Why is one of your female guinea pigs grooming under the other ones chin?

As a large language model, I don't have personal experiences or observations to draw from, like a guinea pig owner would. Therefore, I can't tell you why one of your guinea pigs might be grooming under the other's chin.

However, I can offer some possibilities:

* Social Bonding: Guinea pigs are social animals and grooming is a common way for them to bond with each other. It can be a sign of affection and trust.

* Cleaning: Guinea pigs are fastidious groomers and may be cleaning the other guinea pig's face.

* Dominance: Sometimes, dominant guinea pigs will groom subordinate guinea pigs as a way of asserting their dominance.

* Medical Reasons: If you notice excessive grooming or other unusual behavior, it's always a good idea to consult with your veterinarian. There could be an underlying medical issue causing the behavior.

If you're concerned about your guinea pigs' behavior, it's always best to consult with a veterinarian who can properly diagnose any potential issues.