As the northern winter silence melts, spring greets us with its familiar choir of birds chirruping in the tree tops. Yet, beneath the trees on the floor of Costa Rica's tropical cloud forests, the year-round sound of animal chatter never dies down. Alston's singing mice (Scotinomys teguina) carry out raucous conversations with one another as they defend territory or court a mate. Beyond that, how their back-and-forth communication operates is largely unknown. In a new study published in Science, Daniel Okobi Jr and Arkarup Banerjee in the lab of Michael Long at the New York University School of Medicine, USA, describe how male Alston's singing mice avoid interrupting each other and the brain circuit responsible for their chitchat etiquette.
To test how the mice responded when a conversation started up, the team initially kept some of the rodents in isolation while recording their solo serenades. Then the scientists relocated the loners to a new home, adjacent to another mouse. However, neither mouse could see or smell the other; they could only communicate by singing. After settling in to his new abode, the team noticed that the incomer became more chatty, singing three times more than he had when there was no audience. What was more striking was how polite the mice were when exchanging words: the mouse that had just moved into the neighbourhood waited patiently until after his new neighbour finished singing and then, within a half a second, rapidly responded with his own tune. This precisely choreographed duetting suggests that singing mice actively track conversations and respond only when the social situation is appropriate.
To figure out how singing mice coordinate their colloquy, the researchers investigated the motor cortex, a brain area they suspected to be responsible for polite conversation in mice, and uniquely required for speech production in humans (but not for generating other mammals’ sounds). Knowing that mice become gradually louder and produce notes of longer duration when singing, the team cooled down the motor cortex – to slow down the neural activity. This caused the mice to take longer to spit out what they had to say, but all of the notes were articulated just fine. In contrast, stimulating the motor cortex momentarily interrupted a vocalizing mouse, after which the rodent picked up right where it left off. Lastly, when the team silenced brain activity in the motor cortex with nerve-blocking drugs, the mice still sang as often as before; however, they did not sing back in response to hearing another mouse's song. Taken together, the motor cortex in singing mice seems to be a hub for rapidly perceiving song and precisely orchestrating a vocal reply.
Many creatures across the animal kingdom rely on making sounds to communicate with one another: people use language; birds twitter; Alston's mice sing. These findings from Okobi and his colleagues help extend our understanding of how vocal communication evolved in animals and the shared brain areas that enable animals to chatter, in addition to raising the question of just how quiet mice really are.