Mice establish family groups, territories and complex social structures. They communicate with each other in a variety of ways, using their mouths, noses, ears and bodies. Much of their vocal communication is outside the human hearing range, though you can hear some squeaks in their lower voice range.
Mice usually squeak when talking to each other in their nests or when they are scared. They make plenty of vocalizations that we don't hear. Male mice create a kind of ultrasonic song to attract females who are ready to breed; females sometimes talk to their girlfriends using ultrasonic vibrations as well.
What we consider waste is vital communication for mice. They produce chemicals and pheromones in their urine that tell other mice who's related to whom, whether they have plenty of food to eat, whether they are ready to breed and who's in charge. Dominant males have a different scent to their urine than other males in the family group. If you're a mouse owner, you should clean the cage often, but not every day. Leaving urine-soaked nesting materials for a day or so helps the mice communicate.
Mice can communicate basic emotions, such as aggression or fear, using body language. When a male mouse wags his tail, it's usually a sign of aggression. Males fight for dominance and the right to mate with certain females, and they warn each other of impending fights by wagging tails. Younger or weaker males might cower instead of wagging back, hoping to prevent a fight.
Mice build strong social bonds by grooming each other. When one mouse gently grooms another, she's telling the other mouse that she's a friend. This can take a negative turn when a dominant male mouse decides to groom a submissive one to ensure everyone knows who's the boss: The dominant male might "barber" the other male, grooming him around the face until some of the hair falls out. A lack of bald spots can help other mice spot the dominant male right away.
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