Here's what's likely happening:
* Social Bonding: Rumbling is often a sign of contentment and affection. Your guinea pig may be expressing happiness and wanting to bond with the other pig.
* Dominance Display: Sometimes, rumbling can be used as a dominance display. The guinea pig shaking its butt might be asserting their position in the social hierarchy. This is usually accompanied by other behaviors like mounting or nipping.
How to tell the difference:
* Contentment: If the guinea pig is relaxed, happy, and interacting playfully with the other pig, it's likely a bonding rumble.
* Dominance: If the guinea pig is acting aggressive, holding its head high, and nipping, it's more likely a dominance rumble.
Here are some other things to keep in mind:
* Individual Guinea Pig Personality: Some guinea pigs rumble more frequently than others.
* Context: Pay attention to the overall behavior of your guinea pigs. If there are other signs of aggression or stress, the rumbling might be a sign of something more serious.
If you're concerned about your guinea pigs' behavior, it's always best to consult a veterinarian or experienced guinea pig expert. They can help you interpret the behavior and ensure your guinea pigs are happy and healthy.