A new study has examined the aggressive calls and seductive trills of two groups of singing mice in Costa Rica’s highlands.
The study’s author, Bret Pasch, who attends the University of Texas, focused on the Alston’s brown mouse and the Chiriquí brown mouse. Pasch observed that male mice used high-pitched songs to pierce their often cloud-draped habitats with claims of territory and mating desires.
The communication has helped draw territory lines, benefiting the more aggressive and dominant Chiriquí mice. They live only in Costa Rica and Panama, and are limited to the highest altitudes in Costa Rica, often bushy environments above the tree line.
The Alston live in a wider range of environments stretching from southeast Mexico to Panama. They prefer a mid-range altitude, and overlap with the Chiriquí territory between 2,200 and 2,900 meters above sea level. The Chiriquí’s aggressive instincts are key for maintaining its land, as Pasch’s study found that the Alston will willingly colonize higher-altitude territory when given the opportunity.
With a team, Pasch spent nine weeks and logged 26,880 hours trapping and observing the mice. They visited Irazú Volcano east of San José, Cerro de la Muerte in the south-central interior of Costa Rica, and Barú Volcano in the western interior of Panama.
Pasch captured 192 mice in the field, then set up makeshift laboratories to observe their behavior. He let mice from the different species interact for 10 minutes, reporting that the Chiriquí almost always initiated aggression. He used audio recordings to see how mice would respond. His study showed the high-altitude Chiriquí would vocalize in response to calls from both species, whereas most Alston’s only responded to calls from one of their own.
Pasch tested female mice in a previous study, discovering that they preferred the fastest-paced trills. Female mice would respond faster and migrate to the side of a cage where speakers played the faster songs.
Vocalization for territory are nothing new in the animal kingdom. Birds use it to claim roosting sites and whales use it as well. However, Pasch’s study is one of the few examples where mice exhibit the behavior. Pasch explained vocalization as a way to minimize direct combat.
Travelers keen on taking a day trip to Irazú Volcano will be disappointed at the apparent silence of Alston’s singing mouse. It broadcasts its high-pitched squeals at 22 to 26 kHz, just above the average person’s hearing barrier of 20 kHz. Visitors would have little problem hearing a Chiriquí, which sings at an audible 16 to 20 kHz, provided it is positioned above 2,200 meters.
Climate change could tip the balance in favor of Alston’s. As other studies have shown, rising temperatures are forcing many organisms to migrate up-slope in Central America’s rain forest, to where they are more comfortable. Alston’s is already happy to conquer new territory if it sees no threat. Chiriquí will have nowhere to go.
Pasch has published other videos of the mice here.