from bats’ calls.”
The researchers say that in 90 percent of the cases, they were able to identify the “speaking” bat correctly, and in 65 percent they could also figure out the specific bat to which the message was aimed. They also had an 80-percent success rate in deciphering the content of the vocalizations, or the context in which things were “said.”
The context was initially identified only on the basis of the sound, but was afterward verified by means of the videos. By cross-matching information from the vocal analysis with the identification of bat that emitted the sounds and analysis of the surrounding physical and social situation, they arrived at a relatively precise – and probably unprecedented – level of decipherment of bat language. It would be stretching things to say that the scientists are now fluent in bat talk. However, their research paper presents a new and intriguing element of “conversations” in inter-bat communication.
Yovel and his team identified four different social situations, each of which is characterized by a distinctive vocalization and all of which are related to basic conditions of existence. The first is related to eating and is manifested in fights over food and attempts to grab food from other bats. Second is mating – aggressive courting, or the spurning of such mating attempts. The third social situation occurs during periods of rest: A chirp is emitted that apparently means, “You woke me up!” The fourth situation is one of “general squabbling,” which is naturally common in the crowded cave.
The mapping and identification of contexts are a scientific achievement that advance the understanding of bats. However, they do not describe, for the moment, a very rich world of content or new social phenomena.
“In 70 percent of the cases, we are able to identify the interactions and even predict the response of the bat at which the vocalization was aimed,” Yovel notes. “But it’s clear that the bats’ system of message transmission is more complex and contains more information and more nuances [than we have been able to detect so far]. Sophisticated vocal communication is essential for an animal that lives in the dark. It’s apparently an important method for transmitting social information among the members of the colony, something like gossip, as you find in every community with a highly developed social life.”
The 60 or so bats that inhabit Yovel’s laboratory are free to come and go as they please, flying as far as some of Tel Aviv's suburbs and then coming back; only 22 were chosen for close examination. The small colony roosts in a room that has been specifically adapted for them and decked out as an artificial cave. It is dark, has an entrance that evokes a natural cave, and nets hang from the ceiling for the convenience of the tenants. The lab is equipped with cameras and other sensory devices that allow for monitoring and providing data for several different studies being carried out by Yovel’s team.
The analysis of the vocal communication is not yet able to provide a social map of bat colonies. Questions relating to leadership, parenthood and status within the colony remain unanswered. But by prolonged viewing and analysis of the colony’s activity, the researchers have gleaned some indication of the existence of social phenomena in the bats’ world.
“We saw that the females ‘shout’ more at the males, in terms of both frequency and intensity,” Yovel explains. “The males – at least the ones in our lab – are relatively disciplined and quiet.”
The team has also noticed that there are almost always bats that “win” in squabbles, from which they infer that the vocal information may also contain messages of dominance or status.
The vocal communication between the bats is unrelated to their famous sonar system, based on sound waves of frequencies that are sometimes inaudible to humans, which serve orientation and navigation purposes.
“They maintain two separate sound systems in parallel, and how that works is an interesting question. We don’t know how it is manifested in their brain,” Yovel points out. The scientists are also trying to find out whether bats, like many other mammals, are born with their vocalization ability, or acquire it as part of a learning process.
There are many differences among the 1,300 or so known species of bats, but also much in common. Is language one of the traits they share? Yovel thinks that each species has its own language, though the possibility can’t be ruled out that all bats are able to listen to one another and, at some level, to extract information from what they hear.
“It’s like the way I understand something when I hear two Germans speaking,” he says. “If it’s sufficiently important, evolution will allow that type of learning.”