Born to Roar: Lions' and Tigers' Fearsome Roars Are Due to Their Unusual Vocal Cords

When lions and tigers roar loudly and deeply -- terrifying every creature within earshot -- they are somewhat like human babies crying for attention, although their voices are much deeper.

So says the senior author of a new study that shows lions' and tigers' loud, low-frequency roars are predetermined by physical properties of their vocal fold tissue -- namely, the ability to stretch and shear -- and not by nerve impulses from the brain.

"Roaring is similar to what a baby sounds like when it cries," says speech scientist Ingo Titze, executive director of the National Center for Voice and Speech, which is administered by the University of Utah. "In some ways, the lion is a large replica of a crying baby, loud and noisy, but at very low pitch."

The study of lion and tiger vocal folds and how they produce roaring -- vocalizations used by big cats to claim their territory -- was set for publication on Nov. 2, in the Public Library of Science's online journal PLoS ONE.

While the comparison was not part of the study, Titze says a baby "cries to have people come to help it. The lion uses similar attention-getting sound, but mainly to say, 'I am here, this is my territory, get out of here.'"

"In both cases, we hear loud, grating sounds that grab people's ears. When a baby cries, the sound isn't pretty. The sound is basically rough. The vibration isn't regular."

The same is true of roars by lions and tigers, and, like babies, their vocal folds (commonly called vocal cords) are "very loose and gel-like" and vibrate irregularly to make roars sound rough, Titze says. The main difference: Babies cry at a high-pitched frequency, while big cats have a low-frequency roar.

Roaring Frequency Dictated by Structure of Vocal Folds

The new study's key finding is that lions and tigers can roar loudly and deeply because their vocal folds have a flat, square shape and can withstand strong stretching and shearing. That contradicts a theory that lions roar deeply because the vocal folds are heavy with fat.

Instead, the fat helps give the vocal folds their square shape where they protrude into the airway, unlike triangular vocal folds in most species. The fat also may cushion the vocal folds and provide repair material when they are damaged, the researchers say.

"We were trying to correct a previous assumption that lions and tigers roar at low fundamental frequencies because they have a huge vocal folds," says study co-author Tobias Riede, a research assistant professor of biology at the University of Utah and a research associate at the National Center for Voice and Speech.

"It's true they have large vocal folds, but the shape and the viscoelastic properties [tension and shearing strength] make the roars so loud and deep," he says.

Riede says the scientists "set out to find out the relationship between structure of the vocal folds and how they work to produce the roar in lions and tigers. We tested if the mechanical properties of the vocal folds allowed us to make predictions about the sound."

They did. Measurements of vocal fold resistance to stretching and shearing let researchers accurately predict the "fundamental frequency" ranges at which lions and tigers are known to roar, and the lung pressures needed to produce those roars.

Titze and Riede conducted the research with first author Sarah Klemuk, an adjunct assistant professor of communication sciences at the University of Iowa; and Edward Walsh, director of auditory physiology at Boys Town National Research Hospital in Omaha, Neb. Titze is on the faculty at the University of Iowa and University of Utah, where he is a research professor of otolaryngology and medicinal chemistry. The research was funded by the National Institutes of Health and the National Science Foundation.

"We study a lot of animals -- deer, elk, dogs and cats," Riede says. "Lions and tigers are just interesting examples for very loud and low-frequency vocalization."

These studies have a practical aspect. "If you understand how vocal folds are structured and what effects that structure has on vocal production, then it could help doctors make decisions on how to reconstruct damaged vocal fold tissue" in people such as cancer patients, singers, teachers, coaches and drill sergeants, he says.