Large pocket-winged bats are the dream of field biologists. They hunt insects at dawn and dusk, staying awake much of the day and resting at night.
“They like fairly well-lit places,” says Mirjam Knörnschild, a behavioral ecologist at the Museum of Natural History in Berlin who studies the bats, which rest on trees or the sides of buildings rather than in gloomy, guano-filled caves.
And the bats, about two inches long, keep enough distance between them to tell them apart. “We mark them with colored plastic rings on their forearms,” said Dr. Knörnschild. “We can also use a directional microphone and record the vocalizations of individual bats.”
That’s important because these bats are the only mammals other than humans known to babble like human babies. Bat pups’ babbling includes mature syllables and sounds that only the pups make, and the nature of the babbling changes over time as the bats learn territorial and court songs. Also, their songs are not sung at the high frequencies that bats use for echolocation.
“It immediately reminds you of babies,” said Ahana A. Fernandez, also at the museum, who presented a recent analysis of the babbling with Dr. Knörnschild and other colleagues. dr. Fernandez said the babbling is well known but not thoroughly studied. The researchers wanted to “analyze in detail the pup’s babbling behavior and compare it to the babbling of human babies.”
They analyzed recordings of 216 “babbling attacks” by 20 bat pups from two colonies in Costa Rica and Panama, which lasted about seven minutes on average but lasted as long as 43 minutes. The researchers found that the sounds the puppies make are similar to human babies in their repetition of syllables, the rhythmic nature of the babbling and the universality of the babbling behavior.
As with human babies, all puppies babbled. Other similarities to the babbling of human babies were the early start of the babbling, the long sequences of sounds, and the fact that the pups did not need stimuli from other bats. Like babies, they just kept on chatting and gradually got more sounds. The researchers published their results Thursday in the journal Science.
D. Kimbrough Oller of the University of Memphis, who has studied the vocal development of human infants for decades, said that “there are some remarkable parallels” to human babbling and also to birdsong in the bat pup’s extensive observations and analysis, one of which is. was just “the amount of gibberish going on.”
Like human babies, they made noise all the time.
“When they’re awake, they do it,” said Dr. Oller, who was not part of this research, but is collaborating with some of the authors on future papers. He said the bats babble whether or not there is stimulation, like human babies do. Human babies seem to explore the sounds they make, he said, playing with them as if they were auditory objects, similar to physical objects they manipulate and taste and play with. “I think the greater pocket-winged bat is probably doing the same thing. I think they’re probably exploring those sounds,” Dr. Oller said.
No other mammals are known to do this type of babble, although it is common in songbirds.
Among the bats, both male and female pups babble, but females stop stringing together the syllables they’ve learned when they’re weaned. dr. Knörnschild said that grown women “don’t sing. They don’t produce a territorial song or courtship song and men do.”
Then why babbling? dr. Fernandez said it may be that when the females learn what’s in a courtship song, it makes them better judges of male songs. This is just a hypothesis, but the females definitely rate male songs. Males compete intensely with their bat song to attract a harem of females. The females choose which male they prefer, and the males constantly try to court them, in a sort of constant talent show.
“Female choice appears to play an extraordinarily strong role” in mating behavior, said Dr. Knörnschild. “The males are slightly smaller than the females and they can’t physically force them to do anything.”
And the females may not just be looking for the brightest or most energetic singers, according to Richard Prum, an ornithologist at Yale. In his book “The Evolution of Beauty,” he argued that whether in plumage or dancing or singing, female birds and the females of some other animals make choices based on essentially aesthetic criteria, which pleases them. He said that in the case of the bats, the aesthetic choice could drive the development of song.
dr. Knörnschild said aesthetic choice was certainly a possibility, although she said there were also acoustic qualities in bat song that indicated the fitness of the males.
She also suspects that there are more babbling species. Until now, the science of vocal learning has focused on birds, but among mammals, she said, mole rats, giant otters, dolphins and other cetaceans are good targets for research.
“It would be really interesting to have more babbling descriptions from different species and then maybe pinpoint the evolutionary pressures that cause babbling to occur in one species and not another,” she said.
