The University of Hawaii-Manoa music professor played Terry Woodford's recording in a recent pilot study to try to reduce aggression among young male chimpanzees at Honolulu Zoo.

The heartbeat music has "proven statistically significant in helping very, very small infants fall asleep," including his own 15-month-old child, Ruszkowski said.

The zoo has 10 chimpanzees -- four females and six males, four of whom are infants or juveniles, he said.

"Since the males are entering adolescence, they are causing bodily injury and smashing glass," both of which are costly, he said. One broke a window that will cost about $50,000 to replace, he said.

Brainstorming with Arthur Harvey, UH-Manoa music education coordinator, Ruszkowski said he developed a study for the chimpanzees similar to one Harvey conducted for cardiac patients.

But Harvey was able to hook the patients up to monitoring devices, which he could not do with aggressive male chimpanzees and "keep all my fingers intact," he said.

Ruszkowski said he played music about 30 minutes every morning for a week during the chimpanzees' most aggressive period. He played none in a trial period the next week.

He has not finished analyzing preliminary data, involving 75 variables, but his general observation was the music had a calming effect on the animals within 10 to 15 minutes, he said.

"They were so relaxed, some chimpanzees were falling asleep. That is something that never happened before."

Greg Hamilton, primary chimpanzee keeper, said he is continuing to play Woodford's heartbeat recording or Harvey's "Hawaiian Music with a Heartbeat" if he feels the animals are riled up in the morning.

Videotapes of Disney movies also are popular, particularly with 8-year-old Nalu, he said.

After the chimps are taken from separate pens and put together in a group, there "definitely is increased aggression and excitement," Hamilton said.

"It's all about troop dynamics, the socialization of these guys. We have 8-, 9- and 10- and 13-year-olds. They all have way too much testosterone. They're aggressive to prove themselves."

He said 10-year-old Kona Kona broke the window, "just feeling like he's macho man and needs to prove himself."

Using music to mitigate the aggression is a "win-win situation," he said. "If something happens, great. If it has no effect, we're still in the same situation."

Based on preliminary results, Ruszkowski said he probably will do an expanded study in the summer.