A hawk’s scream, a coyote’s yipping cries, a chorus of spadefoot toads after a summer rain, the bittersweet song of a Cassin’s sparrow – for someone setting out to create a sonic tableau of West Texas, these would be no-brainers. But there’s another animal sound that belongs on the list.
Grasshopper mice are fierce, if diminutive, predators, that routinely dine on scorpions, centipedes and other venomous prey. And as they set out on their nightly hunts, they emit a long, piercing cry. It’s been called “a wolf’s howl in miniature.” Listening closely to these desert mice reveals the surprising world of “bioacoustics.”
Dr. Bret Pasch is a biologist at the University of Arizona.
“And so I guess the overarching question is, what is the role of voice in the lives of the most speciose group of mammals?” Pasch said. “Rodents represent over 40 percent of mammalian diversity, but we really don't know a whole lot about how they're using their voice to go about their daily lives.”
Pasch discovered a passion for sound in his New Jersey youth – listening to wildlife in the Pine Barrens, and to music at home. Pasch first studied “singing mice” in Central America. Then he turned his attention to what he called the “the most famous mouse vocalization” – that of the grasshopper mouse.
Scientists long believed there were two grasshopper mouse species – a northern one centered on the Great Plains, and a southern species in the desert Southwest. But genetics later showed that Chihuahuan Desert grasshopper mice are a distinct species.
All three overlap in New Mexico’s bootheel region, and Pasch focused his research there.
In appearance, the Chihuahuan Desert mice are nearly identical to their kin in the Sonoran Desert. Generally, their “howls” are likewise indistinguishable. But Pasch found that where they coexist, the Chihuahuan Desert mice shift their calls into a higher register.
The mice, it seems, are advertising to potential mates – which need to be of the same species.
“There's a shift in the Chihuahuan grasshopper mouse's voice,” Pasch said. “It's higher frequency than you would expect based on their body size. You get a shift in voice so as to avoid the costs of mating with the wrong species.”
Pasch captured mice, and studied their hearing sensitivity and vocal anatomy in the lab. He found that the mouse’s larynx is specially adapted to produce its “howl.” When they stand on their hind legs, and raise up their snouts, they can be heard by another mouse 160 feet away.
“It's kind of like an opera singer,” Pasch said. “By flaring their mouth at a certain angle, it basically increases the amplitude. It's almost like the shape of an instrument or like a horn. They're cool, they're really cool.”
And, Pasch said, the dramatic cry is only a tiny part of the mouse’s vocal repertoire. At close range, grasshopper mice communicate though a host of ultrasonic vocalizations.
“And those sounds are as diverse and complex as some bird song,” Pasch said, “and it's a little harder to characterize the variation because it's so dynamic. Those calls probably reveal some more information about the nature of the mouse themselves.”
Whatever these ultrasonic exchanges entail, they’re no doubt vital for the mouse’s survival. Bioacoustics, Pasch said, can aid in conservation.
“The closer that we can get to understanding how animals perceive their world and sense their world, I think the better we can serve them,” he said.
Keep your ears open for the cry of the grasshopper mouse in the West Texas night. And remember that there’s a world of creaturely communication beyond our ken.
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