In a nondescript lab building at the southern edge of Northern Arizona University's campus, Bret Pasch is studying a mouse that produces long-distance calls in the same way that humans speak and wolves howl.
The lemon-sized gray and white grasshopper mouse lives in deserts, grasslands and prairies of the western United States, including in the grasslands north of the San Francisco Peaks.
Sometimes called the wolves of the rodent world, the mice eat grasshoppers, scorpions and even other mice, said Pasch, who is an assistant professor in the department of biological sciences at NAU.
Because of that predatory nature, the mice inhabit large home ranges, Pasch said, and that means they have to amp up their calls to make themselves heard across the larger distances. To do so, Pasch and other researchers from Midwestern University at Glendale, and Ritsumeikan University in Japan found mice rely on airflow-induced vocal cord vibrations similar to the way humans speak. In fact, that’s the way most mammals produce sound, Pasch said.
It’s unique among mice and rats though. Instead, other mice and rat species communicate with each other by producing ultrasonic vocalizations in frequencies above what can be heard by humans. They do so via an aerodynamic whistle mechanism where air passes through the vocal cords but they don’t vibrate. Grasshopper mice produce those sounds as well, but only during close-distance social interactions.
To the human ear, the grasshopper mouse’s long distance calls, which last an average of just a second, sound like a high-pitched squeaky wheel. But when slowed down, the sound’s pitch drops and the calls begin to sound eerily like a deep, resonant wolf’s howl.
So why the need for the deeper long distance call?
Pasch compared it to hearing bass music at a party. The low-pitch, low-frequency sound carries better because the longer wavelengths are not as easily absorbed by objects in the environment as short-wavelength, high-frequency sounds, Pasch said.
Scientists have determined that grasshopper mice calls can travel up to 60 meters on the ground level, he said.
So far, Pasch said it appears the calls are a mechanism to facilitate mate detection, but because the mice are also aggressive, he believes they might also serve a purpose of repelling others away from their territories.
“So these long distance sounds can have dual functions" he said.
Along with better understanding how the grasshopper mice produce and perceive sound, Pasch has lots of ideas about human applications for his research. It could, for example, help provide new insights into what happens when people overuse or blow out their voice, he said. Other applications include studying the mice to see what happens to vocal cords as they age or testing potential treatments to help restore the voice, he said.
Pasch and his team of graduate students use several means to study the mice. That includes rows of sound-deadening, ventilated coolers that mice are put in to record their responses when they are played call recordings. Another large room lined with acoustic foam and equipped with cameras, speakers and microphones allows researchers to play the mice recordings of their calls and then track whether they go toward the call or away from it, Pasch said.
“We call it country mouse studios,” he said jokingly.
He also pointed out several ways the mice make their calls more efficient. One adaptation is a tiny flap that extends from the edges of the animals’ vocal cords and serves to lower the energy needed to initiate vibration. Another is the rodents’ posture when they call. Like opera singers, the mice tilt their tiny heads back and open their mouths wide, creating a geometry that amplifies the sound similar to a horn or a loudspeaker, Pasch said.
Another interesting aspect of the mice's vocalization?
As they go through sexual maturity, their voices crack as they try to belt out their calls. Just like human teenagers.
Neil Cobb, director of the Merriam-Powell Center for Environmental Research at NAU, which Pasch is affiliated with, said he thinks the mice’s vocalization system is “part and parcel” of the rodents’ unique predatory lifestyle.
He said he can’t think of any other rodent that is primarily predatory.
Cobb said further research will probably find more similarities in terms of how vocalization and territorial behavior have evolved in separate but parallel tracks with grasshopper mice and predatory canids like wolves and coyotes.