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RACINE — The sweet, yet secret language of a pair of gibbons at the Racine Zoo has Carthage College researchers curious what their playful sounds can reveal about human language development.

Since May 28, Angela Dassow, an assistant professor of biology at Carthage and two student research assistants have been recording the dulcet tones of Yule, a 53-year-old white-handed gibbon, and Robyn, his 38-year-old daughter, as they communicate with each other at the zoological gardens.

On July 2, with temperatures soaring into the 90s, the Hylobates Lars gibbons weren’t in a particularly “calling” kind of mood. Yule contently sat in his green box high above the habitat while Robyn swung on large ropes below the canopy, occasionally foraging.

“These two have been housed together their whole lives. They still actually exhibit the play behavior that we expect out of the infants, even though, obviously, they’re quite old,” Dassow, of Milwaukee, said. “What we’re trying to understand here is why they’re still doing this and what exactly is ‘play’ behavior in these gibbons, specifically with the vocalizations because they make this kind of bleating sound.

“It’s completely different than the rest of their vocal repertoire. The rest of their vocal repertoire is very tonal. Occasionally, they have a few little chirps and whoos that are in there as well, but then the ‘play’ vocalization is very unique,” she said.

Complexities of language

Understanding the gibbons gives the zoo the opportunity to better understand the complexities of language in primates, which in turn, helps researchers draw parallels between animal and human communication, Dassow said. That is also helping researchers understand language development, including how human language disorders occur and progress, she added.

The research crew consists of Carthage students Joy Layton, a senior from Vernon Hills, Illinois, studying biology and Kenosha sophomore Azniv Khaligian, who is studying toward degrees in music and neuroscience. They have hauled their equipment — a directional mic and camcorder — out to the Racine Zoo since May 28, while paying heed to record and collect data on the gibbons’ every vocalization.

“We haven’t been able to record any of these play vocalizations in the wild yet,” Dassow said. “And I don’t suspect we ever will, just because gibbons live higher up in the (forest) canopy. And this vocalization is a really quiet one, so the likelihood to be able to get this in the wild is pretty much non-existent.”

While gibbon infants are known to vocalize play, Dassow suspects that the father and daughter have been able to retain the special dialogue “because they are a related pair.”

“That parental bond has never been broken because they’ve always been together,” she said. “Gibbons that are either in the wild or in other zoos are typically moved out of their family group.”

Different sound types

Dassow said their study of the Racine gibbons will contribute to evolution of vocal communication. “We’ve chosen to work with gibbons because they do produce a very diverse repertoire of sounds,” she said.

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For example, she said, English-speaking humans produce about 40 different sound types and gibbons, 28.

Dassow said that some of the gibbons’ vocalizations are associated with territorial defense, while others are correlated with predator warning calls.

“So, they have a unique sequence of sounds that they use to warn other gibbons if there’s a tiger around,” Dassow said, adding there are individual and unique warning calls if there is a leopard or a snake nearby. “They have a warning in captivity here for their veterinarian.”

The gibbons haven’t yet created a warning call for Dassow.

“They like me so far,” she said. “When they see me when I used to come here more frequently, they would come to the side of the cage and just give a cute little ‘hoo-hoo-hoo-hoo-hoo,’ a little greeting, and then they go back to their business.”

This isn’t the first time Dassow has recorded the gibbon pair. Seven years ago she recorded them, as well. She has also studied gibbons at the Henry Vilas Zoo in Madison, where a pair there had a special warning call for the Good Year blimp, which flies over the nearby Camp Randall Stadium to provide aerial footage of the football games.

“The gibbons (there) just don’t like them,” she said. “They don’t mind hawks flying overhead. They don’t mind planes, helicopters, kites — that’s all fine. But when the Good Year blimp goes by (they may see it as something) that can potentially harm them.”

Intersection of her majors

For Khaligian, the gibbons’ calls are a fascinating intersection of her two majors.

“I can learn things in both disciplines through them and through observing them. It’s really interesting, especially going into the lab and looking at the scientific representation of the sound, which is completely unlike sound as I’ve studied it before,” said Khaligian. “I think my music background (as a violinist) helps me understand the sound and it will help me understand scientifically what’s going on when I create music.”

She said that string instruments or trombones would be the best instruments to mimic the gibbons’ vocalizations because of how the strings and the slides can be manipulated to produce sounds.

“On the violin you can slide the whole way up and same with a trombone,” she said.

The crew will be at the zoo through the end of July.

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