Male mice make ultrasonic vocalizations that, when slowed down enough for us to hear, sound like songs. Listen here.
This study looked at what makes them sing. The answer: female mice.
They sing when they smell the urine of female mice, but not rats or humans.
They sing when they can touch female mice, but not if they can only see them.
And they sing when they are...um...interacting with female mice.
But they don't sing as much when certain brain receptors have been knocked out.
Muscarinic receptors (in this case M2 and M5), are necessary for dopamine release. Dopamine is a neurotransmitter that plays a role in motor control, emotion, sexual behavior, and scads of other functions.
If you don't have dopamine, you are not happy.
So mice without M2 and M5 receptors are not happy and they aren't likely to sing. I wonder if they only sing the blues.
Now let's see what drugs can do. (Mice on drugs!) When they give these guys amphetamines, which activate dopamine, they sing more. Except if they have had the M5 receptor knocked out.
The authors propose that ultrasonic vocalizations can be used to measure positive affect in mice.
In other words, you can tell how happy a mouse is by listening to it sing.
Here's the paper.