Like warring street-corner troubadours, certain mice sing to claim their territory. They may not get any tips in their guitar cases, but by knowing where it's safe to sing, they keep the whole neighborhood harmonious.
Two related species of singing mice share the mountains of Costa Rica and Panama. One, Scotinomys teguina or Alston's singing mouse, lives at lower altitudes and is widespread in the forests of Central America. The other species, Scotinomys xerampelinus or the Chiriquí singing mouse, resides on the tops of the mountains. Males of both mice make chirping calls, unique to their species, that attract mates and advertise to competitors.
But the two tuneful rodents don't exactly meet up for karaoke duets. Bret Pasch, a biologist at the University of Texas at Austin, investigated three mountains where a clear boundary line divides the territory of Alston's mice below from Chiriquí mice above. Why, he asked, is this division so sharp?
Using traps baited with peanut butter and oats, Pasch and his colleagues first documented where the boundary between mouse species was. Then they set up face-offs between males of the two species. Placing pairs of trapped mice in enclosures together, they saw that S. xerampelinus, the higher-altitude mouse, was more aggressive and tended to attack the lower-altitude species. (The lower-altitude mouse is probably wise to retreat, since it's a smaller animal.)
Returning to spots on the mountainside where they knew each species lived, the researchers broadcast recordings of both kinds of mice singing, and listened for responses. Chiriquí singing mice, the more aggressive species, responded to calls of either kind. But Alston's singing mice were more likely to hush up when they heard a song from their rivals. When a male Alston's singing mouse was by itself in an enclosure, the sound of the other mouse's song—played from a speaker—was enough to make it retreat to a far wall and stay there.
Pasch concluded that the higher-altitude mice aren't intimidated by their neighbors, but are restricted to the mountaintops by temperature. The lower-altitude mice, wary of encounters with their larger and more aggressive upstairs neighbors, stay away whenever they hear that mouse's song. When Pasch removed all the Chiriquí mice from certain boundary-zone areas (by trapping them and then carrying them across a river), he saw that Alston's mice quickly moved into the vacant territory.
Alston's singing mice use their relatives' song as a hint to stay away, and Pasch says this sort of interaction could be widespread. "Closely related species often share similar ecological requirements—eating similar foods and living in similar places—as well as similar means of communication," he says. Because of this, communication between species "is probably common." Just don't expect them to appear together in concert.
Image: Alston's singing mouse, by Bret Pasch.
Bret Pasch, Benjamin M. Bolker, & Steven M. Phelps (2013). Interspecific Dominance Via Vocal Interactions Mediates Altitudinal Zonation in Neotropical Singing Mice. The American Naturalist DOI: 10.1086/673263