What makes a big cat roar?

The functional center of mammalian vocalization lies in the larynx, a cartilaginous structure that is connected to the trachea (or windpipe) below, and to the pharynx (or throat) above. The pharynx expands into the nasal and oral cavities and, in some species, also into a collection of other mucosal membrane–lined air chambers. Those interconnected spaces function in the same way the bell of a trumpet helps radiate sound: They collectively shape the output of the cat’s vocal system.

The fundamental structural element of voicing is a bilateral pair of laryngeal protrusions known as vocal folds, commonly referred to as vocal cords. Together they create an opening in the middle of the airway known as the glottis. The epiglottis, shown in figure 1, sits atop the airway, where it acts like a lid that protects the respiratory system from ingested material. It has also slowly evolved into a resonator.

Like all voiced sounds, long-distance roaring is produced during exhalation as warm, moist air flows through the glottal aperture. The air flow sets the vocal folds into vibration at their resonant frequency. Glottal pulses formed by the vibrations are transferred to the oral cavity and associated nasal cavities, where the emergent voice is shaped.

As seen in figure 1, the jaguar’s vocal folds are larger and longer than the snow leopard’s. Indeed, vocal-fold length scales with body mass. Longer and more massive vocal folds produce lower fundamental frequencies, much like how longer strings on a musical instrument produce lower notes.

Two spectrograms displaying tiger advertisement calls are seen in figure 2. They capture the changes in a call’s frequency over time. The amplitudes of component frequencies are shown colorimetrically: White and yellow represent elements with higher sound pressure levels, and red and purple represent lower levels. Perhaps the most evident feature of the call is that expression frequencies are harmonically related—that is, the call is periodic in nature. The harmonic structure is complemented by aperiodic components that make the call spectrally noisy, as seen in the bottom spectrogram in figure 2. The aperiodic component of the call reflects contributions produced by nonlinear intra- and supralaryngeal interactions among various resonating structures scattered throughout the vocal tract and oral cavity. The nonlinear features of a call convey essential information related to the vocalizing animal’s sex, fitness, and identity.

But the size of big-cat vocal hardware is not the only reason why the cats can produce the long-distance roar. Adaptations in the shape of their vocal apparatus are another crucial factor. Roaring-cat vocal folds are square or rectangular in shape. Those of other mammals, including nonroaring cats, are typically triangular. Rectangular-shaped vocal folds, with their thick and compact medial surfaces abutting one another, are more aerodynamically efficient than triangular ones, and that higher efficiency manifests in the form of a wide range of output levels. At the low end of the range, rectangular-shaped folds support the production of remarkably low phonation thresholds. In other words, a relatively small amount of lung pressure can vibrate the vocal folds and produce low-level sounds. At the high end, powerful contractions of thoracic muscles and the diaphragm help produce intense big-cat roars.

Roaring-cat vocal folds are also resilient to the powerful forces produced by intense vocalizations. That is due in part to a unique anatomical feature: fat deposits deep in the body of the vocal folds that not only influence their shape but also function as a cushion to absorb deformations associated with the extreme vibrations that occur during roaring. In addition, the vocal folds’ epithelium intertwines with the underlying connective tissue to produce a large surface area that strengthens the vocal-fold tissue when it undergoes extreme mechanical stress. As a result, roaring-cat vocal folds possess a remarkable capacity to withstand powerful shearing and stretching forces that would injure those of species with less robust vocal hardware.

Our understanding of the physics of roar production is derived in large part from tissue engineering and biomechanics investigations. But roaring also illustrates how slow-moving evolutionary pressures have caused subtle but essential changes in a small cluster of tissues lining a modest stretch of the larynx that enables production of the long-distance roar.

Ed Walsh is the director and JoAnn McGee is the codirector of the Auditory Neurobiology Laboratory at the VA Loma Linda Healthcare System in Loma Linda, California. Their research focuses on the intersection of biomedicine and bioacoustics.